MidLongman wifery(1990)© Group6' 93-98UKLid ]990
Midwifery
o,o0,.,.,0o~oo0o oo.,,,.o oo
CLINICAL REVIEW
Nutritional requirements in pregnancy: What should the pregnant woman be eating? Frank Hytten
It is n o w r e c o g n i s e d t h a t ' e a t i n g f o r two' d u r i n g p r e g n a n c y is n o t a s o u n d practice. H o w e v e r , w h a t s h o u l d a p r e g n a n t w o m a n eat? T h i s article reviews firstly the available l i t e r a t u r e o n the f o o d intake o f h e a l t h y w o m e n p r o d u c i n g a h e a l t h y baby, a n d s e c o n d l y the l i t e r a t u r e o n the calculation o f n e e d s f r o m w h a t is k n o w n o f the physiological d e m a n d s o f p r e g n a n c y .
INTRODUCTION What should a pregnant women eat to ensure optimum nutrition for herself and her fetus? T h e r e have been two main approaches to this question. One is to record the food intake of healthy women having healthy babies; the Other is more theoretical, the calculation of needs from what we know of the physiological demands of pregnancy. T h e first approach is relatively crude because of the great practical difficulties of accurate measurement and is in general only applicable to groups of women rather than to individuals. T h e second is likely to be more precise and also provides more detailed and interesting insights into the biology o f childbearing. But the recent realisation that there is a conspicuous discrepancy between the results provided by the two methods has shed new light on the subject.
Frank Hytten MD, PhD, FRCOG Previously Head of the Division of Perinatal Medicine, Clinical Research Centre, Northwick Park Hospital, Harrow, Middlesex. Current address: 'Blossoms', Cobblers Hill, Great Missenden, Bucks, HP16 9PW.
The physiological basis Taking the physiological approach, there are certain specific nutritional costs of pregnancy which are obvious and can be clearly defined. Like the costs for any enterprise, the nutrient needs may be divided into 'capital g a i n s ' - those that must be acquired but are kept, such as the products of conception and the new maternal tissue, and 'running costs' - those that are acquired and used up for the extra metabolic demands imposed by pregnancy. T h e 'capital gains' can be conveniently looked at in terms of the extra weight gained by the pregnant woman. T h e average healthy woman, eating to appetite, gains about 12.5kg (281b) body weight, and that weight gain is, on average, associated with an optimum reproductive performance. Using that figure as a basis Hytten & Leitch (1971) estimated its components and they are summarised in Table 1. Those average figures are derived from a great deal of published evidence and show that the products of conception account for only about one third of the weight gained at term; the remainder is maternal tissue. Most of that maternal tissue is in obvious sites: the uterus, the enlarged breasts, the expanded volume of circulating blood, and 93
94
MIDWIFERY
in the last trimester a considerable quantity of extracellular fluid which often manifests itself as clinical oedema. But there remains a considerable c o m p o n e n t of the weight, some 3.5kg (almost 81b), which has been accumulated by 30 weeks and contains no water; it is depot fat stored mostly in subcutaneous tissue a r o u n d the lower trunk and u p p e r thighs (Taggart et al, 1967). T h e r e has been considerable discussion in the past about the possibility of the p r e g n a n t woman storing protein, but protein cannot be stored 'dry', only as new tissue. T h e body has no capacity for protein storage of that kind, and although some animals such as the rat enlarge their livers in pregnancy there is no convincing evidence of any such p h e n o m e n o n in h u m a n pregnancy. From these data the energy value of the capital gains can be calculated. Details o f the calculations are given in Hytten & Leitch (1971), but the accumulated fat which is mostly in the m o t h e r and also in fetus during the last weeks of pregnancy, accounts for over 36000 kCal of stored energy. Protein which has been built into fetal tissue and such maternal sites as the enlarged uterus, accounts for a little over 5000 kCal (Hytten, 1980). T o this stored energy, the 'capital gai~ns', must be added the 'running costs' of the pregnancy and these are shown in Figure 1. T h e metabolism o f the fetus and placenta, and of the added maternal tissues, makes an important contribution only in the later stages of pregnancy and at term accounts for only about half
38 36 Cardiac work
34 32 30
Respiration
28 26 24
i
22
o
18
Renal work Breasts
20
Ulerus
16 Placenta
14 12 10 g
Felus
6 4 2 0 10
20 30 Weeks of pregnancy
40
Fig. 1 Components of the 'running costs' of pregnancy (Hytten, 1980)
the metabolic costs. But there are considerable costs incurred throughout pregnancy by the major physiological adaptations of the mother: there is the additional work of the heart and of the respiratory muscles, and in particular the very high cost of recovering sodium f r o m the increased glomerular filtrate. When the oxygen consumption figures are translated into calories, the metabolic, or running costs of the pregnancy amount to a further 36 000 kCal. T h e separate costs, shown together in Figure 2 indicate that the total specific energy cost of pregnancy is about 80000 kCal. Figure 2 also shows two important features of the pattern. One is the remarkably even requirement for
Table 1 Analysis of weight gain in pregnancy (adapted from Hytten, 1980) Increase in weight (g) up to: 10weeks
20weeks
30weeks
40 weeks
Fetus Placenta Amniotic fluid Uterus Mammary gland Blood Extracallular extravascular fluid Maternal fat
5 20 30 140 45 100
300 170 350 320 180 600
1500 430 700 600 360 1300
3400 650 800 970 405 1450
0 310
30 2050
80 3480
1480 3345
Total
650
4000
8500
12 500
MIDWIFERY 80 0 0 0 70 0 0 0 60 000
Maintenance
50 000
8 40 000 30 000
Fat 20 0 0 0 10 000
Protein 0
10
.
20
30
40
Weeks o[ pregnancy
Fig. 2 The cumulative energy cost of pregnancy and its components (Hytten, 1980)
extra energy during pregnancy, about 350 kCal per day during the last two trimesters: almost all capital gains at the beginning when the fetus is small and the mother is storing fat and almost all running costs at the end when the fetal demands are greatest and the mother has stopped storing fat. T h e second is that the r u n n i n g costs of late pregnancy could be totally subsidised, in energy terms, by the fat stored earlier in the pregnancy. This last point makes biological sense of the huge adipose energy store. At the time when this physiological mechanism would have evolved, and even in many third world countries today, women did extremely hard physical work in conditions of uncertain food supplies. It therefore made sense to store a food supply for the fetus if food was available in early pregnancy against possible deprivation later. Such anticipatory energy storage, controlled by hormones, is c o m m o n in the animal world - mammals who hibernate do it before the winter, and birds do it before migration. So far we have considered only the energy requirements of the p r e g n a n t woman; the physiological basis for calculating the needs of individual nutrients is rather less certain.
Protein T h e only major nutrient which need be considered is protein, and from the figures given in Table 2 it can be calculated that the m a x i m u m daily accretion of protein, which occurs in the
95
final weeks o f pregnancy, amounts to about 6 g per day. T h e calculation o f how much maternal dietary protein is needed to supply that 6 g o f tissue protein in such sites as the fetus and placenta is complex and depends on the quality of the dietary protein and the general sufficiency of the diet; for example, in a totally inadequate diet protein might be b u r n e d to supply energy. But for the average European women eating a normal diet the extra dietary protein requirement is about 8.5g per day. This assumes that the metabolism o f protein remains unaltered in pregnancy, an assumption which has been questioned by Naismith (1977) who has shown that rats adapt their liver function in pregnancy to spare protein which might otherwise be degraded. T h e r e are no data which suggest that this is h a p p e n i n g in women, but if it did then the additional protein requirements for pregnancy would be substantially less than 8.5 g per day. An extra 8.5 g o f protein presents no problem; if the woman consumes the extra 350 kCal per day which she needs, then unless she is eating a very unusual and badly balanced diet she will eat much m o r e than 8.5 g o f extra protein. T h e r e are no particular requirements in the diet for extra carbohydrate and fat which will not be met by a normal diet.
Other nutrients Vitamins For most other nutrients the conspicuous lack o f physiological data is matched by the wealth o f strongly held opinions, and that is particularly true of the vitamins. T h e r e is no evidence of a
Table 2 The components of protein and fat accumulated during pregnancy [g] (adapted from Hytten, 1980) Protein
Fat
Fetus Placenta Amniotic fluid Uterus Breasts Blood Maternal fat store
440 100 3 166 81 135 0
440 0 4 4 12 20 3345
Total
925
3825
96
MIDWIFERY
particular extra need for vitamins in a well nourished community which will not be met by the increased overall dietary intake, although certain vulnerable groups may require supplements of folate, or vitamin D. For exam, ple, some Asian women who customarily spend most of their time indoors and whose calcium absorption may be impaired by an excess of wholemeal cereals in the diet should have vitamin D supplements. Minerals T h e extra requirement for Calcium is about 30 g, almost all for the fetal skeleton, and that can be readily acquired from an ordinary diet without recourse to drinking large quantities of milk. Even if the diet were grossly deficient, 30g is a tiny proportion of the mother's skeletal calcium and she could readily provide it from her own stores. The mineral which generates most controversy is iron because there is a widely held view that many women start pregnancy with deficient iron stores, and that the ordinary diet is incapable of supplying the extra needs for iron about 4 mg per day in the last half of pregnancy. T h a t opinion has originated largely because many of the haematological indices such as haemoglobin concentration, serum iron and serum ferritin suggest iron deficiency, whereas those changes, like so many, are part of the widespread modification of homeostasis which occurs in healthy pregnancy and do not indicate iron deficiency. Controlled trials have shown clearly that routine iron supplements in pregnancy, while tending to 'correct' the normal haematological changes, offer no clinical benefit (Mahomed & Hytten, 1989). Indeed there is evidence that iron supplementation can be harmful; it leads to an increase in red cell size which may increase blood viscosity to a degree which could impair utero-placental blood flow. And that may be responsible for the observed increase in low birthweight infants when the mother has been supplemented (Mahomed & Hytten, 1989). That is not to say that true iron deficiency anaemia does not occur in pregnant women. But it should be looked for and treated when it
occurs. It is bad medicine to treat all pregnant women as anaemic, and possibly put some fetuses in jeopardy, in o r d e r to catch a few women who need treatment. T h e same may be said of the new cult metal zinc. There is no good evidence that most pregnant women are deficient, as has been claimed, and universal supplementation is likely to do harm (Hytten, 1985).
The overall dietary needs We have been discussing the specific nutrient needs of pregnancy, and we can conclude that if a healthy woman eats, in addition to her ordinary requirements, a good mixed diet which supplies an extra 350 kCal per day, all the needs of pregnancy will be met. So how big a total diet does the average pregnant woman need? In the past it has been customary to add the specific 350 kCal per day for pregnancy to the usual energy intakes of non-pregnant women, although for an individual much will depend on her usual level of activity. Over the years diet survey data have consistently suggested that pregnant women do not eat 350 kCal more than usual and the strength of that evidence is increasing. For example Durnin et al (1985) found in a weighed diet survey that 67 healthy Glasgow women had a total energy intake in pregnancy only 20000 kCal above their non-pregnant intake, not the calculated 80000, and that their basal metabolism was considerably depressed from early pregnancy. That pregnant women economise on energy expenditure is obvious: they tend to spend more time in bed or sitting at rest, and are more economical in almost everything they do. Even in rural Africa where the demands of agriculture make relaxation difficult, the women manage to reduce their energy output by cutting down on leisure activities (Roberts et al, 1982). But the Glasgow evidence suggests a more fundamental change. T h e r e is certainly a reduction in circulating levels of thyroid h o r m o n e during pregnancy (Franklyn et al, 1983) so that all the maternal cells will be metabolising at a somewhat slower tempo, and there is some evidence that
MIDWIFERY 97
the metabolic response to a meal is reduced in pregnancy (Illingworth et al, 1987). There are other possible ways in which the body could economise on energy expenditure, and they suggest some interesting avenues for research. Meanwhile the addition of 200 kCal daily to the usual non-pregnant diet seems a more appropriate figure than the calculated specific cost of 350 kCal.
Recommended dietary allowances It is necessary to refer to R e c o m m e n d e d Dietary Allowances because they are so often used as a basis o f comparison with measured diets. Numerous official bodies such as the World Health Organization and the British Medical Association have published schedules which specify the amounts of energy and nutrients which should be provided to maintain populations in health. They are not intended for application to individuals and are no more than guides to the needs of populations or large groups. They are based in large part on much of the evidence presented above and it is o f some interest that as physiological evidence has accumulated 'recommended allowances' of energy and nutrients have usually become smaller. But recommended allowances always include a substantial safety margin, and it would be wrong to conclude, without clinical evidence, that because the diet of an individual woman falls somewhat short of the standard it is therefore necessarily inadequate. Indeed in some instances the standards have been set ridiculously high. T h e American r e c o m m e n d e d allowances, set by the US National Research Council, give 20rag as the daily allowance for zinc, and yet in a prosperous and healthy L o n d o n population studied by Campbell-Brown et al (1985), the average intake was only about half that, and no individual reached the recommended intake. Standards for some other nutrients such as calcium and vitamin C which may accord with the intakes of many individuals are also much too high to be regarded as requirements. For the practical midwife official r e c o m m e n d e d allowances of nutrients for pregnant women are more misleading than helpful.
Should any food be avoided? Food taboos in pregnancy have a long history, and there can be few foods that, somewhere or at some time, have not been forbidden to pregnant women. Women often undergo changes of taste and develop aversions to, or cravings for, certain foods sometimes progressing to pica - the craving for such non-nutritive substances as coal, soap or toothpaste. These may he inconvenient, but there is no evidence that they do any harm. Alcohol and caffeine That alcoholism in the mother can seriously damage the fetus is beyond doubt (Jones et al, 1973). But whether 'social drinking' is damaging is uncertain, and the problem is difficult to investigate because women who drink moderate amounts of alcohol often also smoke more and are liable to take other drugs. T h e current best evidence (Wright et al, 1983) suggests that two alcoholic drinks per day, which could be considered as moderately heavy consumption, does not appear to damage the fetus. Nevertheless, there is widespread anxiety among obstetricians that alcohol may cause more subtle damage to the fetus and it seems wise to advise pregnant women to restrict alcohol consumption as much as possible. The effect o f caffeine from such beverages as tea, coffee and some proprietary soft drinks has also been subject to a number of studies (Wright et al, 1983). T h e r e is no good evidence for fetal damage, but the US Food and Drug Administration has advised pregnant women to avoid caffeine-containing foods and drugs, or use them sparingly.
CONCLUSIONS In practical terms the answer to the question which o p e n e d this paper is this: if a healthy woman eating an ordinary diet gains about 450 g (lib) per week during the last two trimesters of her pregnancy, then her nutrition, and that of her fetus is almost certainly secure. That is a slight oversimplification - iron deficiency must be looked for and treated, and women at risk of specific nutrient deficiencies such as vitamin D
98
MIDWIFERY
m a y n e e d specific s u p p l e m e n t a t i o n , b u t in g e n erally well-nourished communities the exceptions are uncommon. Finally it s h o u l d be said t h a t it is c e r t a i n l y much more important that a woman should have b e e n well n o u r i s h e d d u r i n g h e r g r o w i n g years, p e r h a p s e v e n i n c l u d i n g f e t a l life, b u t t h a t is a n o t h e r story.
References Campbell-Brown M, Ward R J, Haines A P, North W R S, Abraham R, McFadyen I R 1985 Zinc and copper in Asian pregnancies - is there evidence of a nutritional deficiency? British Journal of Obstetrics and Gynaecology 92:875-885 Durnin J V G A, McKillop F M, Grant S, Fitzgerald G 1985 Is nutritional status endangered by virtually no extra intake during pregnancy? Lancet 2:823-825 Franklyn J A, Sheppard M C, Ramsden D B 1983 Serum free thyroxine and free triiodothyronine in pregnancy. British Medical Journal 287:394 Hytten F E 1980 Nutrition. In: Hytten F, Chamberlain G (eds) Clinical Physiology in Obstetrics. Blackwell Scientific Publications, Oxford Hytten F E 1985 Do pregnant women need zinc supplements? British Journal of Obstetrics and Gynaecology 92:873-874
Hytten F E, Leitch I 1971 The Physiology of Human Pregnancy 2nd edn. Blackwell Scientific Publications, Oxford Illingworth P J, Jung R T, Howie P W, Isles T E 1987 Reduction in postprandial energy expenditure during pregnancy. British Medical Journal 294:1573-1576 Jones K L, Smith D A, Ulleland C N e t al 1973 Pattern of malformation in offspring of alcoholic mothers. Lancet 2:173-176 Mahomed K, Hytten F E 1989 Iron and folate supplementation in pregnancy. In: Chalmers I, Enkin M, Kierse M (eds) Effective care in pregnancy and childbirth. Oxford University Press, Oxford Naismith D J 1977 Protein metabolism in pregnancy. In: Philipp E E, Barnes J, Newton M (eds) Scientific Foundations of Obstetrics and Gynaecology. Heinemann, London Roberts S B, Paul A A, Cole T J, Whitehead R G 1982 Seasonal changes in activity, birthweight and lactational performance in rural Gambian women. Transactions of the Royal Society of Tropical Medicine and Hygiene 76:668-678 Taggart N R, Holliday R M, Billewicz W Z, Hytten F E, Thomson A M 1967 Changes in skinfolds during pregnancy. British Journal of Nutrition 21 : 439-451 Wright J T, Toplis P J, Barrison I G 1983 Alcohol and coffee consumption during pregnancy. In: Campbell D M, GiUmer M D G (eds) Nutrition in Pregnancy. Royal College of Obstetricians and Gynaecologists, London.
Midwifery
o,o0,.,.,0o~oo0o oo.,,,.o oo
CLINICAL REVIEW
Nutritional requirements in pregnancy: What should the pregnant woman be eating? Frank Hytten
It is n o w r e c o g n i s e d t h a t ' e a t i n g f o r two' d u r i n g p r e g n a n c y is n o t a s o u n d practice. H o w e v e r , w h a t s h o u l d a p r e g n a n t w o m a n eat? T h i s article reviews firstly the available l i t e r a t u r e o n the f o o d intake o f h e a l t h y w o m e n p r o d u c i n g a h e a l t h y baby, a n d s e c o n d l y the l i t e r a t u r e o n the calculation o f n e e d s f r o m w h a t is k n o w n o f the physiological d e m a n d s o f p r e g n a n c y .
INTRODUCTION What should a pregnant women eat to ensure optimum nutrition for herself and her fetus? T h e r e have been two main approaches to this question. One is to record the food intake of healthy women having healthy babies; the Other is more theoretical, the calculation of needs from what we know of the physiological demands of pregnancy. T h e first approach is relatively crude because of the great practical difficulties of accurate measurement and is in general only applicable to groups of women rather than to individuals. T h e second is likely to be more precise and also provides more detailed and interesting insights into the biology o f childbearing. But the recent realisation that there is a conspicuous discrepancy between the results provided by the two methods has shed new light on the subject.
Frank Hytten MD, PhD, FRCOG Previously Head of the Division of Perinatal Medicine, Clinical Research Centre, Northwick Park Hospital, Harrow, Middlesex. Current address: 'Blossoms', Cobblers Hill, Great Missenden, Bucks, HP16 9PW.
The physiological basis Taking the physiological approach, there are certain specific nutritional costs of pregnancy which are obvious and can be clearly defined. Like the costs for any enterprise, the nutrient needs may be divided into 'capital g a i n s ' - those that must be acquired but are kept, such as the products of conception and the new maternal tissue, and 'running costs' - those that are acquired and used up for the extra metabolic demands imposed by pregnancy. T h e 'capital gains' can be conveniently looked at in terms of the extra weight gained by the pregnant woman. T h e average healthy woman, eating to appetite, gains about 12.5kg (281b) body weight, and that weight gain is, on average, associated with an optimum reproductive performance. Using that figure as a basis Hytten & Leitch (1971) estimated its components and they are summarised in Table 1. Those average figures are derived from a great deal of published evidence and show that the products of conception account for only about one third of the weight gained at term; the remainder is maternal tissue. Most of that maternal tissue is in obvious sites: the uterus, the enlarged breasts, the expanded volume of circulating blood, and 93
94
MIDWIFERY
in the last trimester a considerable quantity of extracellular fluid which often manifests itself as clinical oedema. But there remains a considerable c o m p o n e n t of the weight, some 3.5kg (almost 81b), which has been accumulated by 30 weeks and contains no water; it is depot fat stored mostly in subcutaneous tissue a r o u n d the lower trunk and u p p e r thighs (Taggart et al, 1967). T h e r e has been considerable discussion in the past about the possibility of the p r e g n a n t woman storing protein, but protein cannot be stored 'dry', only as new tissue. T h e body has no capacity for protein storage of that kind, and although some animals such as the rat enlarge their livers in pregnancy there is no convincing evidence of any such p h e n o m e n o n in h u m a n pregnancy. From these data the energy value of the capital gains can be calculated. Details o f the calculations are given in Hytten & Leitch (1971), but the accumulated fat which is mostly in the m o t h e r and also in fetus during the last weeks of pregnancy, accounts for over 36000 kCal of stored energy. Protein which has been built into fetal tissue and such maternal sites as the enlarged uterus, accounts for a little over 5000 kCal (Hytten, 1980). T o this stored energy, the 'capital gai~ns', must be added the 'running costs' of the pregnancy and these are shown in Figure 1. T h e metabolism o f the fetus and placenta, and of the added maternal tissues, makes an important contribution only in the later stages of pregnancy and at term accounts for only about half
38 36 Cardiac work
34 32 30
Respiration
28 26 24
i
22
o
18
Renal work Breasts
20
Ulerus
16 Placenta
14 12 10 g
Felus
6 4 2 0 10
20 30 Weeks of pregnancy
40
Fig. 1 Components of the 'running costs' of pregnancy (Hytten, 1980)
the metabolic costs. But there are considerable costs incurred throughout pregnancy by the major physiological adaptations of the mother: there is the additional work of the heart and of the respiratory muscles, and in particular the very high cost of recovering sodium f r o m the increased glomerular filtrate. When the oxygen consumption figures are translated into calories, the metabolic, or running costs of the pregnancy amount to a further 36 000 kCal. T h e separate costs, shown together in Figure 2 indicate that the total specific energy cost of pregnancy is about 80000 kCal. Figure 2 also shows two important features of the pattern. One is the remarkably even requirement for
Table 1 Analysis of weight gain in pregnancy (adapted from Hytten, 1980) Increase in weight (g) up to: 10weeks
20weeks
30weeks
40 weeks
Fetus Placenta Amniotic fluid Uterus Mammary gland Blood Extracallular extravascular fluid Maternal fat
5 20 30 140 45 100
300 170 350 320 180 600
1500 430 700 600 360 1300
3400 650 800 970 405 1450
0 310
30 2050
80 3480
1480 3345
Total
650
4000
8500
12 500
MIDWIFERY 80 0 0 0 70 0 0 0 60 000
Maintenance
50 000
8 40 000 30 000
Fat 20 0 0 0 10 000
Protein 0
10
.
20
30
40
Weeks o[ pregnancy
Fig. 2 The cumulative energy cost of pregnancy and its components (Hytten, 1980)
extra energy during pregnancy, about 350 kCal per day during the last two trimesters: almost all capital gains at the beginning when the fetus is small and the mother is storing fat and almost all running costs at the end when the fetal demands are greatest and the mother has stopped storing fat. T h e second is that the r u n n i n g costs of late pregnancy could be totally subsidised, in energy terms, by the fat stored earlier in the pregnancy. This last point makes biological sense of the huge adipose energy store. At the time when this physiological mechanism would have evolved, and even in many third world countries today, women did extremely hard physical work in conditions of uncertain food supplies. It therefore made sense to store a food supply for the fetus if food was available in early pregnancy against possible deprivation later. Such anticipatory energy storage, controlled by hormones, is c o m m o n in the animal world - mammals who hibernate do it before the winter, and birds do it before migration. So far we have considered only the energy requirements of the p r e g n a n t woman; the physiological basis for calculating the needs of individual nutrients is rather less certain.
Protein T h e only major nutrient which need be considered is protein, and from the figures given in Table 2 it can be calculated that the m a x i m u m daily accretion of protein, which occurs in the
95
final weeks o f pregnancy, amounts to about 6 g per day. T h e calculation o f how much maternal dietary protein is needed to supply that 6 g o f tissue protein in such sites as the fetus and placenta is complex and depends on the quality of the dietary protein and the general sufficiency of the diet; for example, in a totally inadequate diet protein might be b u r n e d to supply energy. But for the average European women eating a normal diet the extra dietary protein requirement is about 8.5g per day. This assumes that the metabolism o f protein remains unaltered in pregnancy, an assumption which has been questioned by Naismith (1977) who has shown that rats adapt their liver function in pregnancy to spare protein which might otherwise be degraded. T h e r e are no data which suggest that this is h a p p e n i n g in women, but if it did then the additional protein requirements for pregnancy would be substantially less than 8.5 g per day. An extra 8.5 g o f protein presents no problem; if the woman consumes the extra 350 kCal per day which she needs, then unless she is eating a very unusual and badly balanced diet she will eat much m o r e than 8.5 g o f extra protein. T h e r e are no particular requirements in the diet for extra carbohydrate and fat which will not be met by a normal diet.
Other nutrients Vitamins For most other nutrients the conspicuous lack o f physiological data is matched by the wealth o f strongly held opinions, and that is particularly true of the vitamins. T h e r e is no evidence of a
Table 2 The components of protein and fat accumulated during pregnancy [g] (adapted from Hytten, 1980) Protein
Fat
Fetus Placenta Amniotic fluid Uterus Breasts Blood Maternal fat store
440 100 3 166 81 135 0
440 0 4 4 12 20 3345
Total
925
3825
96
MIDWIFERY
particular extra need for vitamins in a well nourished community which will not be met by the increased overall dietary intake, although certain vulnerable groups may require supplements of folate, or vitamin D. For exam, ple, some Asian women who customarily spend most of their time indoors and whose calcium absorption may be impaired by an excess of wholemeal cereals in the diet should have vitamin D supplements. Minerals T h e extra requirement for Calcium is about 30 g, almost all for the fetal skeleton, and that can be readily acquired from an ordinary diet without recourse to drinking large quantities of milk. Even if the diet were grossly deficient, 30g is a tiny proportion of the mother's skeletal calcium and she could readily provide it from her own stores. The mineral which generates most controversy is iron because there is a widely held view that many women start pregnancy with deficient iron stores, and that the ordinary diet is incapable of supplying the extra needs for iron about 4 mg per day in the last half of pregnancy. T h a t opinion has originated largely because many of the haematological indices such as haemoglobin concentration, serum iron and serum ferritin suggest iron deficiency, whereas those changes, like so many, are part of the widespread modification of homeostasis which occurs in healthy pregnancy and do not indicate iron deficiency. Controlled trials have shown clearly that routine iron supplements in pregnancy, while tending to 'correct' the normal haematological changes, offer no clinical benefit (Mahomed & Hytten, 1989). Indeed there is evidence that iron supplementation can be harmful; it leads to an increase in red cell size which may increase blood viscosity to a degree which could impair utero-placental blood flow. And that may be responsible for the observed increase in low birthweight infants when the mother has been supplemented (Mahomed & Hytten, 1989). That is not to say that true iron deficiency anaemia does not occur in pregnant women. But it should be looked for and treated when it
occurs. It is bad medicine to treat all pregnant women as anaemic, and possibly put some fetuses in jeopardy, in o r d e r to catch a few women who need treatment. T h e same may be said of the new cult metal zinc. There is no good evidence that most pregnant women are deficient, as has been claimed, and universal supplementation is likely to do harm (Hytten, 1985).
The overall dietary needs We have been discussing the specific nutrient needs of pregnancy, and we can conclude that if a healthy woman eats, in addition to her ordinary requirements, a good mixed diet which supplies an extra 350 kCal per day, all the needs of pregnancy will be met. So how big a total diet does the average pregnant woman need? In the past it has been customary to add the specific 350 kCal per day for pregnancy to the usual energy intakes of non-pregnant women, although for an individual much will depend on her usual level of activity. Over the years diet survey data have consistently suggested that pregnant women do not eat 350 kCal more than usual and the strength of that evidence is increasing. For example Durnin et al (1985) found in a weighed diet survey that 67 healthy Glasgow women had a total energy intake in pregnancy only 20000 kCal above their non-pregnant intake, not the calculated 80000, and that their basal metabolism was considerably depressed from early pregnancy. That pregnant women economise on energy expenditure is obvious: they tend to spend more time in bed or sitting at rest, and are more economical in almost everything they do. Even in rural Africa where the demands of agriculture make relaxation difficult, the women manage to reduce their energy output by cutting down on leisure activities (Roberts et al, 1982). But the Glasgow evidence suggests a more fundamental change. T h e r e is certainly a reduction in circulating levels of thyroid h o r m o n e during pregnancy (Franklyn et al, 1983) so that all the maternal cells will be metabolising at a somewhat slower tempo, and there is some evidence that
MIDWIFERY 97
the metabolic response to a meal is reduced in pregnancy (Illingworth et al, 1987). There are other possible ways in which the body could economise on energy expenditure, and they suggest some interesting avenues for research. Meanwhile the addition of 200 kCal daily to the usual non-pregnant diet seems a more appropriate figure than the calculated specific cost of 350 kCal.
Recommended dietary allowances It is necessary to refer to R e c o m m e n d e d Dietary Allowances because they are so often used as a basis o f comparison with measured diets. Numerous official bodies such as the World Health Organization and the British Medical Association have published schedules which specify the amounts of energy and nutrients which should be provided to maintain populations in health. They are not intended for application to individuals and are no more than guides to the needs of populations or large groups. They are based in large part on much of the evidence presented above and it is o f some interest that as physiological evidence has accumulated 'recommended allowances' of energy and nutrients have usually become smaller. But recommended allowances always include a substantial safety margin, and it would be wrong to conclude, without clinical evidence, that because the diet of an individual woman falls somewhat short of the standard it is therefore necessarily inadequate. Indeed in some instances the standards have been set ridiculously high. T h e American r e c o m m e n d e d allowances, set by the US National Research Council, give 20rag as the daily allowance for zinc, and yet in a prosperous and healthy L o n d o n population studied by Campbell-Brown et al (1985), the average intake was only about half that, and no individual reached the recommended intake. Standards for some other nutrients such as calcium and vitamin C which may accord with the intakes of many individuals are also much too high to be regarded as requirements. For the practical midwife official r e c o m m e n d e d allowances of nutrients for pregnant women are more misleading than helpful.
Should any food be avoided? Food taboos in pregnancy have a long history, and there can be few foods that, somewhere or at some time, have not been forbidden to pregnant women. Women often undergo changes of taste and develop aversions to, or cravings for, certain foods sometimes progressing to pica - the craving for such non-nutritive substances as coal, soap or toothpaste. These may he inconvenient, but there is no evidence that they do any harm. Alcohol and caffeine That alcoholism in the mother can seriously damage the fetus is beyond doubt (Jones et al, 1973). But whether 'social drinking' is damaging is uncertain, and the problem is difficult to investigate because women who drink moderate amounts of alcohol often also smoke more and are liable to take other drugs. T h e current best evidence (Wright et al, 1983) suggests that two alcoholic drinks per day, which could be considered as moderately heavy consumption, does not appear to damage the fetus. Nevertheless, there is widespread anxiety among obstetricians that alcohol may cause more subtle damage to the fetus and it seems wise to advise pregnant women to restrict alcohol consumption as much as possible. The effect o f caffeine from such beverages as tea, coffee and some proprietary soft drinks has also been subject to a number of studies (Wright et al, 1983). T h e r e is no good evidence for fetal damage, but the US Food and Drug Administration has advised pregnant women to avoid caffeine-containing foods and drugs, or use them sparingly.
CONCLUSIONS In practical terms the answer to the question which o p e n e d this paper is this: if a healthy woman eating an ordinary diet gains about 450 g (lib) per week during the last two trimesters of her pregnancy, then her nutrition, and that of her fetus is almost certainly secure. That is a slight oversimplification - iron deficiency must be looked for and treated, and women at risk of specific nutrient deficiencies such as vitamin D
98
MIDWIFERY
m a y n e e d specific s u p p l e m e n t a t i o n , b u t in g e n erally well-nourished communities the exceptions are uncommon. Finally it s h o u l d be said t h a t it is c e r t a i n l y much more important that a woman should have b e e n well n o u r i s h e d d u r i n g h e r g r o w i n g years, p e r h a p s e v e n i n c l u d i n g f e t a l life, b u t t h a t is a n o t h e r story.
References Campbell-Brown M, Ward R J, Haines A P, North W R S, Abraham R, McFadyen I R 1985 Zinc and copper in Asian pregnancies - is there evidence of a nutritional deficiency? British Journal of Obstetrics and Gynaecology 92:875-885 Durnin J V G A, McKillop F M, Grant S, Fitzgerald G 1985 Is nutritional status endangered by virtually no extra intake during pregnancy? Lancet 2:823-825 Franklyn J A, Sheppard M C, Ramsden D B 1983 Serum free thyroxine and free triiodothyronine in pregnancy. British Medical Journal 287:394 Hytten F E 1980 Nutrition. In: Hytten F, Chamberlain G (eds) Clinical Physiology in Obstetrics. Blackwell Scientific Publications, Oxford Hytten F E 1985 Do pregnant women need zinc supplements? British Journal of Obstetrics and Gynaecology 92:873-874
Hytten F E, Leitch I 1971 The Physiology of Human Pregnancy 2nd edn. Blackwell Scientific Publications, Oxford Illingworth P J, Jung R T, Howie P W, Isles T E 1987 Reduction in postprandial energy expenditure during pregnancy. British Medical Journal 294:1573-1576 Jones K L, Smith D A, Ulleland C N e t al 1973 Pattern of malformation in offspring of alcoholic mothers. Lancet 2:173-176 Mahomed K, Hytten F E 1989 Iron and folate supplementation in pregnancy. In: Chalmers I, Enkin M, Kierse M (eds) Effective care in pregnancy and childbirth. Oxford University Press, Oxford Naismith D J 1977 Protein metabolism in pregnancy. In: Philipp E E, Barnes J, Newton M (eds) Scientific Foundations of Obstetrics and Gynaecology. Heinemann, London Roberts S B, Paul A A, Cole T J, Whitehead R G 1982 Seasonal changes in activity, birthweight and lactational performance in rural Gambian women. Transactions of the Royal Society of Tropical Medicine and Hygiene 76:668-678 Taggart N R, Holliday R M, Billewicz W Z, Hytten F E, Thomson A M 1967 Changes in skinfolds during pregnancy. British Journal of Nutrition 21 : 439-451 Wright J T, Toplis P J, Barrison I G 1983 Alcohol and coffee consumption during pregnancy. In: Campbell D M, GiUmer M D G (eds) Nutrition in Pregnancy. Royal College of Obstetricians and Gynaecologists, London.
