Dietary fat and fat types as early determinants of childhood obesity: a reappraisal

(2006) 30, S50 S57 & 2006 Nature Publishing Group All rights reserved 0307-0565/06 $30.00 ORIGINAL ARTICLE www.nature.com/ijo Dietary fat and fat types as early determinants of childhood obesity: a reappraisal K Macé, Y Shahkhalili, O Aprikian and S Stan Department of Nutrition & Health, Nestlé Research Center, Vers-chez-les-Blanc CP 44, 1000 Lausanne 26, Switzerland Background: The growing prevalence of childhood overweight and obesity has renewed interest in determining the influence of the maternal and infant diet on the risk of developing excess fat mass later in life. Approach: Review of available human and animal data reporting the effects of dietary fat and fat types early in life on adipose development. Results: Rodent studies tend to show that maternal high-fat feeding during pregnancy and lactation results in increased adiposity of the offspring. Nevertheless, today there is a lack of population-based studies investigating this potential detrimental effect of maternal high-fat intake. Most epidemiological studies, performed so far, do not find any association between the level of dietary fat intake of infants and children and body weight and/or fatness. Regarding fat types exposure to high levels of dietary n-6 fatty acids during gestation and post-natal life, has been shown to promote obesity in mice. Nevertheless, other rodent studies do not demonstrate such an effect. Conclusion: There is no evidence supporting a restriction of fat intake during the first two post-natal years but the potential detrimental effects of maternal high-fat intake during gestation should be further investigated. The role of dietary fat types as early determinants of childhood obesity has so far been poorly studied. Robust evidence to support the adipogenic effects of n-6 fatty acids enriched-diets is currently lacking but this hypothesis is of importance and should be further evaluated in different animal models as well as in longitudinal human studies. (2006) 30, S50 S57. doi:10.1038/sj.ijo.0803519 Keywords: adipose tissue; polyunsaturated fat; infants; metabolic programming Introduction Dietary fats during early infancy provide not only energy for growth but also supply essential fatty acids (i.e., linoleic and linolenic acids) and allow adequate absorption of fat-soluble vitamins, required for healthy development. Interest in dietary lipid composition early in life has focused mainly on the role of the essential n-6 and n-3 fatty acids and their principal metabolites (arachidonic acid and docosahexaenoic acid, respectively) in the development of the central nervous system, later visual acuity and cognitive functions. 1 The growing prevalence of overweight and obesity has renewed interest in better understanding the role of dietary lipids and their fatty acid composition on early determinants of childhood obesity. The purpose of this article is to review the scientific evidence of the role of fat intake during early life (gestation, suckling and weaning periods) on the Correspondence: Dr K Mace, Department of Nutrition & Health, Nestlé Research Center, PO Box 44, CH-1000 Lausanne 26, Switzerland. E-mail: catherine.mace@rdls.nestle.com development of childhood obesity, with emphasis on the effects of total fat levels and essential fatty acid (EFA) composition on adipose tissue development. Adipose tissue development in infancy Humans diverge from most mammals, including nonhuman primates, by depositing significant quantities of body fat in utero and are consequently one of the fattest species on record at birth. 2 At birth, fat mass represents 15% of body weight in human infants, 3 3% in baboons 4 and 1% in rats. 3 In human infants, peak adiposity is reached during early infancy between 6 and 9 months of age when fat mass represents 26% total body weight. 5 It has been estimated that fat deposition accounts for 40 65% of total body weight gain during the first 4 6 months of life. 5,6 This burst of fat accumulation is followed by a gradual decline in fat mass until 5 to 6 years of age, about 13 and 16% fat for boys and girls, respectively, which corresponds with a lean condition. At this stage, an adiposity rebound takes place. 5

Adipose tissue growth involves the formation of new adipocytes from precursor cells (hyperplasia) and an increase in adipocyte size (hypertrophy) by lipid accumulation. Morphogenic differentiation of fat tissue takes place during the second trimester of gestation in both male and female fetuses. 7 Adipose tissue becomes noticeable at first in the head and neck, and then rapidly progresses to the trunk and finally to the upper and lower limbs. Fat cell numbers vary from one body site to another and some fat deposits appear to grow primarily through hypertrophy while others grow mainly by hyperplasia. 8 More than 90% of the fat deposition occurs in the last 10 weeks of pregnancy. 9 During this period, fat accretion increases exponentially to reach a rate of about 7 g/day close to term. 9 Two main factors contribute to the rapid accumulation of lipids in the human fetus during late gestation: (i) lipogenesis; part of fetal glucose uptake is converted to fat and (ii) maternal fatty acids which are available to the fetus via their placental transfer. 10 The postnatal growth pattern of adipose tissue in human infants has been poorly investigated. As a result of large interspecies differences in time and development of adipose tissue, extrapolation from most animal models to human infants is questionable and may not be relevant at all. The relative contribution of hyperplasia and hypertrophy, during the critical period of post-natal adipose tissue growth in human infants (i.e. first year, adiposity rebound and puberty), is not clear and is still a subject of controversy. Some authors report that the number of adipocytes increases during the first 2 years of age and further increases at the age of 8 10 years, 11 while others have observed that adipose tissue grows essentially by cell enlargement during the first year and then cell multiplication takes place until at least 8 years of age. 12 Nevertheless, all these results should be treated with caution due to the potential lack of reliability of the techniques used for monitoring adipose cell number and size. A sustained excess energy supply clearly triggers adipocyte hypertrophy. As a result adipocytes do not have an unlimited capacity for expansion and an increase in fat cell number may occur in severe forms of obesity. 13 Obese children (identified at 24 months of age) have the same adipose cell size and number as nonobese infants up to the age of 12 months. 11 Nevertheless, at the age of 24 months and subsequently until the age of 17 19 years, both adipose cell size and number were reported to be higher in obese than nonobese children. 14 Interestingly, studies performed in the late 60s and early 70s, but mainly in rats, have led to the adipocyte-number hypothesis (for review see Roche 15 ), which states that the number of fat cells is fixed early in life and predestines an individual to be lean or obese. This theory was a matter of controversy and has not been pursued. Nevertheless, some recent epidemiological studies have demonstrated a correlation between the rates of weight gain during the first 4 months, 16 even the first week of life, 17 and prevalence of overweight later in life. These findings could renew interest in the adipocyte-number hypothesis and the influence of diet during early life on adipose tissue development. Pre- and post-natal recommendations of dietary fat levels The Food and Agriculture Organization (FAO) and the World Health Organization (WHO) acknowledge that an adequate intake of dietary fat is particularly important during pregnancy without giving specific recommendations. Women of reproductive age are advised to consume at least 20% of their energy (%E) from fat, while the upper limits of fat intake depend upon the individual s level of physical activity (35 and 30 %E from dietary fat for active and sedentary individuals, respectively). 18 Nevertheless, other regulatory authorities, for example in France, Germany, Austria and Switzerland provide specific information concerning the recommended fat intakes for pregnant and lactating women (Table 1). During the first 6 months of post-natal life, infants are dependent on human milk or infant formulas which both provide 40 50 %E as fat. The WHO recommends 30 40 %E from fat during weaning up until 3 years of age. 18 Pediatric associations such as the American Academy of Pediatrics (AAP) recommend no fat restriction for infants younger than 2 years. 19 For children older than 2 years, the AAP, the American Heart Association (AHA) and almost 25 other health organizations propose 30 %E from fat, and no less than 20 %E. 18 Presently, the majority of main health authorities agree that fat should not be restricted during infancy and that it should be gradually and safely decreased to no more than 30 %E up to 2 years of age, by adopting healthy dietary habits in the family. Others recommend a transition period after 2 years of age, usually between 2 and 3 years, from a nonrestricted fat diet that characterizes infancy to a modified prudent diet containing 30 %E as fat. 20 Pre- and post-natal dietary fat levels, adipose tissue development and childhood obesity The contribution of maternal nutrition to the risk of obesity of their progeny has been studied in both human populations and in experimental animals. Nevertheless, most investigations were dedicated to better understanding the role of maternal under-nutrition. Indeed, energy or protein deficiency during gestation was shown to increase the risk for the offspring developing obesity later in life. 21,22 To our knowledge, population-based studies investigating the consequences of maternal/fetal exposure to a high-fat diet during gestation on the risk of developing childhood obesity are lacking. Several rodent studies indicate that maternal high-fat feeding, without increase of energy intake, during pregnancy and lactation results in increased adiposity of the offspring, 23 25 while another study shows a decrease in the percentage of body fat. 26 Nevertheless, these studies do not allow one to determine whether exposure to a high-fat diet S51

S52 Table 1 Main suggested recommendations of dietary fat levels Recommending authority Age groups Recommended amounts FAO/WHO 18 0 to 4 6 months 50 60% 6 months 3 years 30 40% a Adults, women of reproductive age, pregnancy and lactation 20 35% (active individuals) USA, DRIs 35 0 6 months 31 g/day (55%) 7 12 months 30 g/day (40%) 1 18 years 30 40% a Adults, pregnancy and lactation F American Heart Association b,53 o 2 years No restriction 2 18 years 30% (no less than 20%) Adults (no specific indication for pregnancy and lactation) 30% (no less than 20%) American Academy of Pediatrics 54 o 2 years No restriction 2 18 years Gradual decrease to p30%, but not less than 20% France: Recommended dietary allowance 36 0 to 4 6 months F 6 to 1 2 years F From 3 years to adolescence 30 35% Adults 30 35% Pregnancy 30% Lactation 30% ESPGHAN 55 0 to 4 6 months 45 50% 6 12 months F 12 24 months No restriction 24 36 months 30 35% From 3 years to adolescence 30 35% Germany, Austria, Switzerland (D-A-Ch) 37 0 4 months 45 50% 4 12 months 35 45% 1 o4 years 30 40% 4 15 years 30 35% Adults (15 over 65 years) 30% Pregnancy: from the fourth month of pregnancy 30 35% Lactation 30 35% %: percentage of the total energy intake; no recommended value. a No specific recommendation is actually available, but an acceptable distribution range is suggested. b Also applied by other major US Health organizations, such as: American Academy of Pediatrics (1992), National Cholesterol Education Panel (NCEP) (1992), Healthy People (2000), United States Department of Agriculture (USDA) (1995), National Institutes of Health (1985), as well as 26 other health-related organizations. during both the pre-natal and suckling period is required to program these effects. Recently, Buckley et al. 27 demonstrated that a high-fat diet provided to rat dams, during the gestation period only, induced an increase in percent body fat (both total and abdominal fat) in the offspring at 3 months of age, when compared with offspring from dams fed a chow diet. Unfortunately, the energy intake of the two groups of dams was not mentioned and therefore it is not possible to conclude whether this effect is due to dietary fats per se or an increase in energy intake. Human milk is the recommended mode of infant feeding with current emphasis on exclusive breast-feeding for at least the first 6 months of age. The fat content of milk can provide up to 52% total milk energy. Fat is the most variable macronutrient component of human milk and the milk fat content varies between women. 28 It is well established that the fat content is higher in hind-milk (53.3 g/l) than in foremilk (24.5 g/l) 29 but that it does not change dramatically with stage of lactation. 28,29 Information available on the impact of the variation of fat and energy content of breast milk on growth and adiposity during infancy and later in life are limited to small, short-term studies due to the practical constraints of appropriate frequent milk sampling and longterm follow-up. In a study with a small group of only six infants, growth rate was not related to fat and energy intake during the first 6 months of life. 28 In animals, by using artificial rearing techniques 30 it is possible to manipulate both the quantity and the composition of dietary intake during the suckling period in rat pups. This model, known as the pup in the cup, was used to investigate the influence of a low- vs high-fat/carbohydrate ratio on the development of metabolic disorders later in life. Rat pups (4-day-old) fed a low fat/carbohydrate ratio (20/56 %E) become, within the first 24 h, hyperinsulinemic and develop obesity after puberty. 31 In contrast, pups fed a high fat/carbohydrate formula (68/8 %E), equivalent to the ratio found in rat milk, do not develop hyperinsulinemia or adultonset obesity. 31 However, the possible relevance of this

Table 2 Relationships between fat intake (E%) and subsequent weight and/or fatness in infants and children (p9 years) S53 Country, study name Study design Age groups Cohort size Measurements Association between fat intake and measurements Infant diet Denmark (1991) 56 Longitudinal 5 10 months 399 Weight and weight gain 0 Denmark Prospective/ 0 1 year 146 Weight and weight gain, skinfolds 0 COPENHAGEN study (1987 1989) 32 Observational Infant/children diet USA 57 Prospective 3 months 2 years 78 Weight, skinfolds, fat mass 0 USA 58 Prospective 0 2 years 76 Weight and weight gain, skinfolds, fat mass and fat mass gain + (weight gain) 0 (fat mass gain) Italy (1991 1996) 59 Prospective 0 5 years 147 BMI 0 UK, ALSPAC study (1991 1996) 60 Longitudinal 0 5 years 889 BMI, age of adiposity rebound 0 USA, Bogulasa Heart Study 61 Observational 0 7 years 50 (125) Weight, skinfolds 0 Australia, Adelaide Nutrition Study 62 Retrospective 3 months 9 years 140 Weight, skinfolds 0 France (1985 1995) 63 Observational 10 months 8 years 112 BMI, skinfolds, adiposity rebound 0 Children diet UK, National diet and Nutrition survey 64,65 Cross-sectional 1.5 4.5 years 1444 BMI 0 77 Fat mass 0 USA (1994 2002) 66 Observational 2 8 years 70 BMI + USA 67 Longitudinal 3 6 years 146 BMI + Abbreviations: BMI, body mass index. 0, no associations; +, positive association. finding to human childhood obesity is questionable because (i) newborn rodents represent a model of extreme prematurity (ii) the carbohydrate content of rodent and human milk is very different (about 8 and 40 %E, respectively) and (iii) during the first 6 months of life, infants are mainly dependent on human milk or formulas both of which have a high-fat content. Generally, fat intake decreases substantially with the introduction of complementay foods from 52 to 45 %E at 4 months and 30 %E at 9 12 months. 32 It has been proposed that changes in weaning practices during the last decades could have contributed to the observed increase in prevalence of childhood obesity 31 even if today there is no clinical or epidemiological evidence to support this hypothesis. Several population-based studies have evaluated the relation between dietary fat intake and body fat in infants and children. Most of these studies (11 out 13) did not find any association between fat intake during infancy and later indices of adiposity (Table 2). Those that established a positive association between later body weight and fatness emphasize that the relation is potentially stronger with fat intakes after 2 years of age. Many studies have characterized the role of dietary fat on obesity in experimental animals. With few exceptions, obesity is induced by high-fat diets in a variety of mammals including nonhuman primates, dogs, pigs, rats and mice (for review see West and York 33 ). In rodents, the interventions are generally performed just after weaning (28 days after birth) or later. As a high-fat diet induces hyperphagia in rodents, it has been proposed that a higher energy intake is responsible for increased adiposity. Nevertheless, several studies have demonstrated that body fat can increase on a high-fat diet when an increase in energy intake is prevented. 33,34 Pre- and post-natal recommendations for PUFA and LC-PUFA The EFA, that is, linoleic acid (LA, 18:2n-6) and a-linolenic acid (ALA, 18:3n-3), belong to the family of polyunsaturated fatty acids (PUFA). LA and ALA serve as essential precursors for producing their respective long-chain products (LC- PUFA), arachidonic acid (ARA, 20:4n-6) and both eicosapentaenoic acid (EPA. 20:5n-3) and docosahexaenoic acid (DHA, 22:6n-3). In contrast with the WHO guidelines, 18 the most recent recommendations, US DRIs, 35 the French RDAs 36 and the D- A-Ch references 37 provide specific EFA reference intakes for pregnant and lactating women (Table 3). For infants and young children, the WHO as well as the other major current references, recommend a daily diet containing similar levels of EFA to those found in breast milk. The WHO also recognizes the significant importance of ARA and DHA, but no specific levels have been suggested. However, recommendations related to formula composition for term infants: 600 mg LA, 50 mg ALA, 40 mg ARA and its associated LC-n-6 PUFA, and 20 mg of DHA per kg of body weight, is provided. The French RDAs are the only references that give particular importance to recommended levels of LC-PUFA in the diet. Based on limited studies in animals, infants, children and adults, a reasonable ratio of total n-6/n-3 or LA/ALA varying between 5:1 and 10:1 is now globally recommended for all age groups.

S54 Table 3 Main suggested recommendations of n-6 and n-3 PUFA and LC-PUFA composition n-6 n-3 Ratio FAO/WHO 18 0 to 4 6 months 8% or 4 10% a 0.8 1% a LA/ALA: 5 10 a ARA/DHA+EPA: 1 1.5 a 6 months 3 years 8% or 4 10% a 0.8 1% a LA/ALA: 5 10 a ARA/DHA+EPA: 1 1.5 a Pregnancy and lactation 4 10% LA/ALA: 5 10 USA DRIs 35 LA (g/day) ALA (g/day) 0 6 months 4.4 0.5 7 12 months 4.6 0.5 1 3 years 7 0.7 4 8 years 10 0.9 9 13 years 12 (M), 10 (F) 1.2 (M), 1.0 (F) 14 18 years 16 (M), 11 (F) 1.6 (M), 1.1 (F) Pregnancy 13 1.4 Lactation 13 1.3 France, RDAs 36 0 to 4 6 months LA: 2 4.5% ALA: 0.45 1.5% LA/ALA: 4 10 AA: 0.1 0.25% EPA: 0.05 0.15% ARA/DHA: 1 1.5 DHA: 0.1 0.4% 6 months to 1 2 years LA: 2 5% ALA: 0.4 1% From 3 years to adolescence LA: 2 5% ALA: 0.4 1% Adults LA: 4% ALA: 0.8% DHA: 0.05% Pregnancy and lactation LA: 4.4% ALA: 0.9% DHA: 0.1% D-A-Ch 37 n-6/n-3 0 4 months 4% 0.50% 5 15 b 4 12 months 3.50% 0.50% 5 1 o4 years 3.00% 0.50% 5 4 15 years 2.50% 0.50% 5 Adults 2.50% 0.50% 5 Pregnancy and lactation 2.50% 0.50% 5 Abbreviations: AA, arachidonic acid (20:4 n-6); ALA, a-linolenic acid (18:3 n-3); DHA, docosahexaenoic acid (22:6 n-3); EPA, eicosapentaenoic acid (20:5 n-3); LA, F, female; linoleic acid (18:2 n-6); M, male. %, Percentage of the total energy intake. a Similar levels as found in human milk. Based on average amount of essential fatty acids provided by human milk in 14 European countries and 10 African countries as well as in Canada and USA. b Recommendations pertaining to infant formulas. Pre- and post-natal dietary PUFA and LC-PUFA levels, adipose tissue development and childhood obesity ARA and EPA/DHA are formed through a series of common desaturation and elongation enzymatic reactions. As the same enzymes are utilized, metabolic competition exists between LA and ALA. Therefore, manipulation of the dietary LA/ALA ratio has a strong impact on the proportion of both ARA and EPA/DHA accretion in the tissues. EFA and LC-PUFA are supplied to the infant through the placenta during pregnancy, and human milk after birth. Human neonates, even pre-term, are able to synthesize small amounts of LC-PUFA. 38,39 However, the rate of synthesis may be insufficient, especially in pre-term infants, to allow normal or optimal LC-PUFA accretion in body tissues. The importance of dietary PUFA and LC-PUFA on fetal and infant growth as well as brain development has been the subject of intense investigation. 40 The interest in the role of these fatty acids, especially the n-6 species, on the early development of adipose tissue and their potential impact on childhood obesity is more recent. ARA (20:4n-6) is the precursor of prostaglandins (PG) a family of compounds that play an important role in cellular processes. 41 PG, including PGI 2 (prostacyclin), PGJ 2 and PGE 2 promote, in vitro, adipocyte hyperplasia or hypertrophy 42 while others, such as PGF 2a, are anti-adipogenic. 43 To address whether dietary intake of high levels of n-6 fatty acids could promote adipose tissue development, in vivo, Massiera et al. 44 have carried out nutritional interventions in wild-type (wt) and prostacyclinreceptor knockout (ip-r / ) mice. In this study, mother mice were fed a high fat diet with a LA/ALA ratio of either 2 or 60, during the mating-pregnancy-lactation period. From weaning onward, male pups were maintained on the same diet as that consumed by their mothers until 22 weeks of age. Interestingly, at 8 weeks of age the offspring of wt dams fed the high LA/ALA ratio diet had a greater body weight and total fat mass than those of dams fed the low LA/ALA ratio of 2 or fed a low-fat chow diet. 44 On the other hand, the body weight and fat mass of ip-r / pups on either type of diet were similar. The authors concluded that the consumption of large amount of LA, the precursor of AA, during gestation,

the suckling period and early infancy, promotes adipose tissue development and obesity through prostacyclin signaling. An association between the increasing levels of LA content in mature breast milk of US women during the last 60 years and the prevalence of childhood obesity has been also proposed. 45 Nevertheless, the causality of such an association remains to be demonstrated. Moreover, other animal studies investigating the role of n-6 and n-3 PUFA in the maternal diet did not find an adipogenic effect of LA in the offspring. 23,46 Rat offspring of dams fed a high-fat diet with a LA/ALA ratio of 41, before mating, during pregnancy and lactation, had similar body weight and fat mass at weaning as those of dams fed a LA/ALA ratio of 9, 23 which represents an optimal ratio for maintaining normal tissue fatty acid concentrations in rats. 47 Similarly, rat pups from dams fed a LA/ALA ratio of 216, during the gestation and suckling period, showed similar body weight and fat mass as those of dams fed a ratio of 9. 46 In this study, the exposure of the pups to a very low LA/ALA ratio (i.e., 0.4) led to a decrease in growth rate. 46 The role of dietary n-3 and n-6 fatty acids on early development of adipose tissue and their potential impact on childhood obesity has not been clearly investigated in human infants. Children (2.5 years old) from mothers supplemented during the first 4 months of lactation with fish oil (low in LA and rich in EPA/DHA), showed a higher BMI and waist circumference than those from mothers supplemented with olive oil. 48 One recent study evaluated the growth and body composition of premature infants who were fed formulas supplemented or not with 0.42% ARA and 0.26 % DHA. 49 Interestingly, at 12 months of age, infants fed the LC-PUFA supplemented formula had similar body weight, length and head circumference, but significantly less fat mass and greater lean body mass, than infants who were fed the nonsupplemented control formula. In summary, the in vivo adipogenic effects of the n-6 fatty acids have not been clearly established and conflicting results exist. In addition there is today no human data supporting a causal association between high n-6 fatty acid intake early in life and an increased risk of developing childhood obesity. Further investigations using epidemiological and intervention studies in infants as well as animal models are needed. The need for more relevant animal models If and how adipose tissue is programmed for hyperplasia and/or hypertrophy by early nutrition that ultimately results in an individual becoming overweight or obese later in life are important questions to be answered to combat childhood obesity. To resolve such questions long-term studies in humans would be necessary, since early biomarkers of obesity are lacking. Moreover, in order to test different hypothesis and to unravel potential mechanisms of action, there is a clear need to use animal models. Nevertheless, as previously mentioned, adipose tissue development in the human infant follows a quite unique pattern that distinguishes it from other species (see adipose tissue development in infancy). Interestingly, unlike other rodents, the guineapig (Cavia porcellus) shares with the human infant the deposition of a significant amount of body fat in utero. By using a NMR whole body composition analyzer system (EchoMRI, Echo Medical Systems, Houston, Texas, USA), we found that the newborn guinea pig (about 110 g body weight) has 14.270.3% (n ¼ 15) body fat. A previous evaluation, 3 using the carcass analysis method, found a value of 10.1% body fat in a newborn guinea-pig weighing 80 g. Data that confirms that the post-natal body fat mass of the guinea-pig is similar to that of a human infant. By 3 months of age, the body fat mass of the developing guineapig had increased to 2370.3 % (n ¼ 15). Moreover, the postnatal development of guinea-pig brown adipose tissue is analogous to that of the tissue in man, with a rapid disappearance after birth. 50 Finally and importantly, especially for investigating the role of n-6 and n-3 PUFA and their long chain derived products (ARA and DHA/EPA), desaturase activities are quite limited in the guinea-pig, and humans, while very active in other rodents. 51,52 Therefore, the guineapig appears to be a more relevant animal model than mice or rats for studying the effect of early nutrition, especially maternal nutrition, on adipose tissue development and the metabolic consequences later in life. Nevertheless, some limitations exist since few molecular and biochemical tools are available for this animal model. Conclusions and perspectives To date, the role of dietary fat and fat types as early determinants of childhood obesity has been poorly studied. As human adipose tissue development and fat accretion takes place partly during gestation, it is important to evaluate the contribution of the maternal diet on the risk for the infant of developing obesity later in life. Populationbased studies investigating the influence of maternal fat intake (fat level and fat type) on the risk for the offspring developing overweight or obesity are lacking. Rodent studies tend to show that maternal high-fat feeding during pregnancy and lactation results in increased adiposity of the offspring. Robust evidence to support the adipogenic effects of n-6 PUFA enriched-diets is, to our point of view, currently lacking but this hypothesis is of importance and should be further evaluated in different animal models as well as in longitudinal studies. Most of the epidemiological studies performed so far have not found any association between the level of dietary fat of infants and children and body weight and/or fatness. Rat studies have demonstrated that under very artificial conditions of low-fat high-carbohydrate feeding in the early S55

S56 post-natal suckling period it is possible to program insulin resistance and obesity later in life. As the weaning period represents the transition between a high-fat diet (breast milk or formula) and a carbohydrate rich diet (recommended adult diet), it is of interest to study the influence, during this period, of different fat/carbohydrate ratio intakes on the propensity to develop obesity later in life. Interestingly, the EC-funded Early Nutrition Programming Project (http://www.metabolic-programming.org) is currently evaluating some of these research topics through clinical trials as well as epidemiological and animal studies. Acknowledgements The authors are grateful to Dr K Acheson for critically reading the manuscript. References 1 Milner JA, Allison RG. The role of dietary fat in child nutrition and development: summary of an ASNS workshop. American Society for Nutritional Sciences. JNutr1999; 129: 2094 2105. 2 Kuzawa CW. 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