The past decades have seen a substantial increase in the global incidence of obesity and its related metabolic disorders. In addition to the health and quality of life implications of these diseases, the direct and indirect costs of these conditions represent a significant economic burden to countries worldwide . As a result, identifying the causes of this epidemic and strategies to overcome it has become a major public health priority, and numerous antiobesity health campaigns have been launched. To date, however, these campaigns have done little to successfully combat the problem.
The importance of quantity and quality of nutritional intake for the regulation of body weight and fat mass in individuals is widely acknowledged, and has led to suggestions that changes in the nutritional quality of the typical Western diet is an important driver of the expanding waistlines of populations worldwide. Although good nutrition is important at all life stages, it is increasingly recognized that the nutritional environment an individual experiences before birth and in early infancy is of particular importance for their later metabolic health, and that exposure to an inappropriate nutritional supply during critical windows of development can predispose an individual to obesity and type 2 diabetes later in life [2▪,3]. By extension, the diet consumed by pregnant and breast-feeding women is a key determinant of the metabolic health of future generations .
In this review, we will present the temporal and biological evidence underlying the hypothesis that excess intake of omega-6 polyunsaturated fatty acids (omega-6 PUFA) is associated with increases in body fat mass, with a focus on existing evidence from animal and human studies that exposure to elevated intakes of omega-6 PUFA before birth or in early infancy can programme an increased susceptibility to obesity throughout the life course. We will highlight the current paucity of human studies which have examined the long-term consequences of perinatal exposure to high omega-6 PUFA intakes, and emphasise the need for increased research in order to establish conclusively whether there is a true causative association.
SETTING THE SCENE: THE GLOBAL EPIDEMIC OF OBESITY AND METABOLIC DISEASE
The global incidence of overweight, obesity and metabolic disease nearly doubled in the period from 1980 to 2008 and continues to increase. In 2008, more than 1.4 billion adults (20 years and older) and 40 million children under the age of 5 were overweight and, of these, over 200 million men and nearly 300 million women were obese . This increase in the number of overweight and obese individuals has been accompanied by a dramatic increase in the incidence of its associated comorbidities, including type 2 diabetes and cardiovascular disease. The significant impact of obesity and its associated metabolic disorders on both the health and quality of life of sufferers and on the health budgets of economies worldwide has prompted extensive research to identify factors which have contributed to the epidemic and strategies for reversing the current trend.
Both genetic and environmental factors have been implicated in the aetiology of obesity and metabolic disease. There appears little doubt that genetics plays a role in predisposing certain individuals to obesity and metabolic disease, and there is an ever-growing list of single nucleotide polymorphisms that confer increased susceptibility to obesity and type 2 diabetes [5▪▪]. However, it is unlikely that there has been any substantial shift in the gene pool of humans over the relatively short time frame that the obesity epidemic has taken hold, suggesting that environmental, rather than genetic, factors are likely to play the more important role. Of these environmental factors, poor dietary habits play a significant role in promoting weight gain and the accumulation of body fat mass in individuals . However, not all dietary components are equal in this regard. There are some which contribute more to fat accumulation than others, and there have been numerous attempts to identify those components of the diet which are the major contributors to weight gain on a population level.
EXAMINING THE CAUSES: FOCUS ON FAT
It was initially postulated that excessive intake of saturated fat was a key driver of the obesity epidemic. However, it is now clear that the epidemic of obesity in the USA, Australia and other Western countries has in fact coincided with a significant decline in the per capita intake of saturated fat, creating somewhat of a problem with this hypothesis. However, as saturated fatty acid intake has fallen, there has been a corresponding increase in the intake of omega-6 PUFA in Westernised nations around the world, leading to a significant increase in per capita omega-6 PUFA intake over this time . This shift was initially prompted by the limited availability of animal-based fats during the Second World War, which led to their replacement with plant-based alternatives, and has since been reinforced by health recommendations which favour polyunsaturated over saturated fats . In addition, changes in the lipid composition of formulated animal feeds have led to changes in the fatty acid composition of animal products, including meat and eggs .
There is growing concern that this increasing dominance of omega-6 PUFA may have negative consequences for metabolic health. These concerns are based on the biological actions of omega-6 [linoleic acid; 18:2(n-6) and derivatives thereof], which are largely proinflammatory, prothrombotic and proadipogenic. It has been suggested that increases in omega-6 PUFA intake over the past few decades may be an important factor contributing to the current obesity epidemic. In particular there is evidence that exposure to excess omega-6 PUFA before birth or in early infancy may be responsible for promoting fat cell formation early in life and thereby predisposing individuals to excess accumulation of body fat as children and adults .
THE EARLY LIFE ORIGINS OF OBESITY
Although good nutrition is important at all life stages, a number of critical periods in development have been identified during which exposure to a sub-optimal nutritional environment are particularly detrimental, as they can have lasting effects on an individual's propensity to develop obesity and metabolic disease later in life. Of these critical windows, the most important appear to be those that coincide with the major periods of development of the key metabolic systems, that is before birth and during the first 2 years of life in humans, and during the foetal and suckling periods in rodents. Both human and experimental animal studies have shown that exposure to an inappropriate nutrient supply during these critical windows of development have lifelong consequences for an individual's health . The early studies of this ‘biological programming of metabolic disease’, focussed primarily on the effects of sub-optimal nutrition, either global caloric restriction or low protein, and showed that these exposures were associated with an increased accumulation of visceral adipose tissue in the offspring and, consequently, a predisposition to insulin resistance and type 2 diabetes . More recently, attention has turned to the more common situation in most Western countries; that of maternal overnutrition, and these studies have demonstrated that exposure to a ‘high-fat’; and/or ‘high-sugar’ diet before birth or in the early neonatal period predisposes the offspring to obesity and metabolic disease after birth [2▪].
Numerous studies in both large and small animal models have explored the mechanisms underlying this association, and have demonstrated that altered nutrient supply to the developing foetus/neonate plays a predominant role. These studies demonstrate that exposure to either excessive calories or an increased supply of fat and/or sugar during critical developmental windows leads to altered development of key systems involved in the regulation of energy balance and metabolism which permanently affects their structure and function [2▪,13▪▪]. These systems include, but are not limited to, the central neural network for appetite regulation, the fat cell or adipocyte, the mesolimbic reward system and insulin signalling pathways in skeletal muscle [9,13▪▪]. As a result of these programmed changes in development, these offspring are hyperphagic, have an increased propensity to accumulate body fat, have a preference for high-fat and high-sugar foods and are less insulin sensitive .
However, despite the extensive work which has been done in this area, fundamental questions remain about which specific components of the diet are responsible for this programming effect, and questions have been raised about the relevance of the common model of high-fat, high-sugar feeding to typical human diets. In particular, the high-fat diets that are commonly used in animal studies to induce maternal obesity are high in a number of fatty acid classes, but there have been limited attempts to dissect out which of the individual fatty acids is responsible for the programming effects. In the majority of these diets, saturated fat is the main fat component of the high-fat mix which, from what we have seen in the previous section, may not in fact be truly reflective of current trends in fatty acid intakes in humans. However, in our hands, in addition to increasing saturated fat, cafeteria diet feeding in rodents was associated with an increase in omega-6 PUFA in the maternal milk and offspring plasma (Vithayathil and Muhlhausler, unpublished observations), and may be playing a role in the adverse outcomes of offspring born to mothers who consume high-fat diets during pregnancy and lactation.
OMEGA-6 POLYUNSATURATED FATTY ACIDS AND THE ADIPOCYTE
The hypothesis that increased maternal intake of omega-6 PUFA could have consequences for fat deposition for the foetus or breast-fed offspring has a clear biological basis. A series of studies have demonstrated the capacity of linoleic acid and its long-chain derivative, arachidonic acid [20 : 4(n-6)] to promote the differentiation of preadipocytes in vitro, suggesting that increased exposure to omega-6 PUFA during critical windows in the development of the adipocyte could result in a permanent increase in the number of adipocytes in an individual, and thus their propensity to accumulate body fat [15,16]. These in-vitro studies are supported by experimental animal studies, in which rats provided with diets containing higher linoleic acid levels and/or higher ratios of linoleic acid/alpha-linolenic acid (ALA) exhibit an increased expression of lipogenic genes, increased fat mass and greater adipocyte size and adipocyte number compared with rats fed a diet containing lower linoleic acid levels [17,18]. Thus, omega-6 PUFA promoted the expansion of fat depots by upregulating both hyperplasia and hypertrophy. In a study by Hibbeln and coworkers, mice were fed diets which contained either 1% energy linoleic acid or 8% energy linoleic acid, similar to the 7–8% energy linoleic acid found in the typical US diet. Mice consuming the high linoleic acid diet exhibited increased food intake, increased body weight and higher fat mass compared with those on the 1% linoleic acid diet [19▪▪]. This provided new evidence that omega-6 PUFA could promote increases in fat deposition at levels of dietary linoleic acid which are commonly encountered in human diets.
OMEGA-6 POLYUNSATURATED FATTY ACIDS AND THE EARLY ORIGINS OF OBESITY
The major period of fat development in human infants occurs before birth and in the first year of life, with nutritional exposures during this time having permanent consequences for the regulation of body fat mass throughout life. The established proadipogenic role of the omega-6 PUFA forms the basis of the hypothesis that exposure to an increased supply of these fatty acids during the period of fat cell development could result in permanent programming of increased body fat mass. The potential role of omega-6 PUFA in the programming of obesity is supported by numerous animal studies. In one such study, the offspring of mice fed an linoleic acid-rich diet during pregnancy were 40% heavier at weaning than offspring of dams fed on diets with a balanced linoleic acid/ALA ratio . Importantly, this occurred in conjunction with a significant increase in body fat mass, and was still present in adult life . The potential importance of the omega-6 PUFA in the intergenerational cycle of obesity has been emphasised by a recent study by Massiera et al. , which demonstrated that feeding rats a diet in which linoleic acid made up 55% of the lipid fraction (19% of total energy) over four generations, led to a progressive increase in body fat mass in each successive generation, in the absence of any difference in the intake of saturated fat between the groups. The accumulation of body fat mass was due to an increase in both the hyperplastic and hypertrophic expansion of the adipose depots, driven by the upregulation of genes implicated in the hyperplastic and hypertrophic development of adipose tissue . Importantly, in the guinea pig, which is considered as the best animal model of adipose tissue growth, increasing the linoleic acid/ALA ratio from 2 : 1 to 30 : 1 during the preweaning period also resulted in increased fat mass in adulthood .
Since the period of fat cell development extends into the postnatal period in the majority of species, including humans, the fatty acid composition of the infant diet is also likely to contribute to the programming of the adipocyte and propensity to obesity. In lactating women, the fatty acid composition of the maternal diet is reflected in the composition of the breast milk. Previous studies have demonstrated that women supplemented with omega-3 LCPUFA accumulate n-3 LCPUFA in their breast milk in proportion with their level of intake . Consequently, the fatty acid composition of breast milk in Western countries has undergone a shift in line with that seen in the diet of the general population . Infant formulas have also undergone a marked evolution in their fatty acid composition over the past 3 decades. Those formulas which are now manufactured provide an adequate content of ALA and have a more balanced linoleic acid/ALA ratio, generally ranging between 5 and 10 to 1  (Philippe Guesnet, personal communication). The Global Standard for the composition of infant formula by a coordinated group of international experts suggested that the minimum linoleic acid level of 2.7% energy would be adequate to meet requirements . However, due to the exclusive use of vegetable oils in the formulation of infant formulas, the contents of linoleic acid in many formulas are well above physiological requirements, and may be high enough to interfere with omega-3 PUFA metabolism (Philippe Guesnet, personal communication). The International Society for the Study of Fatty Acids and Lipids, in their 2008 Statement on Dietary Fats in Infant Nutrition, stated that the linoleic acid content of formulas ranged from between 6 and 25% of total fatty acids, however, acknowledged that, given the potential negative effects of high omega-6 PUFA exposure, further research on high omega-6 formulas was needed .
Thus, Western infants, whether breast or formula-fed, are exposed to elevated levels of omega-6 PUFA not only before birth, but also during the early infant period, which could further exacerbate the effects of these fatty acids on the development of adipose tissue in these children.
EVIDENCE FROM HUMANS
In humans, most studies have focussed on the effects of increasing omega-3 intake, and there are currently no clinical studies which have directly investigated the effects of increasing maternal omega-6 PUFA intake in a randomized controlled trial. In the Project Viva cohort, a higher omega-6:omega-3 PUFA ratio in umbilical cord blood phospholipids was associated with a high subscapular skin-fold thickness at 3 years of age [26▪▪]. In contrast, a recent intervention study involving supplementation with n-3 LCPUFA and instruction to lower arachidonic acid intake during pregnancy and lactation did not show any effect on infant fat mass and fat distribution during the first year of life [27▪▪]. Further studies should help to solve this issue.
The hypothesis that increased maternal intake of omega-6 PUFA could be associated with adverse metabolic outcomes in her offspring is certainly not new. A series of studies in the mid-2000s focussed on the potential role of the omega-6 PUFAs in the origins of childhood obesity, and raised several of the points outlined in this review [7,28–30]. The increase in dietary intakes of omega-6 PUFA has been documented in several large studies, and has occurred over a time when the prevalence of obesity in the population has risen sharply, despite declines in the per capita intake of saturated fat. There is evidence supporting the hypothesis that omega-6 PUFA have proadipogeneic and prolipogenic properties, and recent work in animals has demonstrated that exposure to a high omega-6 PUFA diet during early life is sufficient to programme an increased body fat mass in the offspring.
Thus far, the work implicating omega-6 PUFA in the programming of obesity appears to have been largely ignored by health agencies, which continue to advocate the health benefits of polyunsaturates without identifying the functional differences between omega-6 and omega-3 types . In addition, the potential link between omega-6 PUFA and obesity does not appear to have challenged the popular belief that saturated fats are ‘bad’ and polyunsaturated fats are ‘good’. It is difficult, as scientists, to understand the reason for this. It is clear, however, that there is an urgent need for human clinical studies, in particular randomized controlled trials, to conclusively demonstrate whether there is a causal link between maternal omega-6 PUFA intakes and health outcomes in children, including obesity and insulin resistance. If causality is established, then it will be critical to use this as an evidence base for modifying existing dietary fat recommendations, particularly in light of the fact that the full impact of current high omega-6 PUFA intakes on future generations will not yet be apparent.
B.S.M. is supported by a Career Development Award from the National Health and Medical Research Council of Australia. The authors wish to thank Dr Christopher Ramsden and Dr Philippe Guesnet for their valuable comments, and Dr John Carragher for editorial assistance.
Conflicts of interest
There are no conflicts of interest.
REFERENCES AND RECOMMENDED READING
Papers of particular interest, published within the annual period of review, have been highlighted as:
- ▪ of special interest
- ▪▪ of outstanding interest
Additional references related to this topic can also be found in the Current World Literature section in this issue (pp. 77–78).
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