The role of dietary fat in obesity

Current scientific evidence indicates that dietary fat plays a role in weight loss and maintenance. Meta-analyses of intervention trials find that fat-reduced diets cause a 3-4-kg larger weight loss than normal-fat diets. A 10% reduction in dietary fat can cause a 4-5-kg weight loss in individuals with initial body mass index of 30 kg m (-2). Short-term trials show that nonfat dietary components are equally important. Sugar-sweetened beverages promote weight gain, and replacement of energy from fat by sugar-sweetened beverages is counterproductive in diets aimed at weight loss. Protein has been shown to be more satiating than carbohydrate, and fat-reduced diets with a high protein content (20-25% of energy) may increase weight loss significantly. There is little evidence that low-glycemic index foods facilitate weight control. Evidence linking certain fatty acids to body fatness is weak. Monounsaturated fatty acids may even be more fattening than polyunsaturated and saturated fats. No ad libitum dietary intervention study has shown that a normal-fat, high-monounsaturated fatty acid diet is comparable to a low-fat diet in preventing weight gain. Current evidence indicates that the best diet for prevention of weight gain, obesity, type 2 diabetes, and cardiovascular disease is low in fat and sugar-rich beverages and high in carbohydrates, fiber, grains, and protein.     

Effect of very high-fat diets on body weight,lipoproteins, and glycemic status in the obese

Given the increased prevalence of obesity in the United States, despite reduced fat intake, there has been increasing interest in the effect of dietary fat on body weight, lipoproteins, and glycemic status. Despite predictions from epidemiologic and physiologic studies, recent prospective trials have demonstrated equivalent weight loss on high-fat versus low-fat diets. Nevertheless, the type of dietary fat consumed has substantially different effects on lipoproteins. Saturated fat raises high-density lipoprotein cholesterol but has unfavorable effects on total cholesterol, and has been associated with increased cardiovascular events. In contrast, unsaturated fats, and particularly omega-3 fatty acids, have the combined benefits of lowering serum cholesterol and raising high-density lipoprotein, as well as favorable effects on insulin resistance and inflammation; they also lower cardiovascular events in high-risk patients. Although current national guidelines modestly liberalize unsaturated fat consumption, important questions still remain about the optimal percentage of unsaturated fats in the diet.     

Pathways to obesity

The prevalence of obesity is increasing worldwide, which indicates that the primary cause of obesity lies in environmental and behavioural changes rather than in genetic modifications. Among the environmental influences, the percentage of fat energy of the everyday diet and the lack of physical activity are two important factors, which contribute to explain the rising prevalence of obesity. In this review, several lines of evidence are presented to illustrate why dietary fat does affect obesity development. There are four factors which support a link between dietary fat and obesity development:The thermic effect of nutrients, expressed as percentage of their energy content, is 2-3% for lipids, 6-8% for carbohydrates and 25-30% for proteins. This means that the efficiency of nutrient utilization (calculated as 100%-the thermic effect of the nutrient) is higher for fat than for carbohydrate or protein.Postingestive fuel selection favours the oxidation of dietary proteins and carbohydrates, whereas dietary fats are preferentially stored as triacylglycerol in adipose tissue. Alcohol, by inhibiting lipid oxidation, indirectly favours the storage of dietary fats.High-fat diet promotes excessive energy intake by passive overconsumption; the fat-induced appetite control signals are too weak or too delayed to prevent excessive energy intake from a fatty meal. The only proof that dietary fats contribute to weight gain is to test the long-term effect of ad libitum low-fat diets. Most studies on low-fat diets show that they induce a modest weight loss in obese individuals, but their long-term effect from a public health perspective is limited, probably due to a low compliance to the dietary advice.     

The role of the Mediterranean diet on weight loss and obesity-related diseases

Pandemic obesity is a major public health problem because of its association with non-communicable diseases and all-cause mortality, which can be improved/delayed with weight loss. Thus, several scientific societies and governments have launched guidelines to reduce body weight and adiposity or, at least, to avoid weight gain. In spite of the abundant literature on the topic, there is still controversy on the relative roles of fat and carbohydrate in the diet on weight gain. Present recommendations to avoid weight gain and obesity are directed to reduce intake of total energy variably and of total fat to <30% of energy, in spite on the lack of evidence of protection against cardiovascular disease (CVD) and mortality. By contrast, both high and low carbohydrate diets are associated with CVD and all-cause mortality in prospective studies, with a safe intake level at ≈50% of energy. Many popular diets with widely different macronutrient composition, including the Mediterranean diet, have been used in obesity; when energy-restricted, all result in similar modest weight loss at 6 months, but the effects are largely lost at 12 months. The Mediterranean diet is a plant-based, high-fat, high-unsaturated fat dietary pattern that has been consistently associated with lower rates on non-communicable diseases and total mortality in prospective studies and with reduced CVD in the PREDIMED trial. For this merits above other diets, this dietary pattern might also be used advantageously for weight loss. The results of the PREDIMED and PREDIMED-Plus randomized controlled trials on adiposity variables in high-risk populations are discussed.

     

Effect of dietary fat content on the incidence of obesity among ad libitum fed mice

Graded increments in the fat-to-carbohydrate ratio of the diet elicited a gradual, but reversible increase in the average mass of body fat maintained by adult female (CDI) albino mice under ad libitum feeding conditions. In addition, the inter-individual variability in the animals' fat mass was greatly magnified by diets with a substantial fat content (greater than 30 percent of calories). As a result, the incidence of obesity (defined as one third or more of body weight as fat) increased progressively from 0 percent to 35 percent when the diet's fat content was varied from 1 percent to 64 percent of its total energy content. A state of weight maintenance can only become established when the relative rates of glucose and fatty acid oxidation are proportional, on average, to the carbohydrate-to-fat ratio of the diet. When diets with a relatively high fat content are consumed, a considerable enlargement of the adipose tissue mass appears to be necessary in many animals before weight maintenance becomes spontaneously established. It is proposed, therefore, that changes in the adipose tissue mass, along with shifts in the range in which glycogen levels are maintained, and other adaptive changes, contribute to bring about rates of fat oxidation commensurate with a diet's fat content. This impact of dietary composition on body composition may be a factor contributing to the increased incidence of obesity in affluent populations consuming diets with a substantial fat content.     

The low-carbohydrate diet and cardiovascular risk factors: Evidence from epidemiologic studies

AimsObesity is an important public health issue because of its high prevalence and concomitant increase in risk of cardiovascular diseases. Low carbohydrate diets are popular for weight loss and weight management but are not recommended in leading guidelines due to the perception that increases in dietary fat intake may lead to an adverse cardiovascular risk profile. To clarify the effects of a low-carbohydrate diet for weight loss on cardiovascular disease risk factors as compared to a low fat diet for weight loss, we systematically reviewed data from randomized controlled clinical trials and large observational studies.Data synthesisWe searched the MEDLINE database (Jan 1966–Nov 2013) to identify studies that examined a low-carbohydrate diet as compared to a low-fat diet for weight loss or the improvement of cardiovascular disease risk factors.ConclusionsRecent randomized controlled trials document that low-carbohydrate diets not only decrease body weight but also improve cardiovascular risk factors. In light of this evidence from randomized controlled trials, dietary guidelines should be re-visited advocating a healthy low carbohydrate dietary pattern as an alternative dietary strategy for the prevention of obesity and cardiovascular disease risk factors.     

Low-fat, high-carbohydrate (low-glycaemic index) diet induces weight loss and preserves lean body mass in obese healthy subjects: results of a 24-week study

BACKGROUND: The traditional treatment for obesity which is based on a reduced caloric diet has only been partially successful. Contributing factors are not only a poor long-term dietary adherence but also a significant loss of lean body mass and subsequent reduction in energy expenditure. Both low-fat, high-carbohydrate diets and diets using low-glycaemic index (GI) foods are capable of inducing modest weight loss without specific caloric restriction. The purpose of this study was to investigate the feasibility and medium-term effect of a low-fat diet with high (low GI) carbohydrates on weight loss, body composition changes and dietary compliance. METHODS: Obese patients were recruited from two obesity outpatient clinics. Subjects were given advise by a dietician, then they attended biweekly for 1-hour group meetings. Bodyweight and body composition were measured at baseline and after 24 weeks. RESULTS: One hundred and nine (91%) patients completed the study; after 24 weeks the average weight loss was 8.9 kg (98.6 vs. 89.7 kg; p < or = 0.0001). There was a significant 15% decrease in fat mass (42.5 vs. 36.4 kg; p < or = 0.0001) and a decrease in lean body mass of 5% (56.1 vs. 53.3 kg; p < or = 0.0001). DISCUSSION: In this 6-month study, a low-fat, low-GI diet led to a significant reduction of fat mass; adherence to the diet was very good. Our results suggest that such a diet is feasible and should be evaluated in randomized controlled trials.     

Prevalence of obesity in Bombay.

Obesity has been reported in developed as well as developing countries. However, data on a large sample of the Indian population are lacking. This study analysed the prevalence of obesity among 1,784 adults in Bombay from various sections of society. Since the prevalence of obesity depends upon the criteria used, prevalence was judged by three criteria viz. percentage excess of body weight, body mass index, and body fat content. The data were classified and analysed according to occupation, age group, income, diet type, and also with respect to family history of obesity. The three methods gave a different prevalence of obesity. In general, the criterion of body mass index under-estimated, and body fat content over-estimated the prevalence as compared to that obtained by percentage excess body weight. An extremely high prevalence of obesity was found in all sub-groups of the sample. As judged by a body mass index of 25 and above, male students had the lowest (10.7%) and male medical doctors had the highest (53.1%) prevalence of obesity. Prevalence was highest for the age group 31-50 years for males and females, and declined on either side of this age range. Prevalence was directly proportional to financial income, and subjects with a family history of obesity had a greater prevalence of obesity compared to those without. This study indicates the gravity of the problem of obesity in Bombay, and provides directions for nutritional planning in the future.     

Contemporary strategies for weight loss and cardiovascular disease risk factor modification

Because of the overweight/obesity epidemic and attendant increase in risk of chronic disease worldwide, there is a pressing need to identify effective weight loss strategies. Current recommendations for weight loss advocate a reduced-calorie dietary pattern (lower in total fat) in conjunction with regular physical activity and behavior change. This combined approach is important for long-term adherence and weight maintenance. Several large-scale clinical trials implementing different reduced-calorie dietary patterns have shown that 4 to 10 kg of weight is typically lost. The different hypocaloric dietary patterns shown to be effective include reduced-fat diets (10%–30% of calories from fat), a Mediterranean-style diet, and diets that are reduced in energy density. The weight loss achieved is accompanied by beneficial changes in cardiovascular disease risk, including the lipid and lipoprotein profile and blood pressure. This article summarizes our current understanding of effective weight loss programs.     

Contemporary strategies for weight loss and cardiovascular disease risk factor modification

Because of the overweight/obesity epidemic and attendant increase in risk of chronic disease worldwide, there is a pressing need to identify effective weight loss strategies. Current recommendations for weight loss advocate a reduced-calorie dietary pattern (lower in total fat) in conjunction with regular physical activity and behavior change. This combined approach is important for long-term adherence and weight maintenance. Several large-scale clinical trials implementing different reduced-calorie dietary patterns have shown that 4 to 10 kg of weight is typically lost. The different hypocaloric dietary patterns shown to be effective include reduced-fat diets (10%–30% of calories from fat), a Mediterranean-style diet, and diets that are reduced in energy density. The weight loss achieved is accompanied by beneficial changes in cardiovascular disease risk, including the lipid and lipoprotein profile and blood pressure. This article summarizes our current understanding of effective weight loss programs.     

The role of low-fat diets in body weight control: a meta-analysis of ad libitum dietary intervention studies

OBJECTIVES: Low-fat high-carbohydrate diets are recommended to prevent weight gain in normal weight subjects and reduce body weight in overweight and obese. However, their efficacy is controversial. We evaluated the efficacy of ad libitum low-fat diets in reducing body weight in non-diabetic individuals from the results of intervention trials. DESIGN: Studies were identified from a computerized search of the Medline database from January 1966 to July 1999 and other sources. Inclusion criteria were: controlled trials lasting more than 2 months comparing ad libitum low-fat diets as the sole intervention with a control group consuming habitual diet or a medium-fat diet ad libitum. MAIN OUTCOME MEASURES: Differences in changes in dietary fat intake, energy intake and body weight. Weighted mean differences for continuous data and 95% confidence intervals (CIs) were calculated. RESULTS: Two authors independently selected the studies meeting the inclusion criteria and extracted data from 16 trials (duration of 2-12 months) with 19 intervention groups, enrolling 1910 individuals. Fourteen were randomized. Weight loss was not the primary aim in 11 studies. Before the interventions the mean proportions of dietary energy from fat in the studies were 37.7% (95% CI, 36.9-38.5) in the low-fat groups, and 37.4% (36.4-38.4) in the control groups. The low-fat intervention produced a mean fat reduction of 10.2% (8.1-12.3). Low-fat intervention groups showed a greater weight loss than control groups (3.2 kg, 95% confidence interval 1.9-4.5 kg; P < 0.0001), and a greater reduction in energy intake (1 138 kJ/day, 95% confidence interval 564-1712 kJ/day, P = 0.002). Having a body weight 10 kg higher than the average pre-treatment body weight was associated with a 2.6 +/- 0.8 kg (P = 0.011) greater difference in weight loss. CONCLUSION: A reduction in dietary fat without intentional restriction of energy intake causes weight loss, which is more substantial in heavier subjects.     

Should we recommend low‐fat diets for obesity?

Dietary fat intake has been blamed for the increase in adiposity and has led to a worldwide effort to decrease the amount of fat in the diet. However, the comparative efficacy of this approach is debatable. Whilst short‐term dietary intervention studies show that low‐fat diets lead to weight loss in both healthy and overweight individuals, it is less clear if a reduction in fat intake is more efficacious than other dietary restrictions in the long term. The purpose of this systematic review was to determine the effectiveness of low‐fat diets in achieving sustained weight loss when used for the express purpose of weight loss in obese or overweight people. A comprehensive search identified six studies that fulfilled our criteria for inclusion (randomized controlled trial, participants either overweight or obese, comparison of a low‐fat diet with another type of weight‐reducing diet, follow‐up period that was at least 6 months in duration and inclusion of participants 18 years or older without serious disease). There were a total of 594 participants in the six trials. The duration of the intervention varied from 3 to 18 months with follow‐up from 6 to 18 months. There were no significant differences between low‐fat diets and other weight‐reducing diets in terms of sustained weight loss. Furthermore, the overall weight loss at the 12–18‐month follow‐up in all studies was very small (2–4 kg). In overweight or obese individuals who are dieting for the purpose of weight reduction, low‐fat diets are as efficacious as other weight‐reducing diets for achieving sustained weight loss, but not more so.     

Should obese patients be counselled to follow a low‐glycaemic index diet? Yes

A reduction in dietary fat has been widely advocated for the prevention and treatment of obesity and related complications. However, the efficacy of low‐fat diets has been questioned in recent years. One potential adverse effect of reduced dietary fat is a compensatory increase in the consumption of high glycaemic index (GI) carbohydrate, principally refined starchy foods and concentrated sugar. Such foods can be rapidly digested or transformed into glucose, causing a large increase in post‐prandial blood glucose and insulin. Short‐term feeding studies have generally found an inverse association between GI and satiety. Medium‐term clinical trials have found less weight loss on high GI or high glycaemic load diets compared to low GI or low glycaemic load diets. Epidemiological analyses link GI to multiple cardiovascular disease risk factors and to the development of cardiovascular disease and type 2 diabetes. Physiologically orientated studies in humans and animal models provide support for a role of GI in disease prevention and treatment. This review examines the mechanisms underlying the potential benefits of a low GI diet, and whether such diets should be recommended in the clinical setting.     

Protein leverage and energy intake

Increased energy intakes are contributing to overweight and obesity. Growing evidence supports the role of protein appetite in driving excess intake when dietary protein is diluted (the protein leverage hypothesis). Understanding the interactions between dietary macronutrient balance and nutrient‐specific appetite systems will be required for designing dietary interventions that work with, rather than against, basic regulatory physiology. Data were collected from 38 published experimental trials measuring ad libitum intake in subjects confined to menus differing in macronutrient composition. Collectively, these trials encompassed considerable variation in percent protein (spanning 8–54% of total energy), carbohydrate (1.6–72%) and fat (11–66%). The data provide an opportunity to describe the individual and interactive effects of dietary protein, carbohydrate and fat on the control of total energy intake. Percent dietary protein was negatively associated with total energy intake (F = 6.9, P < 0.0001) irrespective of whether carbohydrate (F = 0, P = 0.7) or fat (F = 0, P = 0.5) were the diluents of protein. The analysis strongly supports a role for protein leverage in lean, overweight and obese humans. A better appreciation of the targets and regulatory priorities for protein, carbohydrate and fat intake will inform the design of effective and health‐promoting weight loss diets, food labelling policies, food production systems and regulatory frameworks.     

High-fat diet feeding reduces the diurnal variation of plasma leptin concentration in rats

To investigate the response of plasma leptin and its diurnal variation to graded levels of dietary fat intake, adult (486.8+/-10.8 g), male rats (N = 52) were fed diets containing 12%, 28%, 44%, and 60% fat for 4 weeks. The body weight gain and abdominal fat pad weight were higher (P < .05) in groups fed diets containing 44% and 60% fat compared with the two diets containing less fat. There were no significant differences in terms of body weight or fat pad weight between animals fed the two diets with higher fat content or between animals fed the two lower-fat diets. Twenty-four-hour energy expenditure was not different among the dietary fat groups. After 3 days on the experimental diets, plasma leptin increased (P < .03) in all dietary groups. The increases in leptin in animals fed 12% and 28% fat diets occurred primarily in the morning. In contrast, in groups fed the two diets containing higher fat content, leptin levels increased mainly in the afternoon. As a result, the daily variation in leptin increased (P < .05) in the two groups fed lower-fat diets, but decreased (P < .04) in animals fed the two higher-fat diets. These data demonstrate that short-term high-fat diet feeding abolished the diurnal fluctuation of plasma leptin levels, which may prevent proper leptin function and eventually contribute to the development of obesity.     

The pediatric obesity epidemic: causes and controversies

Obesity in children and adolescents has reached alarming proportions in the United States. Nutritional surveys do not indicate a significant increase in caloric intake in children and adolescents over the last 3 decades, although caloric intake has increased recently in adolescent females. Dietary fat has also been falling. There is no conclusive evidence linking physical inactivity to the obesity epidemic, and longitudinal studies indicate that physical inactivity may be the result of obesity rather than its cause. Hence, attention should be focused on dietary carbohydrate. Carbohydrate intake has increased as a result of the decrease in dietary fat. Indirect evidence also indicates that the quality of carbohydrate has been changing, so that American children are eating more carbohydrates with a higher glycemic index. It is proposed that high-glycemic-index diets lead to excessive weight gain as a consequence of postprandial hyperinsulinemia. Low-glycemic-index diets lower postprandial insulin levels and insulin resistance. It seems likely that diets restricted in sweetened sodas and noncitrus juices and containing ample whole grains, vegetables, and fruit could have a major impact on the prevalence of pediatric obesity.     

Influence of dietary composition on energy expenditure during recovery of body weight in the rat: Implications for catch-up growth and obesity relapse

The influence of dietary composition on whole-body energetics was examined during the first 2 weeks of isocaloric refeeding after low food intake in a rat model. The high energetic efficiency and energy partitioning toward fat accretion characteristic of this refeeding period were unaltered by (1) dietary fat levels varying between 6% and 30% of energy intake; (2) protein levels between 15% and 40%; (3) carbohydrate types (glucose v fructose v sucrose v starch v unrefined carbohydrate); and (4) diets containing 30% fat but differing in fatty acid composition (long-chain triglycerides [LCT] v medium-chain triglycerides [MCT] v oleic v linoleic v alpha-linolenic metabolites eicosapentaenoic acid [EPA] and docosahexaenoic acid [DHA] omega-3 fatty acids). Changes were only observed for extreme diets, ie, those deficient in protein or very high in fat. Low-protein diet was the only condition in which the high metabolic efficiency characteristic of the refeeding period was partially suppressed, and this occurred despite a lack of concomitant reduction in body fat deposition. On the contrary, with high-fat diets (> 30% of dietary energy consumption) the elevated efficiency was further increased, an effect that was only partially accounted for by the lower energy cost of body fat gain from high-fat diets. These studies indicate that during body weight recovery, the mechanisms underlying the adaptive increase in metabolic efficiency favoring the replenishment of body fat stores override any effect of food type on thermogenesis, and suggest some convergence in the controlling neural pathway. The implications of these findings vis-a-vis nutritional rehabilitation (catch-up growth) and obesity relapse are discussed.     

Sex differences in obesity and the regulation of energy homeostasis

Obesity prevalence is generally higher in women than in men, and there is also a sex difference in body fat distribution. Sex differences in obesity can be explained in part by the influence of gonadal steroids on body composition and appetite; however, behavioural, socio-cultural and chromosomal factors may also play a role. This review, which evolved from the 2008 Stock Conference on sex differences in obesity, summarizes current research and recommendations related to hormonal and neuroendocrine influences on energy balance and fat distribution. A number of important gaps in the research are identified, including a need for more studies on chromosomal sex effects on energy balance, the role of socio-cultural (i.e. gender) factors in obesity and the potential deleterious effects of high-fat diets during pregnancy on the foetus. Furthermore, there is a paucity of clinical trials examining sex-specific approaches and outcomes of obesity treatment (lifestyle-based or pharmacological), and research is urgently needed to determine whether current weight loss programmes, largely developed and tested on women, are appropriate for men. Last, it is important that both animal and clinical research on obesity be designed and analysed in such a way that data can be separately examined in both men and women.     

Comparison of overweight and obesity prevalence in school-aged youth from 34 countries and their relationships with physical activity and dietary patterns

The purposes of this systematic review were to present and compare recent estimates of the prevalence of overweight and obesity in school‐aged youth from 34 countries and to examine associations between overweight and selected dietary and physical activity patterns. Data consisted of a cross‐sectional survey of 137 593 youth (10–16 years) from the 34 (primarily European) participating countries of the 2001–2002 Health Behaviour in School‐Aged Children Study. The prevalence of overweight and obesity was determined based on self‐reported height and weight and the international child body mass index standards. Logistic regression was employed to examine associations between overweight status with selected dietary and physical activity patterns. The two countries with the highest prevalence of overweight (pre‐obese + obese) and obese youth were Malta (25.4% and 7.9%) and the United States (25.1% and 6.8%) while the two countries with the lowest prevalence were Lithuania (5.1% and 0.4%) and Latvia (5.9% and 0.5%). Overweight and obesity prevalence was particularly high in countries located in North America, Great Britain, and south‐western Europe. Within most countries physical activity levels were lower and television viewing times were higher in overweight compared to normal weight youth. In 91% of the countries examined, the frequency of sweets intake was lower in overweight than normal weight youth. Overweight status was not associated with the intake of fruits, vegetables, and soft drinks or time spent on the computer. In conclusion, the adolescent obesity epidemic is a global issue. Increasing physical activity participation and decreasing television viewing should be the focus of strategies aimed at preventing and treating overweight and obesity in youth.