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Is Fat Intake a Risk Factor for Fat Gain in Children?

Institute of Physiology, University of Lausanne, 1005 Lausanne, Switzerland

Address correspondence and requests for reprints to: Dr. Eric Jéquier, Institut de Physiologie, University of Lausanne, Case postale, CH-1000, Rue du Bugnon 7, CH-1005 Lausanne, Switzerland.

	Introduction

Top Introduction Nutrient-induced thermogenesis:... Effects of dietary fat... Does fat intake promote... Does a reduction in... Other advantages of low-fat... Conclusion References

 
The prevalence of obesity in children and adults is increasing worldwide. This demonstrates that the primary cause of obesity lies in environmental and behavioral changes rather than in genetic modifications. Among the environmental influences on body weight regulation, the percentage of fat energy of the everyday diet plays an important role. In many low-income countries, the percentage of fat energy in the everyday diet increases in people who reach a higher socioeconomic level, and this is accompanied by an increased prevalence of overweight and obesity. The passive over-consumption of energy-dense, high-fat diets combined with the decline in physical activity are the two main factors that account for the rising prevalence of obesity. The latter probably explains why several epidemiological studies report a trend of a decreased fat consumption in United States, while obesity prevalence is rising (1). The well-documented fact of dietary underreporting by obese people (2), however, is to be taken into account when assessing the results of such epidemiological studies, because worldwide the prevalence of overweight and obesity is significantly related to the percentage of fat energy in the diet (3).

Here, several lines of evidence are presented to illustrate why dietary fat does affect obesity development. There are four questions that are relevant to the relationship between dietary fat and obesity development: 1) Is there a difference in the efficiency of energy utilization from carbohydrate vs. fat? 2) What are the effects of dietary fat and carbohydrate on postingestive fuel selection? 3) Does a high-fat diet promote excessive energy intake by passive over-consumption? and 4) Does a low-fat diet influence the regulation of body weight?

	Nutrient-induced thermogenesis: efficiency of energy utilization from carbohydrate vs. fat

Top Introduction Nutrient-induced thermogenesis:... Effects of dietary fat... Does fat intake promote... Does a reduction in... Other advantages of low-fat... Conclusion References

 
After food ingestion, there is an increase in energy expenditure, a phenomenon called "nutrient-induced thermogenesis" or the thermic effect of food. This rise in energy expenditure mainly results from the absorption, the processing, and the storage of nutrients (4). The thermic effect of carbohydrate was found to be greater than that of fat (6–8% vs. 2–3% of the energy content of ingested nutrients for carbohydrate and fat, respectively). The reason for this difference in the thermic effects of fat and carbohydrate is the higher energy cost for storing glucose as glycogen than the cost for processing and storing fatty acids as fat. In practical terms, these results show that the efficiency of energy utilization is greater for fat than for carbohydrate, particularly when either fat or carbohydrate is consumed in excess. Because the thermic effect of nutrients is a loss of energy for the body, the lower the thermic effect, the higher the proportion of nutrient energy that is available for useful energy requiring processes or for weight gain. Maffeis et al. (5) report that a high-fat meal induces a lower thermogenic response than a isocaloric, isoproteic low-fat meal given to children. Although the meal-induced thermogenesis was 30% lower with the high-fat meal compared with the low-fat meal, this energy saving only represents about 2% of total 24-h energy expenditure.

It is important to assess whether the small differences in the efficiency of carbohydrate and fat energy utilization that are measured within 5 h following a meal are also observed when meals are given under eucaloric conditions for prolonged periods of time. Administration of isoenergetic formulas to adult individuals with various percentages of carbohydrate (15–85% energy as carbohydrate with 15% of energy as protein and the balance of energy as fat) did not induce any significant variation in energy needs as a function of percentage fat intake (6). Other studies on the effects of imposed isoenergetic low- or high-fat diets in obese and nonobese women showed that isoenergetic shifts from dietary fat to dietary carbohydrate within the generally recommended range had no effect on energy metabolism. Only diets with a very high carbohydrate and a very low fat content induced significant increases in either sleeping metabolic rate or in the thermic effect of a high carbohydrate meal (7).

Thus, imposed isoenergetic diets with various percentages of carbohydrate and fat modify energy metabolism only slightly and can be considered as comparable energy sources to cover energy needs. It is important to emphasize that the concept of a comparable efficiency of energy utilization between carbohydrate and fat calories arises from experimental conditions of imposed isoenergetic diets with various fat and carbohydrate contents. This concept, however, does not take into account two aspects of nutrient physiology that may affect the subjects eating behavior under conditions of everyday life (i.e. the postingestive fuel selection and the specific effects of nutrients on food intake regulation). Evidence has accumulated recently showing that high-fat, energy-dense meals favor passive over-consumption, a mechanism that very likely contributes to explain the increasing prevalence of obesity (8).

	Effects of dietary fat and carbohydrate on postingestive fuel selection

Top Introduction Nutrient-induced thermogenesis:... Effects of dietary fat... Does fat intake promote... Does a reduction in... Other advantages of low-fat... Conclusion References

 
After a meal, the metabolic fuel mix oxidized depends on the plasma concentration of glucose, free fatty acids, amino acids, and on hormonal responses, among which insulin secretion plays a major role. The carbohydrate content of a meal is the major determinant of insulin secretion, the main hormone that controls fuel disposal. Insulin promotes glucose uptake and oxidation in insulin-sensitive tissues. By stimulating glycolysis at several steps and by activating the pyruvate dehydrogenase complex, insulin controls the conversion of glucose to acetyl-CoA and its entry into the Krebs cycle in insulin-sensitive tissues. In addition, insulin suppresses the activity of the hormone-sensitive lipase and, therefore, inhibits lipolysis and lipid oxidation. As a result, after a high carbohydrate meal, glucose oxidation is promoted whereas fat oxidation is inhibited (7). The main driving mechanisms, which influence fuel selection between carbohydrate and fat oxidation, are glucose availability and the postprandial insulin response.

The metabolic priority for carbohydrate oxidation accounts for the fact that carbohydrate balance is well controlled; under habitual stable food intake conditions, carbohydrate balance is reached over a 24-h period. This means that nearly all the amount of carbohydrates ingested over a whole day is oxidized within 24 h. The fact that carbohydrate balance is well controlled is also related to the relative limited amount of body glycogen storage (maximal storage capacity, 500–800 g glycogen in an adult man). Furthermore, the daily carbohydrate intake corresponds to about 50% of the glycogen storage capacity. Thus, the half-life of dietary carbohydrate after ingestion is about 24 h, whereas that of dietary fat is much longer because of the very large body pool of adipose tissue triglycerides.

A biochemical process that could invalidate the concept of carbohydrate balance is de novo lipogenesis (i.e. the conversion of glucose into fatty acids and triglycerides). It is commonly believed, and often stated in biochemical textbooks, that hepatic de novo lipogenesis from glucose is an important metabolic pathway in humans. If it was true, dietary carbohydrate might be an indirect source of fat accumulation in adipose tissue through hepatic de novo lipogenesis. The concept of nutrient balance would then be invalidated because one dietary macronutrient (i.e. carbohydrate) could influence the balance of another macronutrient (i.e. fat). Two different methods of investigation have been used to assess de novo lipogenesis in humans: indirect calorimetry and stable isotopes techniques. Indirect calorimetry allows measuring net de novo lipogenesis (i.e. the difference between de novo fat synthesis and fat oxidation). A significant net lipogenesis has been observed in humans only with experimental massive carbohydrate overfeeding (9), a condition that does not occur in everyday life. Under these exceptional conditions, de novo lipogenesis is strongly stimulated and hepatic lipogenesis only accounts for a small portion of de novo fat synthesis, suggesting that adipose tissue lipogenesis may play a role.

Under spontaneous feeding conditions, however, isotopic measurements of de novo lipogenesis show that only 2–3% of glucose carbon atoms are converted into fatty acids; the latter are secreted by the liver, after esterification into triglycerides, as very low-density lipoprotein. Even with a high carbohydrate diet, hepatic de novo lipogenesis does not exceed 5–10 g fatty acids synthesized per day (10). Thus, human obesity does not result from the conversion of glucose into lipids, and dietary carbohydrates cannot be considered as nutrients directly responsible for the development of obesity. However, dietary carbohydrates indirectly induce a reduction in fat oxidation, and, for this reason, they may play a role in the excessive weight gain leading to obesity. This may occur in children who drink large quantities of soft drinks with a high sugar content.

The influence of carbohydrate on body weight regulation is also dependent on the effects of nutrients on food intake control. It is not possible to eat excessive amounts of carbohydrates because of the bulk effect of most carbohydrate rich meals that promotes satiation and satiety.

	Does fat intake promote excessive energy intake by passive over-consumption?

Top Introduction Nutrient-induced thermogenesis:... Effects of dietary fat... Does fat intake promote... Does a reduction in... Other advantages of low-fat... Conclusion References

 
The effects of nutrients on satiation and satiety have been much studied recently. Satiation corresponds to the suppression of hunger after the ingestion of a certain amount of food whereas satiety describes the period of time of absence of hunger between meals. It is important to assess the satiating capacity of the nutrients because the ability of the different macronutrients to stimulate satiety and to suppress subsequent food intake is not equal. There is a hierarchy in the ability of the three macronutrients to suppress subsequent food intake. Proteins have the greater satiating capacity; carbohydrates, which are also able to decrease the amount of food ingested at the next meal, follow them. By contrast, lipids have a less potent satiating effect than proteins and carbohydrates (11). Meals with a high lipid content favor passive over-consumption because the high-energy density promotes energy intake. In addition, the fat-induced appetite control signals are too weak to prevent excessive energy intake from a fatty meal, and the satiating effect is relatively small in relation to the amount of the ingested energy.

According to the concept of oxidation hierarchy, carbohydrate and protein intake elicits an acute autoregulatory increase in their oxidation, with a suppression of fat oxidation. It is interesting to emphasize that the hierarchy in the capacity of the macronutrients to elicit satiety (protein > carbohydrate > fat) is similar to the priority of fuel selection of macronutrients following a meal. It has been hypothesized that a stimulus generated at the level of fuel oxidation, presumably in the liver, provides a feedback signal that links the oxidative metabolism of fuels to the control of food intake. The effect of high-fat meals on spontaneous food intake has been studied in short-term and long-term covert manipulation studies (11). Both types of studies show a positive relationship between dietary fat content and energy balance. An interesting finding of long-term manipulation studies of dietary fat is the absence of compensation of energy intake with high-fat diets even after 14 days of positive energy balance. In addition, it has been established that passive overconsumption is related to the energy density of foods. High-fat diets are more energy dense than high carbohydrate diets, and the former favor hyperplagia.

The passive over-consumption of high-fat diets is also due to the fact that people tend to consume a similar bulk of food regardless of its composition (11). With high-fat, energy-dense diets more calories are passively ingested than with high carbohydrate diets. The improved taste and texture of fatty foods further enhance the increased energy consumption of high-fat diets.

	Does a reduction in the fat content of the diet influence the regulation of body weight?

Top Introduction Nutrient-induced thermogenesis:... Effects of dietary fat... Does fat intake promote... Does a reduction in... Other advantages of low-fat... Conclusion References

 
It is remarkable that only a very small difference between energy intake and energy expenditure can account for weight gain in the development of obesity. If energy intake would exceed energy expenditure by 1% everyday, this would result in a gain of 1 kg adipose tissue after 1 yr! This explains why it is so difficult, or even impossible, to demonstrate the precise mechanism of weight gain in a given individual. By contrast, it is possible to test whether a decrease in the percentage of fat energy (combined with a concomitant increase in carbohydrate energy) may be a useful approach to induce long-term weight loss in overweight and obese patients.

In a meta-analysis of low-fat diets (12), it was found that these diets induced about 5 kg weight loss in comparison with the control groups. The amount of weight loss was related to the degree of reduction in dietary fat and to the pretreatment body weight. An advantage of this dietary approach in the treatment of obesity is the implementation of behavioral changes: the patients have to change their habitual food choice, and there is some evidence that the benefit of the weight loss may be long-lasting in patients who adhere to these dietary recommendations.

	Other advantages of low-fat diets

Top Introduction Nutrient-induced thermogenesis:... Effects of dietary fat... Does fat intake promote... Does a reduction in... Other advantages of low-fat... Conclusion References

 
It is fortunate that low-fat diets are not only useful for preventing or treating obesity, but they are also recommended to reduce the risk of coronary heart diseases and of certain types of cancer. Low-fat diets are relatively rich in carbohydrates, and this has created some concern about their nutritional value (13). The isoenergetically replacement of fats by carbohydrates has been shown to lower both high-density lipoprotein (HDL) and low-density lipoprotein cholesterol plasma levels. Because the lowering of HDL cholesterol is considered to be associated with an increased risk of coronary heart diseases, the recommendation to favor low-fat, high carbohydrate diets has been criticized. It is interesting to note that the drop in HDL cholesterol was found to be very small when the higher carbohydrate diet is fed ad libitum instead of isoenergetically. Furthermore, the physiologic reduction of HDL cholesterol plasma levels with a low fat diet is not detrimental; on the contrary, the incidence of atherosclerosis is reduced in populations with low-fat diets. Low-fat diets associated with more fruits, vegetables, and fibers have also been shown to reduce blood pressure.

	Conclusion

Top Introduction Nutrient-induced thermogenesis:... Effects of dietary fat... Does fat intake promote... Does a reduction in... Other advantages of low-fat... Conclusion References

 
Low-fat, high carbohydrate diets are to be recommended for the prevention of obesity. The two main criticisms, which have been reported against the implementation of low-fat, high-carbohydrate diets, are their alleged lack of efficacy in eliciting weight loss and their potential adverse effect in cardiovascular disease prevention. These criticisms are not justified for the following reasons: low-fat diets have been shown to promote moderate weight loss over 1 yr or more, and no study has ever reported an increased incidence of cardiovascular diseases with low-fat diets. In addition, an improved insulin sensitivity and a decrease in impaired glucose tolerance accompany even a modest weight loss in obese subjects. It is, therefore, appropriate from a public health perspective to promote a reduction in fat intake as an important goal for the prevention of obesity and obesityinduced diabetes. Although the beneficial effects of low-fat diets have been mainly studied in adult subjects, there are many reasons to believe that the main concepts described above do also apply to adolescents. For infants and children, the nutritional advice must, of course, take into account the energy cost of growth and development; long-term studies for obesity prevention using low-fat diets need to be carried out in children. A practical issue is to improve the poor compliance of obese-prone individuals to adhere to a low-fat diet on a long-term basis. Another reason for concern with high carbohydrate, low-fat diets is the amount of highly refined and processed carbohydrate in the diet, because an excess of sugar intake has been associated with the development of diabetes mellitus. Therefore, diets should contain a sufficient amount of cereal fiber and grains, and the amount of refined carbohydrate must be kept low.

It is relevant to emphasize that the control of nutrient intake only concerns one side of the energy balance equation. The other side, energy expenditure, is highly dependent on the degree of physical activity. A low level of daily physical activity is a factor that contributes to the positive energy balance, which may lead to obesity. This is particularly well documented in children who spend several hours per day watching television programs. Because exercise of moderate intensity mostly stimulates fat oxidation, one understands why a lack of physical activity favors a positive fat balance and body weight gain. It is, therefore, important to stimulate fat oxidation by promoting physical activity.

Received November 3, 2000.

Accepted November 15, 2000.

	References

Top Introduction Nutrient-induced thermogenesis:... Effects of dietary fat... Does fat intake promote... Does a reduction in... Other advantages of low-fat... Conclusion References

 

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