Short term effects of energy restriction and dietary fat sub-type on weight loss and disease risk factors

Abstract

Background and aims

Decreasing energy intake relative to energy expenditure is the indisputable tenet of weight loss. In addition to caloric restriction modification of the type of dietary fat may provide further benefits. The aim of the present study was to examine the effect of energy restriction alone and with dietary fat modification on weight loss and adiposity, as well as on risk factors for obesity related disease.

Methods and results

One-hundred and fifty overweight men and women were randomized into a 3 month controlled trial with four low fat (30% energy) dietary arms: (1) isocaloric (LF); (2) isocaloric with 10% polyunsaturated fatty acids (LF-PUFA); (3) low calorie (LF-LC) (−2 MJ); (4) low calorie with 10% PUFA (LF-PUFA-LC). Primary outcomes were changes in body weight and body fat and secondary outcomes were changes in fasting levels of leptin, insulin, glucose, lipids and erythrocyte fatty acids. Changes in dietary intake were assessed using 3 day food records. One-hundred and twenty-two participants entered the study and 95 completed the study. All groups lost weight and body fat (P < 0.0001 time effect for both), but the LC groups lost more weight (P = 0.026 for diet effect). All groups reduced total cholesterol levels (P < 0.0001 time effect and P = 0.017 intervention effect), but the LC and PUFA groups were better at reducing triacylglycerol levels (P = 0.056 diet effect). HDL increased with LF-LC and LF-PUFA but not with LF-PUFA-LC (0.042 diet effect). The LF and LF-LC groups reported greater dietary fat reductions than the two PUFA groups (P = 0.043).

Conclusion

Energy restriction has the most potent effect on weight loss and lipids, but fat modification is also beneficial when energy restriction is more modest.

Introduction

Worldwide, more than one billion adults are overweight and more than 350 million are obese. Many will suffer from obesity related diseases, such as cardiovascular diseases and diabetes, and in turn premature death [1]. The mechanisms behind obesity development are complex and multifactorial but diet and physical activity are central. Decreasing energy intake relative to energy expenditure is the indisputable tenet of weight loss, but success with obesity management remains practically elusive. One of the many ways forward lies with addressing metabolic links between body fat and food components, in particular dietary fat. Lifestyle intervention, including a low calorie, low fat diet has proven to promote weight loss and reduce the incidence of diabetes [2]

In addition to caloric restriction, modifying the type of dietary fat may be beneficial. With respect to cardiovascular risk, focusing on dietary polyunsaturated fat improves circulating triacylglycerol levels, a benefit additional to weight loss [3], [4]. Likewise, replacing dietary carbohydrate [5] or saturated fat [6] with unsaturated fats produces favourable changes in circulating lipids. With obese insulin resistant participants, higher proportions of dietary unsaturated fats can reduce central fat distribution [7] and improvements in insulin sensitivity can be observed with modified fat diets where the total fat level is kept below 37% energy [8]. One small study of normal healthy adults, found that replacing 6 g/day of dietary fat with fish oil reduced body fat mass and increased lipid oxidation under isocaloric conditions [9].

Despite this knowledge, the level of obesity appears resistant to interventions. Mechanisms have been put forward to explain why obese people do not lose much weight, suggesting that metabolic adaptations might be worth exploring [10]. For example, feeding studies have shown that fat induced thermogenesis is reduced in obesity, and this could reflect an adaptive response to weight gain [11]. Manipulating the type of dietary fat might ameliorate this effect because polyunsaturated fatty acids (PUFA) are known to suppress the expression of genes associated with lipogenesis, but activate genes involved in fat oxidation [12]. Increasing fat oxidation might help to reduce body fat, although it might also affect further weight loss because the energy deficit required to lose a kilogram of body weight depends on fat mass [13]. Either way, whether dietary manipulation affects the energy deficit required for weight loss over time is a question worth pursuing.

In the end, these are theoretical positions that need to be translated to practice under ‘free living’ conditions. In this study we used a food guidance system to compare the effect on weight loss and adiposity of a standard low fat diet with a low fat diet inclusive of PUFA rich foods, either under isocaloric or low calorie (−2 MJ/day) conditions. Secondary outcomes included biomarkers of cardiovascular disease and diabetes risk, and changes in energy deficit.