Elsevier

Nutrition

Volume 20, Issues 7–8, July–August 2004, Pages 716-727
Nutrition

Review article
Optimizing fat oxidation through exercise and diet

https://doi.org/10.1016/j.nut.2004.04.005Get rights and content

Abstract

Interventions aimed at increasing fat metabolism could potentially reduce the symptoms of metabolic diseases such as obesity and type 2 diabetes and may have tremendous clinical relevance. Hence, an understanding of the factors that increase or decrease fat oxidation is important. Exercise intensity and duration are important determinants of fat oxidation. Fat oxidation rates increase from low to moderate intensities and then decrease when the intensity becomes high. Maximal rates of fat oxidation have been shown to be reached at intensities between 59% and 64% of maximum oxygen consumption in trained individuals and between 47% and 52% of maximum oxygen consumption in a large sample of the general population. The mode of exercise can also affect fat oxidation, with fat oxidation being higher during running than cycling. Endurance training induces a multitude of adaptations that result in increased fat oxidation. The duration and intensity of exercise training required to induce changes in fat oxidation is currently unknown. Ingestion of carbohydrate in the hours before or on commencement of exercise reduces the rate of fat oxidation significantly compared with fasted conditions, whereas fasting longer than 6 h optimizes fat oxidation. Fat oxidation rates have been shown to decrease after ingestion of high-fat diets, partly as a result of decreased glycogen stores and partly because of adaptations at the muscle level.

Introduction

An inability to oxidize lipids appears to be an important factor in the etiology of obesity and type 2 diabetes. An elevated 24-h respiratory quotient in Pima Indians has been associated with a high rate of weight gain.1 It has also been reported that individuals with obesity and non–insulin-dependent diabetes mellitus have a decreased capacity to oxidize fatty acids (FAs) and this is related to increased insulin resistance.2 Further, some of the benefits of performing regular exercise, such as decreased insulin resistance, reduced hypertension, and reduced plasma low-density lipoprotein (LDL) concentration, are likely to be related to enhanced fat oxidation. This effect could be direct by adaptations in the fat metabolism pathways or indirect by reducing fat mass.3, 4 Exercise training and regular physical activity have been shown to increase fat oxidation in healthy individuals5, 6 and in obese populations.7 Having a better understanding of the factors that influence the rate of fat oxidation at rest and during exercise is therefore important. Interventions aimed at increasing fat metabolism could potentially reduce the symptoms of the disease in these groups of patients and might have tremendous clinical relevance.

One of the main training adaptations observed in endurance athletes is increased fat oxidation, and it has been suggested that an increased capacity to oxidize fat is related to endurance capacity and exercise performance.8 Training or diet interventions that optimize fat metabolism could therefore, at least in theory, also benefit endurance athletes.

Understanding the mechanisms behind the changes in fat metabolism that might occur as a result of various interventions and knowledge of the different sources of fat that may be utilized is important. Long-chain fatty acids (LCFAs) are a major source of energy at rest and during low- to moderate-intensity exercise. The source of FA utilized during exercise (FA from adipose tissue, FA in circulating lipoproteins (plasma triacylglycerol [TAG]) or muscle TAG, may vary depending on the conditions. Several sites have been suggested at which FA oxidation can be regulated: 1) adipose tissue lipolysis and FA delivery to the muscle, 2) FA movement across the muscle membrane, 3) hydrolysis of intramuscular TAGs (IMTAG), and 4) FA movement across the mitochondrial membranes. In Figure 1 these sites have been graphically displayed.

This review focuses on several interventions that influence fat oxidation, including the selection of the exercise intensity or the type of exercise, exercise training, and the influence of diet. In addition, the mechanisms behind the effects of these interventions on fat metabolism are discussed. This discussion is structured around the sites, displayed in Figure 1, where regulation of FA metabolism is believed to take place. For a more detailed discussion of these mechanisms, the reader is referred to several recent reviews.9, 10, 11, 12, 13, 14 This review is based predominantly on human studies in healthy individuals.

Section snippets

Acute exercise and fat oxidation

An important question from a practical point of view is: At what exercise intensity can the highest rates of fat oxidation be found? In 1932, Christensen15 showed, using respiratory exchange ratios (RERs), that changes in exercise intensity induce changes in substrate utilization. It was also observed that, with increasing exercise duration, fat oxidation progressively increased.16 Later this was attributed to a reduction in muscle glycogen breakdown and total carbohydrate (CHO) oxidation.

The

Short-term CHO intake and fat oxidation

Fatty meals have been suggested as a way to increase fat oxidation and the ingestion of medium-chain triacylglycerols (MCTs). The effectiveness of these nutritional interventions have been discussed elsewhere in this issue.93 Here we discuss the effects of CHO intake before and during exercise.

It has been demonstrated that the ingestion of CHO before or during exercise can result in a marked reduction in FA oxidation. The magnitude of the effect that CHO intake has depends on several factors

Large variation in fat oxidation still unexplained

Although a number of important factors regulating fat oxidation have been identified, It is apparent from many studies that a considerable degree of intersubject variability in substrate utilization persists. This is especially apparent in a study by Venables et al.20 in which fat oxidation was measured in a large group of healthy individuals. Peak fat oxidation rates ranged from 0.18 to 1.01 g/min. This variation still exists (although to a lesser degree) when all factors such as training

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