Compared with the finite stores of carbohydrate (CHO), endogenous fat depots in humans are large, and represent a potentially unlimited source of fuel for skeletal muscle metabolism during aerobic exercise. However, fatty acid (FA) oxidation by muscle is limited, especially during the high power outputs and intensities sustained by athletes in training and competition. It has been proposed that strategies that promote FA oxidation during exercise could attenuate the rate of muscle glycogen utilisation and improve sports performance; this is one of the key adaptations achieved by aerobic training. This chapter reviews a variety of approaches that have been attempted to achieve this goal, with particular focus on the low-carbohydrate high-fat (LCHF) diet, which has been enthusiastically revived in recent times. As background to this interest, it is important to understand the role of endogenous fat as an energy substrate for skeletal muscle during exercise, the effect of exercise intensity on the regulation of fat metabolism, and the processes that could limit FA during exercise.
This chapter provides an overview of fat metabolism during exercise, which can be combined with the information in Chapter 1 and several excellent reviews of metabolism during exercise (e.g. Hargreaves & Spriet 2018, 2020), for a greater understanding of the regulation and integration of various fuel pathways.
SOURCES OF FAT AS A MUSCLE FUEL
Lipids provide the largest nutrient store of chemical energy that can be used to power biological work (see Figure 16.1). As an energy source, triacylglycerol (TG) has several advantages over CHO: the energy density of lipid is higher (37.5 kJ/g for stearic acid versus 16.9 kJ/g for glucose), while the relative weight as stored energy is lower. TG also provides more adenosine triphosphate (ATP) per molecule than glucose (147 versus 38 ATP). However, the complete oxidation of FA requires more oxygen than the oxidation of CHO (6 versus 26 mol of oxygen per mole of substrate for glucose and stearic acid oxidation, respectively), making CHO a more economical fuel source when oxygen delivery to the muscle becomes limited. Indeed, the oxidation of CHO provides about 5–10% more ATP per litre of oxygen than fat; and higher CHO stores have been associated with greater gross efficiency of exercise (power production per litre of oxygen consumption or work done per kJ of energy expenditure) (Cole et al. 2014).
Endogenous fuel stores for skeletal muscle contraction in a well-trained endurance athlete
The size of the adipose tissue TG pool is difficult to estimate and obviously depends on the fat mass of each individual, but is likely to range from 50 000–100 000 kcal (200–400 MJ) in men and women with 10–30% body fat (see Figure 16.1). In order for this TG to be ...