Many experts and laypersons alike believe that weight gain is caused by eating too much and doing too little. This ‘eat-too-much, do-too-little’ theory of weight gain is so ingrained in the mindset of the general population that it is almost impossible to convince anyone to question this mantra. Because it is believed that weight gain is caused by eating too much food and doing too little exercise, the response of most people to getting fat is to perform endless aerobic exercise like a hamster on a wheel, while at the same time starving themselves of food. The poor track record of low calorie diets and aerobic exercise regimens at causing successful long term fat loss should be a warning to all. However, like a moth to the flame, people return to this tired old paradigm every time they need to lose weight, and most fail to make any permanent improvements. In fact cycles of dieting and exercise often lead to deteriorations in body composition such that the individual actually gets fatter and unhealthier over time.
The real problem is that obesity is a disease of perceived starvation caused by hypothalamic leptin resistance, secondary to peripheral insulin resistance. These metabolic changes have two main effects. Firstly the insulin resistance prevents to correct utilisation of energy for work. Instead energy is sequestered in fat tissue where it is unavailable for use. Secondly, the leptin resistance causes the feedback signal pertaining to energy reserves that is received by the hypothalamus to become inhibited, and this means that the hypothalamus is informed that fat reserves are low. The low energy status of the skeletal muscle along with the weak afferent signal from fat cells is akin to a starvation state, and as a result the body increases energy efficiency through increased appetite and a decrease desire to perform activity. This to some extent explains the hyperphagia and ‘laziness’ seen in the obese individual. Think of a starving man carrying around a bag of fat he cannot eat and you have a pretty good model of obesity.
Performing aerobic exercise and restricting calories under such starvation conditions only leads to further increases in energy efficiency that prevents meaningful or healthy fat loss. In addition, as forced calorie restriction proceeds, the skeletal muscle of the individual is sacrificed and used as energy and this lowers the resting metabolic rate considerably. This not only detrimentally affects the health of the individual and deteriorated their body composition, but also increases the risk of weight regain if energy intake is increase to ‘normal’ following the low calorie period. This occurs because the lower amount of skeletal muscle and the lower resting metabolic rate necessitates fewer calories to maintain the structures of the body and as such the homeostatic energy level for the individual is reduced. This latter point gets to the route cause of why most low calorie diets fail, and that is because they cause a decrease in skeletal muscle mass and resting metabolic rate while doing little to address the underlying insulin resistant state.
The belief that exercise is beneficial for fat loss is a good lie based on a grain of truth. The mainstream opinion is that exercise causes weight loss through increases in energy expenditure because activity ‘burns calories’. However, when we look at the amount of energy consumed during even a long period of performing aerobic exercise we see that it is only a very small percentage of the total energy consumed during the course of the day. The average person burns more calories sleeping that they do during hours of intense aerobic exercise. The resting metabolic rate makes up by far the largest contribution to energy expenditure (here) and those with the greatest skeletal muscle burn by far the largest amount of energy when at rest. This brings us nicely to the reason that exercise may really cause fat loss, and that is because it increases skeletal muscle mass and its sensitivity to insulin. It is not the actual exercise per se, but the metabolic changes that occur after exercise that are the key to successful fat loss.
In fact it is easy to show that it is the muscle mass gained during exercise, and the improved insulin sensitivity, that are the reason that exercise causes fat loss. This effect becomes apparent if we look at studies that have used testosterone injections on healthy men. For example in one study1, healthy men aged 60 to 75 years old were injected with varying doses of testosterone (as testosterone ethanoate in 25, 50, 125, 300 or 600 mg per week) for 20 weeks. The results showed that testosterone above 25 mg per week cause significant increases in fat free mass (+1.7, +4.2, +5.6 and +7.3 for 50, 125, 300 and 600 mg per week, respectively), and that the body fat loss was inversely related to testosterone doses. The mean fat loss was -0.9, -1.5, -2.2 and -3.0 kg in the 50, 125, 300 and 600 mg testosterone per week, respectively. The improvements in body composition were all the more remarkable because the subjects were informed to perform no exercise and were provided with a standard diet of 2700 kcal per 75 kg body mass.
Therefore exercise is not a requirement for fat loss, but the hormonal milieu and the skeletal muscle induced by exercise may be so. Increases in muscle mass explain the fat loss effects of exercise, and why resistance training is so good at improving the body composition. Resistance training causes increases in testosterone levels, and this hormonal shift in turn builds muscle and raises the resting metabolic rate thereby increasing the energy consumption to higher levels than could be achieved even through prolonged bouts of aerobic exercise. Of course aerobic exercise builds muscle too in the untrained individual, but generally not to such great extent as resistance training, which explains its less efficacious nature at improving body composition. Exercise (and testosterone) is also known to sensitise the skeletal muscle to insulin, and this reverses insulin resistance, allowing correct energy utilisation and enabling the oxidation of fatty acids through reductions in fasting insulin levels (because high insulin levels inhibit fat oxidation).
So exercise is beneficial at causing weight loss, perhaps not because it ‘burns calories’ while being performed, but because it causes metabolic changes that reverse insulin resistance and increase resting metabolic rate. Of course these changes are dependent on creating the correct internal hormonal milieu of raised testosterone levels, as well as the provision of adequate energy and protein in the diet to provide a positive energy and nitrogen balance, respectively. That the men in this study lost fat, gained muscle and looked better, while performing no exercise and while undertaking no energy restriction at all (in fact they ate more than their normal maintenance calorie levels) should be an eye opening experience to anyone stuck in the ‘eat-too-much, do-too-little’ paradigm. A starving man carrying a bag of fat cannot be expected to do much work, but allow him to eat that fat (akin to reversing insulin resistance) and his work capacity will improve considerably.
Dr Robert Barrington’s Comments: The key to weight loss is the reversal of insulin resistance and the building of skeletal muscle. Reversing insulin resistance is possible through dietary changes alone, although exercise can speed this process. High quality diets based on traditional eating practices have been shown to reverse insulin resistance and cause weight loss without the need to exercise and without the need for calorie restriction. Exercise is beneficial to fat loss because it causes increases in skeletal muscle and this not only improves the composition of the body but also increases the resting metabolic rate. The best way to lose fat therefore appears to be a high quality diet without calorie restriction, in combination with resistance training. Any aerobic activity should be kept to a minimum and should not be viewed as a calorie burning exercise, but rather as an adjunct to a healthy lifestyle through improvements in cardiovascular conditioning. The fact that testosterone has insulin sensitising effects further supports a role for post-exercise metabolic regulation, rather than exercise itself, as the primary driver of fat loss through increased physical activity. In fact low testosterone levels may be one of the causes of insulin resistance in men2.
RdB
