From the book’

This is especially true when we look at longer timeframes of several days or weeks. In the case of the Hadza(tribe in Tanzania), when they rest, they really rest. And it’s true for athletes and everyone else who is active too. We can be very active for a period of time, but we claw back that energy debt later. It’s this reduction of energy usage in other ways inside the body that may explain why exercise is associated with improved physical health, even if it doesn’t lead to weight loss.
Pontzer’s model posits that going for a long walk or run results in simply scaling back on routine non-essential bodily processes, reducing the amount of energy spent on your immune, endocrine, reproductive and stress systems. That may sound bad, but a bit of downtime actually seems to help to restore those systems to a healthier level of function. And it makes sense evolutionar-ily: throughout hominid history, there will have been significant periods during which food was scarce. Under the conventional model of calorie burning, that would mean using the most calories when food was least available because you would inevitably work harder to hunt or gather those calories. The fixed energy model means that energy use is consistent even if we do have to walk further to get food. And in a time of scarcity, it makes sense to borrow from – for example – the reproductive system to reduce fertility.
According to Ponzter’s data, we burn around 2,500 calories per day at desk jobs, the same number of calories as if we were walking a long distance. Since we’re not spending that energy on walking, we spend it elsewhere, on things like being stressed. The hypothesis says that office workers will likely have increased levels of adrenaline, cortisol and white blood cells, all of which make us anxious and inflamed. 53 , 54 A sedentary life (of the kind you probably live if you’re reading this – although not necessarily) leads to higher levels of testosterone and oestrogen, which might sound good to some people, but which can increase risks of cancers. By contrast, the Hadza – who do around two hours of moderate and vigorous physical activity every day, many times more than typical people in the UK and the USA – have morning salivary testosterone concentrations that are roughly half those of western populations. 55This is a good thing, and it may explain why exercise is such an important treatment for many chronic conditions
and seems to reduce depression and anxiety. 56