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The simple but difficult physics of losing weight

Summary:
Following up this CT post on health living,, everyone has their own story and their own health. That’s true, but we are all subject to the same physical laws. So, here’s my story and some thoughts on the physics. I managed to lose about 12 Kg over a couple of years, almost entirely through exercise. The basic physics is simple(1) weight loss = (kilojoules burnt – kilojoules consumed)*k,(2) kilojoules burnt = base metabolism + work done where k ≃ 0.025 is a constant reflecting the rate at which your body converts kilojoules of food energy into kilograms of fat. If you can alter the right hand side of (1) through any combination of diet and exercise then you will lose weight. The problem is that altering either of these, or even altering while holding the other constant is

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Following up this CT post on health living,, everyone has their own story and their own health. That’s true, but we are all subject to the same physical laws. So, here’s my story and some thoughts on the physics.

I managed to lose about 12 Kg over a couple of years, almost entirely through exercise.

The basic physics is simple
(1) weight loss = (kilojoules burnt – kilojoules consumed)*k,
(2) kilojoules burnt = base metabolism + work done

where k ≃ 0.025 is a constant reflecting the rate at which your body converts kilojoules of food energy into kilograms of fat. If you can alter the right hand side of (1) through any combination of diet and exercise then you will lose weight.

The problem is that altering either of these, or even altering while holding the other constant is really hard. Dieting makes you tired and slows your metabolism. Exercise increases your appetite, and also encourages you to flop once you stop exercising. All that’s because your body isn’t evolved to lose weight easily. Hunger and fatigue are both adaptations to stop you doing that. And, even if you can shift (1) enough to lose some weight, (2) puts a limit on how much you can lose. Balance is restored by the fact that your lighter body takes less energy to maintain and move around.

The crucial thing is to find some change for which you have both the willpower to adopt it initially and the willingness to maintain it indefinitely. For me, as I said, that’s been exercise. I aim to burn 4000 kj (about 1000 calories) a day in addition to base metabolism, which implies about 100 minutes of vigorous aerobic exercise. That’s logistically feasible for someone with flexible working hours and no kids at home, but very difficult otherwise. And it takes a long while to get to the point where you really enjoy it. That’s why the experts mostly recommend working on diet. But, if you can manage it, I think exercise is the better way to go.

John Quiggin
He is an Australian economist, a Professor and an Australian Research Council Laureate Fellow at the University of Queensland, and a former member of the Board of the Climate Change Authority of the Australian Government.

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