Eating less will not make us weigh less

Similarly, doing more will not make us weightless.

It is an almost universally held belief that people who are overweight just need to eat less and/or do more. The idea that eating less will make you weigh less assumes that the body cannot and does not adjust. It can and it does. To think that if you eat 500 fewer calories the body will give up 500 calories of fat, to make up the difference, is the ultimate naivety in the world of dieting. The body is not a cash machine for fat.

Let us say that our average person has a basal metabolic rate (BMR) requirement for 1,500 calories a day (the number of calories the person would need if they were ill in bed all day – just to run all the activities done by the body). Let us then say that they have a requirement for 500 additional calories if they are up and about (this is a realistic estimate – the BMR is the main determinant of the calorie need for the day by a margin).

The idea that a reduction of 500 calories leads to the body giving up 500 calories of fat assumes that neither the BMR requirement (1,500 calories) nor the additional requirement (500 calories) changes. In reality both change. The person who eats less has less energy and they will likely do a less additional activity that day – they won’t go to the gym or walk to the post box – they will be too tired. The body will also cut back on its maintenance for the day – it can save cell repair, fighting infection and building bone density for another day – you haven’t eaten enough, so it can cut back.

Think about it – you lose your job – you don’t automatically dip into savings – you cut back on expenditure and the body does exactly the same.

The exact same applies for doing more. If you think that you can eat the 2,000 calories needed for the day and then try to do 500 more calories worth of exercise, with the body making no adjustment elsewhere, you are wrong. The body is highly likely to cut back on the additional calorie expenditure above the BMR. If you go to the gym, you may then sit on the sofa all evening – too tired to do the housework. The body can also reduce the maintenance it had planned to do for that day.

Furthermore, exercise and BMR require quite different calories. Exercise is arguably best fuelled by carbohydrates (it provides glucose quickly for the body to use). BMR activities need fat, protein, vitamins and minerals. Carbohydrates can be useful for the vitamins and minerals they provide, but the macronutrient, the carbohydrate itself, can only be used for energy – not cell repair and fighting infection. Hence – if you eat 1,500 calories of carbohydrate (as the average citizen of the developed world currently does) – it can’t be used for body maintenance – you need to burn if off down the gym or you will gain weight.

Both the eating less and doing more beliefs also make the massive and wrong assumption that the body is able to burn fat. The body will always use carbohydrates for fuel first. Hence, if our average person has any glucose in the bloodstream or any glycogen (stored glucose) in the body – this will be used to cover any gap in food eaten or activity done. The body can only burn fat when there is no glucose/glycogen available. A modern man rarely, if ever, allows his body to get to the state where it can burn its own fat – let alone will.

Eating less makes us want to eat more and/or do less. Doing more makes us want to eat more and/or do less. Neither eat less nor do more has worked, can work, or ever will work as a solution for the obesity epidemic.