Calories-in, Calories-out

A failed principle of dieting

4 min readOct 6, 2021


Photo by Piret Ilver on Unsplash

Most people intuitively understand energy balance, particularly when it comes to weight management. This is known as the ‘calories-in calories-out’ (CICO) model, and its premise is that weight loss will occur when calories-out (CO) are greater than calories-in (CI). This is the basis for most diets, which rely on reducing CI to create an energy imbalance that favours weight-loss. The problem is, we know that these diets work in the short-term but almost always fail in the long-term. Why?

Lets look more closely at what happens to metabolism when CI is reduced. Take a hypothetical dieter (HD) who starts a CICO diet to lose weight.

Assume that before starting, HD is consuming 2000 calories (CI) and expending 2000 calories (CO) daily. HD is not gaining weight, but is dissatisfied with being overweight and starts a diet to reduce CI to 1500.

HD begins to lose weight because CO (2000) is greater than CI (1500). However, the body soon realises this is not sustainable in the long-term and that it cannot continue to expend 2000 calories of energy a day as though nothing is happening. Its reaction is to reduce its resting metabolic rate so as to reduce CO. Eventually, CO is reduced to 1500 and CI = CO again. HD is now in exactly the same situation as before the diet and weight loss stops.

However, HD is now running at a much lower energy level and is likely to be lethargic and pretty miserable. HD’s brain, the ultimate regulator, needs more energy and releases irresistible hormonal signals to eat. HD sensibly starts to eat a bit more. CI now becomes greater than the new CO, and weight starts to go back on again. As HD increases CI and more calories are available for the body, CO will start to rise again. However, the increase in CO lags the increase in CI, so there is a steady calorie surplus — weight continues to increase. This weight increase should stop when CO finally catches up with CI. If this happens, HD’s weight is stable again, but probably back where it started.

But, here’s the problem. For many people CO does not catch up. In fact, CO can remain below its initial value for years. HD starts to put on even more weight, gets worried and starts another diet that will end in the same way. Sound familiar? Sadly, everyone will silently blame HD for not having the willpower to stick with diets. The reality is that this failure has nothing to do with HD’s willpower. It is because CICO is not an appropriate model for long-term weight reduction.

There is nothing new in any of this, the science is known. However, the CICO model is so intuitive and compelling that it is an easy sell by the multi-billion dollar diet industry. When the diets they promote inevitably fail, it doesn’t matter — the industry can blame the dieter or urge the dieter to try a new diet. Worse, the dieter blames themselves and feels bad about it.

So, here is a message I want you to hear if you have tried CICO diets but not been able to keep weight off. I think it is important enough to shout: IT’S NOT YOUR FAULT.

Is there an alternative? Yes. The alternative is to increase CO, that is, increase metabolism to burn energy. That’s what happens with a ketogenic diet (KD).

On a KD, the ratio between macronutrients (carbohydrates, proteins and fats) gets altered severely away from carbohydrates in favour of fats, leaving proteins alone. No other change is made to the diet — total CI can stay the same or be reduced according to hunger signals. No deprivation is necessary or desirable. Exercise need not be increased and no effort need be made to increase CO.

If CI and CO are kept the same, the CICO model predicts this would have zero effect on weight. In fact, it has a dramatic effect on weight. Burning 1g of carbohydrate will release 4 calories of energy. Burning 1 g of fat will release 9. The extra energy available from fat burning fuels metabolism that results in more fat burning. Resting metabolic rate is not adjusted downwards because we have not created an energy deficit — total CI is not restricted. In fact, the KD increases metabolism (CO) and that results in weight loss. A recent study showed that energy expenditure can increase by about 100 calories a day. This is the equivalent to the energy burned during 30 minutes of moderate exercise 3 times a week.


CICO triggers an energy conservation response whereas KD triggers an energy expenditure response. That’s why a CICO diet will maintain weight despite heroic attempts by the dieter, whereas a KD can reduce weight, effortlessly.

Eating fat won’t make you fat. Eating fat can make you lean.




Science of cooking, eating and health. Retired neuroscientist.