Physics, Twinkies and Why You Can't Ignore Calories

Energy cannot be created or destroyed, it can only be transformed from one form to another.
— First Law of Thermodynamics

Weight loss isn't rocket science - it's actually thermodynamics. 

Applying the first law of thermodynamics to weight loss:

Change in energy stores = Energy intake - Energy expenditure

If your energy intake is greater than your energy expenditure, you have a calorie surplus and the body stores it as fat. If your energy intake is less than your energy expenditure, your body loses energy stores i.e. it has to burn fat or muscle to generate energy.

This principle has governed weight loss advice from medical professionals for decades. It is the theory that underpins the practice of recommending a 500 calorie deficit each day to lead to a 0.5kg weekly weight loss. It has led to glib phrases such as "A Calorie is a Calorie". 

Some people have tried to argue that numbers of calories was less important than source of calories when eating for weight loss. Some people have become convinced that low carbohydrate, high protein and ketogenic diets allow you to eat as much as you want - just as long as you don't eat carbohydrates or significantly reduced your carbohydrate intake.

The concept of unlimited eating still leading to weight loss is massively appealing, but the reality is slightly different. All these diets actually work by creating a calorie deficit. They've achieved it surreptitiously by removing or restricting certain food groups, but it's still a calorie deficit.


The Twinkie Diet of Professor Mark Haubs shows that calories really can't be ignored - you can lose weight eating only sugary cake if you just reduce your calorie intake. 

But the human body isn't simply an oven that burns food and creates energy. The human body is a fuelled by a complex set of metabolic pathways and controlled by complex hormone pathways. It doesn't process all food types through the same metabolic pathways or with the same degree of efficiency. 

What does this mean? It means that you could increase your chances of success by eating quality food with the right macronutrient ratios. A lot of people give up diets because they go into starvation mode and get uncontrollably hungry. The Twinkie Diet would certainly not have been a filling diet.

So what's the conclusion? You can't eat unlimited amounts of "the right" type of food and expect to lose weight. You just can't ignore calories and there has to be a calorie deficit. But the source and quality of your calories is really important if you want to succeed over the long term. 

Over the coming weeks we'll look in more detail at macronutrient ratios and how to optimise your chances of success.