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When you eat, your body converts the calories in your food into usable energy. We get calories from three main energy sources: carbohydrates, protein and fat. To work out how much energy is in your food, you just apply the 4-4-9 rule: 1 gram of carbs or protein contains 4 calories, and 1 gram of fat contains 9 calories.

Unfortunately, this rule is a bit of an oversimplification.

Usable Calories

Your body cannot actually use every calorie in your food. The amount of useable calories in foods varies quite a lot from food to food.

For example, vegetable protein has 2.6 usable calories per gram, legume protein contains 3.5, while meat and egg protein have 4.3. Fat from nuts contain 8.4 calories per gram while fat from meat is closer to 9. Carbs from fruit work out to 3.6 calories per gram while those from cereals are closer to 4.1[1].

If you average out the sources of protein, carbs and fat found in a normal diet, you get the 4-4-9 rule. However, if you eat foods with fewer usable calories, you reduce your caloric intake. For example vegetarians or people who just eat very little meat will be consuming protein containing somewhere between 3 and 3.5 calories per gram, while the fat they eat will contain around 8.5 usable calories per gram.

The Thermic Effect of Food (TEF)

There’s a further complication. The problem is that not all the energy your body digests is actually available to you. It requires energy to metabolise food. This energy loss is known as the Thermic Effect of Food or TEF.

Your body is good at metabolising fat. For every 100 calories of energy your body digests, it obtains about 98 calories of usable energy. Similarly, calories from low-fibre carbohydrates are almost entirely available for use by the body.

However, for every 100 calories of protein your body digests, your body uses about 20 calories for metabolic processes, leaving you with just 80 calories of energy[2]. This can make a big difference. For example, if you measure your food and work it out so that you eat exactly 2000 calories a day, if you get 50% of your calories from protein your body will actually be getting about 1800 calories, while a 10% protein diet results in 1960 usable calories.

How can you use this to your advantage

If you want to lose weight, you need to eat fewer calories than you burn off. One way to do this is to eat less. But if you have trouble with self control (who doesn’t), an easier way is to eat foods with a lower calorie density and fewer available calories.

If you switch processed foods with a high number of available calories for foods with a lower number of useable calories, such as fruits, vegetables, nuts and legumes, even if your calorie counter says you’re eating the same number of calories you will actually be eating fewer, making it much easier to lose weight. An added bonus is that these foods are much better for your health.

A second thing you can do is try to include more protein in your diet. Not only is protein more filling[3], but the thermic effect of food means you effectively end up getting fewer calories, also making it easier to lose weight.