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Why a Calorie is Not Just a Calorie…

6 September 2021


So often in clinic I discuss the intricacy of what my patients eats in detail. Often patients will tell me that they keep to a very strict, low calorie intake. However very often I find that although the intake of calories is low and kept within their planned deficit, the type of calories are problematic and are directly effecting their weight loss efforts.  This is when understanding the nutritional value of a food comes into play. For example, a Mars Bar contains around 230 calories per bar, similarly 150g of avocado also contains around 230 calories. If a patient consumes the Mars Bar but is still keeping within the calorie deficit, I can guarantee both the success of the on-going weight loss goals as well the patients overall health status will be impacted with this choice of food versus if they were consuming the avocado option instead. This sounds obvious but let me break down some calorie myths. The bottom line is that the sources of calories does matter.

It’s true that all calories have the same amount of energy. One dietary calorie contains 4,184 Joules of energy. In that respect, a calorie is a calorie. But when it comes to your body, things are not that simple. The human body is a highly complex biochemical system with elaborate processes that regulate energy balance. Different foods go through different biochemical pathways, some of which are inefficient and cause energy (calories) to be lost as heat. Even more important is the fact that different foods and macronutrients have a major effect on the hormones and brain centres that control hunger and eating behaviour. The foods you eat can have a huge impact on the biological processes that control when, what and how much you eat.


You will find that the food you eat contains two main sugars – Glucose and Fructose. Although gram for gram, the two provide the same number of calories they behave very differently in the body. Glucose can be used by all of your body’s tissues, but fructose can only be used by the liver in any significant amount. Studies have found that Fructose does not stimulate the satiety centres. Please don’t be discouraged to eat plenty of fruits. While they contain fructose, they’re also rich in fibre, water and provide significant chewing resistance, which mitigates the negative effects of the fructose. Limit to 2 – 80g portion per day.

The Thermic Effect of Food

Different foods go through different metabolic pathways and some of these pathways are more efficient than others. The more efficient a metabolic pathway is, the more of the food’s energy is used for work and less is dissipated as heat. The metabolic pathways for protein are less efficient than the metabolic pathways for carbs and fat.

The thermic effect of food is a measure of how much different foods increase energy expenditure, due to the energy required to digest, absorb and metabolise the nutrients.

Here is the thermic effect of the different macronutrients:

Fat: 2–3%

Carbs: 6–8%

Protein: 25–30%

Sources vary on the exact numbers, but it’s clear that protein requires much more energy to metabolise than fat and carbs.

If you go with a thermic effect of 25% for protein and 2% for fat, this would mean that 100 calories of protein would end up as 75 calories, while 100 calories of fat would end up as 98 calories. Therefore high-protein diets boost metabolism by 80–100 calories per day, compared to lower-protein diets. Also Protein calories are less fattening than calories from carbs and fat, because protein takes more energy to metabolise. Whole foods also require more energy to digest than processed foods. This leads to significantly reduced appetite, making you eat fewer calories.

The Satiety Index

Different foods have different effects on satiety. This means some foods will give you a greater feeling of fullness. It’s also much easier to overeat on some foods than others.

For example, it may be quite easy to eat 500 calories or more of ice cream, while you’d have to force feed yourself to eat 500 calories of eggs or broccoli. The satiety index is a measure of the ability of foods to reduce hunger, increase feelings of fullness and reduce calorie intake for the next few hours. I highlight this in all my NT sessions with my patients. Its key to not over eat by staying fuller for longer.

If you eat foods that are low on the satiety index, then you will be hungrier and end up eating more. If you choose foods that are high on the satiety index, you will end up eating less and losing weight.

Examples of foods that are high on the satiety index are boiled potatoes, beef, eggs, beans and fruits. Foods that are low on the index include donuts and cakes.

Low-Carb Diets Lead to Automatic Calorie Restriction

Low-carb diets lead to more weight loss than low-fat diets, often 2–3 times as much. We are so very caught up on the 80’s drive to eat everything Low Fat but the reality is many of these products remove fat but replace it with sugars.

Also, low-carb diets also cause significant water loss. Excess bloat tends to go away in the first week or two and moreover low-carb diets tend to include more protein than low-fat diets. As highlighted earlier, Protein takes energy to metabolise, and the body expends energy turning protein into glucose.

The Glycemic Index

There is no argument that refined carbohydrates cause weight gain, blood sugar fluctuation and cravings for more refined food. Sugars like sucrose and high-fructose corn syrup, as well as refined grain products like white bread contain very little nutrient value. Refined carbohydrates tend to be low in fibre and are digested and absorbed quickly, leading to rapid spikes in blood sugar. They have a high glycemic index (GI), which is a measure of how quickly foods raise blood sugar. When you eat a food that spikes blood sugar fast, it tends to lead to a crash in blood sugar a few hours later. When that happens, you get cravings for more high-carb snacks. In clinic I refer to this as the “blood sugar roller coaster.”

If you’re on a high-carb diet, it’s crucial to choose whole, unprocessed carb sources that contain fibre. The fibre can reduce the rate at which the glucose enters your system. It dampens the speed at which the energy is released.

Annie Gill (Nutritional Therapist at Weightmedics)

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