A simplistic reason for this would be to suggest that because “Calories in – Calories out = total calorie deficit” then the slim friend must be using up more energy (either by having a faster metabolism or by doing more exercise). The solution might be to suggest that you either have to “eat less, or move more” if you want to keep up with your friend - but what if there was another reason which wasn’t related to exercise or speed of metabolism? What if despite eating the same food, containing the same amount of energy you were not actually absorbing the same amount of energy – therefore meaning that there is a difference between the “Calories in” of you and the slim friend?
In order to understand this more we have to first appreciate how we know anything about the calories contained in food and drink at all. The calorific content of any substance is determined by incinerating it in a bomb calorimeter in a laboratory – this will tell you exactly how many calories are potentially available. When we eat or drink, we do not actually incinerate the food – we digest it – and the calories that are not used up during the digestive processes or passed out in the indigestible foodstuffs as waste are those that are actually absorbed – and this is your true “Calories in”. So, if there is a discrepancy between the actual calories absorbed by your slim friend and you then it is reasonable to assume that the reason behind this lies in the differences between each of your digestive systems.
Our digestive system extends from our mouth with food passing through the stomach, small and large intestine before being expelled as waste from the anus. The digestive system is known more commonly as the gut. The gut is home to literally trillions of microorganisms (bacteria/ yeasts and microbes) collectively known as the microbiome and these play an important role in digestion. The microbiome is a mix of “good” and “bad” bacteria – “good” bacteria are those which have been shown to have a positive effect on health and optimal health depends on having enough good bacteria to counteract the bad bacteria.
When food is eaten, it is digested as it passes through the gut by both mechanical forces (such as chewing) and chemical processes which involve various enzymes breaking the particles of food into molecules that can be absorbed into the body. These enzymes are produced by both the cells in the gut and the microbiome.
Twin studies, which involve studying the differences between identical twins, have produced some interesting results; you can assume that the inherited intrinsic gut cells will be the same so on discovering that the composition of the microbiome was very different between an overweight twin and a slim twin, it would be reasonable to wonder if these differences would account for the difference in weight. The major differences in the microbiome were found to be in their diversity – the slim twin had a large diverse population of bacteria with a high proportion of “good” bacteria. Studies involving mice have confirmed the link between the composition of the microbiome and obesity.
So perhaps this helps to explain a reason that consuming exactly the same food and drink as your slim friend does not mean you will be getting the same number of calories and therefore the impact on your weights will also be different.
Food intake does not mean the same as food uptake.