The Calories-In-Calories-Out theory of fat mass regulation relies on the first law of thermodynamics and treats the body as a bomb calorimeter. It essentially states that body weight, and in essence body fat, is simply an equation of “what you eat” versus “what you expend”.
The Hormone Theory of Fat Mass Regulation (often called the carbohydrate-insulin theory of obesity) suggests that calories are not what dictate fat mass, it is the hormones in your body, mainly insulin, that predicts it.
Let us distill these to theories down to a sentence each:
1) CICO: A calorie is a calorie, it does not matter what type of calorie it is, they are all equivalent.
2) Hormone Theory: The accumulation of fat mass is a result of dietary carbohydrates leading to elevated insulin levels which shifts metabolism into fat storage and away from fat oxidation.
We are going to be unpacking these and their respective roles in body fat regulation in a future post but for now lets move on.
Both of these models lead to testable predictions, the hallmark of science. I think it is most appropriate to borrow the words from someone far smarter and well versed on this topic here, “A logical consequence of the carbohydrate-insulin model is that decreasing the proportion of dietary carbohydrate to fat without altering protein or calories will reduce insulin secretion, increase fat mobilization from adipose tissue, and elevate oxidation of circulating free fatty acids. The altered metabolic and endocrine milieu is therefore predicted to relieve the state of cellular internal starvation resulting in decreased hunger, increased body fat loss and increased energy expenditure. In contrast, a more conventional model asserts that a calorie is a calorie, meaning that isocaloric exchanges between dietary carbohydrate and fat will not substantially influence energy expenditure or body fat”.
Essentially, if the Hormone Theory is right, right lowering carbohydrates and equally increasing fat will have a greater advantage for fat loss; if the CICO model is right then there will be no difference.
This is, in essence, the whole impetus for the Ketogenic Diet
How does the Ketogenic Diet stack up in the real world? What does the evidence say?