The following text started off as a commentary on an article, requested by several readers. But, as it rose in popularity, I saw the chance to explain the common misconceptions about fat accumulation.
I'll try to explain it as simple as possible, without going too deep into mechanisms of how it all works. Some things might remain insufficiently explained, but a too detailed analysis will lower the number of readers. The same will happen if I continue stalling. So...
What does "getting fat" mean? When are we fat?
First of all, I'd like to emphasize that getting fat and being fat are not correct terms, but they are so widespread it's impossible to avoid them. Also, while medicine and nutrition science use obesity instead of being fat, getting fat has no better term except fat tissue gain, which is too long of a phrase. So, getting fat it is. Unless there are some people who think „obesiying“ is a good term?
Getting fat means storing fat tissue. If this is a continuous process, in a certain point the person crosses the healthy body weight border and this starts to have a negative effect on health. To determine this point and the border between healthy and unhealthy body weight is not easy. Of course, this border is fictional, but necessary, to make it easier to identify the people with amplified health risks due to enhanced body weight. Traditionally, it is determined through Body mass index (BMI).
A healthy BMI is one between 18 and 25. When it gets over 25, we say that the person is overweight. Over 30, we call this person obese. BMI over 35 is a category 2 obesity, and BMI over 40 is a morbid obesity.
There is a significant problem with this scale, noticeable already if you glance at the formula used to calculate it. BMI does not take into account the composition of your body. For this reason, it is not applicable to athletes or bodybuilders who have a higher BMI due to greater muscle mass, and a lower fat tissue ratio.
In elderly people, acceptable BMI is somewhat higher, because muscle mass (lost with age) is easier to maintain if the total body weight is somewhat higher. Higher body weight in elderly people is connected to lower mortality rates.
For categorization of children, we also use adjusted numbers.
The group of people that the traditional numbers apply to, and they shouldn't, are people with lower muscle mass. These are most commonly girls and women, even though men are not excluded – the group that mostly utilizes reduction diets due to social pressure or a wrong perception of their bodies, all of which can be drastic, and is not followed by strength exercises necessary to maintain muscle mass. Because of sudden drops in body weight, muscle mass is reduced as well. The problem is worsened by a quick return of those kilograms, but this time in form of fatty tissue, because people go back to old habits which caused the weight gain in the first place. After a few of these spikes and drops, a person can have a normal BMI, but a lower percentage of muscle mass and a higher percentage of fatty tissue – in lack of a better term, I'll call this „skinny fat“.
Also, some genetic differences in non-fat mass (muscle + connective tissue) are present among people with normal body weight, and that is why the borderline BMI defining obesity should be assessed individually.
Due to all of these exeptions and reasons, I think that using only BMI to categorize obesity is not the best solution for everyone. Even though obesity in most people is visible at first sight by a professional (or an honest person), with people suspect of low muscle mass it would be far more precise to categorize them by qualitatively measuring their fat tissue. The most practical way to determine this would be to visit your doctor or nutritionist and have them measure the skinfolds – subcutaneous fat – using a caliper.
How and why do we get fat?
Every organism needs energy to sustain life. People don't receive energy directly from the sun like plants, but from food. Also, we do not receive energy from vitamins and minerals (which is often suggested by some supplement brands) but from macronutrients – protein, fats, carbs, alcohol.
Energy value of macronutrients is expressed through kilocalories (kcal), but to make it simpler we use the term calorie (Cal), which is actually 1000 cal. Same units are used to express the energy consumption as well.
Since the lack of food is a characteristic of human life on earth so far, storing energy from food is an evolutionary advantage because it allows survival in periods with no food. Today, when the food in developed countries is omnipresent and easily accessible, this advantage is of little importance, and is primarily a burden.
When the energy intake is greater than the consumption, the body stores it in form of macronutrients – carbs are stored as limited muscle and liver glycogen, and fats are stored as adipose (fat) tissue.
On the other hand, if the energy requirements are not met through food – the body will start using internal sources. Even though the main function of protein from muscle tissue is to build muscle, not provide energy, when you lack the two primary internal energy sources (mostly glycogen), the body will use those proteins to produce glucose in the process of gluconeogenesis. Why this happens is obvious if you think of the fact that glucose is the primary source of energy in all body cells, and the only source for the brain and the nervous system. I'd like to point out that the brain can function in extreme conditions by using ketones made by fat decomposition, but there are certain doubts about its low efficiency levels. (1) (EDIT: Previous version of this article this statement was formulated with certainty. After I consulted a few books, I determined there was no reason for me to say that. More in the comments.)
Every calorie excess in the body will be stored in some way, regardless of its origin. In low glycogen reserves, the body will „direct“ the excess into its restoration (provided enough carbs), and the fats will be used as a direct energy source. Since glycogen reserves are limited to 800-3000 kcal, they are easily restored and unwanted fatty tissue is easily gained. The basics of this process will be explained in my next article.
So, an individual nutrient nor food do not make you fat, what makes you fat is a constant calorie sufficit (intake greater than consumption). Of course, it's simpler to reach that sufficit through eating chips, cookies, and drinking soda, than it is through eating boiled potato and leek, and drinking unsweetened tea, but that's a completely different issue which does not change the laws of physics.
In my next article I will talk more about transforming carbs into fat, and I'll mention the role of insulin in the process of getting fat. You'll also find out if it's possible to gain weight from air, and the answer to the eternal question of whether all calories are equal.
1. Wing, R. R., J. A. Vazquez, and C. M. Ryan. "Cognitive effects of ketogenic weight-reducing diets." International journal of obesity and related metabolic disorders: journal of the International Association for the Study of Obesity 19.11 (1995): 811-816.