In summary:
- X calories from equal meals can have significantly different fattening effect on different individuals, even when we burn the same amount of calories by exercise, because our basal metabolic rates can differ by as much as 140%.
- X calories from different meals consumed by the same individual can also have different fattening effect, because the thermic effect of various macronutrients can differ by as much as 30%.
- The "calories in, calories out" principle still stands in the sense that you won't gain weight if you consume only as many calories as you burn them by BMR + exercise. If two individuals consume the same amount of calories, the one with low BMR will need to exercise more to burn them than the one with high BMR. So, a calorie is always a calorie, only that some people can burn them "easier" by having higher BMR.
- The calories on the nutrition facts labels quite accurately tell us how much usable energy we get with food.
What do calories on the nutrition labels mean?
The calories on the nutrition facts labels tell you how much metabolizable energy the food contains:
Bomb calorimetry directly measures the heat of combustion of a food
and thus gives values for gross energy (23), whereas stated food
values are metabolizable energy estimations (ie, gross energy
corrected for assumed obligatory energy losses before energy is
available to the body). (American Dietetic Association)
Calories not available to the body are mainly in undigested nutrients, like dietary fiber, lost in stool.
The metabolizable energy can be used for:
- Maintaining physiological functions
- Physical activity
- Heat production
When are calories fattening?
When your calorie intake exceeds the calories lost by your basal metabolism, physical activity and produced heat.
1) Differences in basal metabolic rate (BMR)
The BMR between individuals can vary greatly, for example, from 1,027 to 2,499 kcal/day (4,301-10,4555 kJ/d), which is a 140% difference, as measured in one study in 150 adults (AJCN, 2005). The main contributor to BMR was lean body mass (all the mass, except fat), so a short thin woman will have much lower BMR than a tall muscular man.
2) Calories burnt by physical activity
They are charts that estimate how many calories you burn by a certain type of exercise available. According to Harvard Medical School, a 70 kg (155 lbs) man in 30 minutes burns:
- 167 kcal by walking 4 mph
- 220 kcal by swimming
- 300 kcal by running 5 mph
- 450 kcal by bicycling 16-19 mph
- etc.
3) Thermic effect of food (diet induced thermogenesis)
Different macronutrients have different thermic effects, which refers to the loss of consumed energy due to increased metabolism triggered by food (Nutrition and Metabolism, 2004):
- protein: 15-30%
- alcohol: 10-30%
- carbohydrates: 5-10%
- fat: 0-3%
- a mixed diet: 10%
This means that a calorie from proteins and alcohol and large meals should be a bit less fattening than a calorie from carbohydrates and fats and small meals:
In conclusion, it is evident that overfeeding on carbohydrate and/or
fat results in body composition alterations that are different than
overfeeding on protein. It is commonly believed that 3,500 kcal is
equivalent to 0.45 kg (1 pound) of fat and that changing energy
balance in accordance with this will produce predictable changes in
body weight. However, the overfeeding literature to date does not
support this assertion. Dietary protein appears to have a protective
effect against fat gain during times of energy surplus, especially
when combined with resistance training (International Journal of
Exercise Science, 2017).
In conclusion, the main determinants of diet-induced thermogenesis are
the energy content and the protein-and alcohol fraction of the diet.
Protein plays a key role in body weight regulation through satiety
related to diet-induced thermogenesis. (Nutrition & Metabolism, 2004)
The following review focuses mainly on various percents of fat and carbohydrates: "Calories in, calories out” and macronutrient intake: the hope, hype, and science of calories (American Journal of Physiology, 2017):
Weight gain or loss is not primarily determined by varying proportions
of CHO and fat in the diet, but instead by the number of calories
ingested. Changes in EE, which metabolic pathways are used and other
considerations are quite modest when compared with caloric intake.
Until high-quality, metabolic ward primary data become available
indicating otherwise, a calorie is still a calorie. (CHO = carbohydrates, EE = energy expenditure)
In general, the review suggests that by far the most important factor contributing to weight gain is the total calorie intake and that food composition is less important.
