According to a study discussed on this website, people lose more weight on a calorie restricted fat reduced diet than on a similar calorie restricted carb reduced diet.

  1. Is this study a fair dinkum, non-bogus study?
  2. If so, shouldn't fat calories' numerical values be increased to better reflect the new scientific findings?
  3. And if so, how should this be done?
  • 3
    $\begingroup$ Because the calories of food are not estimated with a dice but with a calorimeter. $\endgroup$
    – Chris
    Jul 25 '21 at 12:06
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    $\begingroup$ I am not saying the study is false, but a certain amount of food has a measurable amount of calories. This will not change because a social construct demands changing it. $\endgroup$
    – Chris
    Jul 25 '21 at 13:50
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    $\begingroup$ I’m voting to close this question because it is about the articulation of dietary advice, rather than a problem in biology. $\endgroup$
    – David
    Jul 25 '21 at 14:55
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    $\begingroup$ That single definition is fine as heat is a form of energy. The digestion of food involves oxidation of molecules but the energy released by this is not only in the form of heat but also chemical energy that can be used to build new large molecules including, but not exclusively, fat, or mechanical energy to do muscle work. The new molecules synthesised will depend to some extent on the constitution of the food — what small units it is broken down to. So although calorific value may be a general indicator of whether a foodstuff will make you put on weight it cannot be an exact measure. $\endgroup$
    – David
    Jul 26 '21 at 7:08
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    $\begingroup$ It should be pointed out that the study doesn't conclude that "people lose more weight on a calorie restricted fat reduced diet than on a similar calorie restricted carb reduced diet." More like, some people in study lost more weight. But a study sample size of 19 without a non-intervention control group isn't nearly enough evidence to make inferences about the human population as a whole. $\endgroup$
    – MikeyC
    Jul 29 '21 at 1:47

Calories is a unit of energy, which cannot be changed. The efficiency of using the energy coming in different forms is however different. The original article in which the study was reported points out that indeed the matter at stake is the balance between the fat consumed and the fat burned:

The most sensitive method for detecting the rate of body fat change requires calculating daily fat balance as the difference between fat intake and net fat oxidation (i.e., fat oxidation minus de novo lipogenesis) measured by indirect calorimetry while residing in a metabolic chamber. At the end of the diet periods, our study had a minimum detectable difference in daily fat balance of 220 kcal/day (or 23 g/day) and the cumulative fat loss had a minimum detectable difference of 110 g. The observed differences in fat balance and cumulative body fat loss between RC and RF diets were substantially larger than these values and were statistically significant. While the fat balance method does not determine the anatomical location of lost fat, decreased adipose tissue triglyceride likely makes up the majority. Any additional loss of ectopic fat from liver or skeletal muscle would likely be even more beneficial.

What makes me personally skeptical is comparing the fat intake with net fat oxidation, since the former does not really replace the latter. It is better to say that carbohydrates are used more efficiently, i.e., they satisfy energetic needs of the organism better than the fatty foods, and hence the energy shortage that needs to be replenished at the expense of burning the fat stocked in the body is lower, and the weight loss os lower.

This could be modeled by the simplest energy balance equation: $$ \frac{d M(t)}{dt} = \alpha E - \beta M^{3/4}, $$ where $M(t)$ is the body mass measured in the energy units, $E$ is the energy intake (in calories), $\alpha$ is the efficiency of energy extraction, and the last term describes the energy expenditure due to metabolism, according to the Kleiber's law.

The equilibrium solution of this equation is $$ M=\left(\frac{\alpha E}{\beta}\right)^{\frac{4}{3}} $$ Now, if we assume that $\alpha_{fat} < \alpha_{carb}$, the fatty foods should result in lower body mass, provided the same energy intake $E$.

In principle, one could label foods by the effective caloric content relative to that of pure fat, that is by value $\alpha_{food}E/\alpha_{fat}$.


  • since the argument in the comments is about the meaning of calories, let me also point out that calories defined as "the amount of heat needed to raise the temperature of 1 kilogram of water from 0 to 1 degree Celsius" are actually kilo-calories - a rather common misuse of this word.
  • I also agree with the comments pointing out the shortcomings of the study in question: the small sample size and its short duration.
  • $\begingroup$ +1. Thx for your contribution. I especially like the paragraph at the end of your answer; that begins: “In principle...” $\endgroup$ Jul 28 '21 at 3:06

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