Intermittent Fasting: Why is exactly *zero* calories special?

Question

Is there something special about exactly 0 calories vs 1 calorie?

When researching Intermittent Fasting, the "magic" seems to come from the body's response to insulin dropping and its response to secrete glucagon, causing glyconeolysis, catabolism, gluconeogenesis, and lipolysis - the latter three becomes dominant once the liver runs out of glycogen.

One thing that's mentioned several times by online articles and books (such as The Obesity Code) is that "reduced calorie" diets do not cause the same effects. However, they rarely go into what effects specifically they're talking about, or how reduced is "reduced."

If you are fasting at 0 calories, does the addition of one calorie somehow cause the glycogen to stop causing the lipolysis, gluconeogenesis, and catabolism? What about 100 calories? 500 calories?

What happens in the transitional region between 0 calories ("fasting") and 1500 calories ("reduced calories")? If you're fasting for 16 hours, eat 100 calories, then fast 16 more, how significantly different is that from fasting 32 hours?

Answer 1

In summary, intermittent fasting could help decrease appetite and thus make weight loss easier for some people, but there is no convincing evidence that it would stimulate catabolism more than other methods of weight loss.

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If you're fasting for 16 hours, eat 100 calories, then fast 16 more, how significantly different is that from fasting 32 hours?

If you burn, let's say, 2,400 Calories/day, that is 100 Calories/hour, you can see that consuming 100 Calories will pause the fasting metabolic processes only for about an hour and then you will be again in the fasting mode. If at least a part of those 100 calories come from carbohydrates, they can prevent you to go into ketosis, which could otherwise occur toward the end of your second part of fast - you can see this answer for explanation. But again, ketosis as such is not associated with increased catabolism.

Is there something special about exactly 0 calories vs 1 calorie?

No, consuming 1 vs 0 Calories will make only the effect that 1 Calorie can make on your metabolism, that is providing energy for about half of a minute and then continuing as if on zero calorie diet.

According to Do Intermittent Diets Provide Physiological Benefits Over Continuous Diets for Weight Loss? A Systematic Review of Clinical Trials (Molecular and Cellular Endocrinology, 2015), intermittent fasting can reduce appetite and thus help in weight loss, but not necessary better than other diets:

While intermittent fasting appears to produce similar effects to continuous energy restriction to reduce body weight, fat mass, fat-free mass and improve glucose homeostasis, and may reduce appetite, it does not appear to attenuate other adaptive responses to energy restriction or improve weight loss efficiency.

Intermittent fasting thus represents a valid--albeit apparently not superior--option to continuous energy restriction for weight loss.