Clarifying some misconceptions about short-term fasting

16 07 2010

Fasting is to most Dietitians as garlic is to vampires. And breakfast is a must if you want to lose fat (or be healthy). Omitting breakfast is supposed to make your metabolism sluggish and contribute to fat gain because:

a. It implies a fasting period, which reduces your metabolic rate.


b.  When you dont eat breakfast you are reducing the number of meals of the day and an increased meal frequency makes you burn more calories.

It has been repeatedly shown that the latter is a fallacy and that energy expenditure related to food intake is determined by overall calorie intake, not by the number of meals. You can read more about this here. But the myth of fasting = reduced metabolism = weight/fat gain is the one that bothers me the most.

Metabolism during short-term fasting

Because in real life most IFers (those who use an Intermittent Fasting approach) use fast periods between 12 and 36 hours, I will consider short-term fasting as a fasting period that lasts a maximum of 36 hours.

The “reduced metabolic rate” fallacy comes from the fact that during prolonged fasting metabolic rate is reduced in order to survive, which seems to be related to the degree of body fat depletion (less fat, less thermogenesis). But the same doesnt apply for shorter periods of fasting, as in IF or Alternate Day Fasting (ADF) protocols.

Surprisingly for many, quite the opposite happens during short fasts: energy expenditure is increased. For instance, one study showed a significant increase in Resting Energy Expenditure (REE), associated with an increased serum concentration of norepinephrine, an increased rate of lipolysis and ketogenesis, and a shift from glucose oxidation towards fatty acid oxidation (measured by RQ), during the first 48 hours of fasting. Others have shown a rise in REE after 72 hours of fasting, peaking at the 36h mark. In these subjects, both plasma adrenaline and noradrenaline levels increased during the study. In addition, adrenoceptor sensitivity seems to be increased during this period, so adrenergic-stimulated lipolysis is potentiated.

There seems to be a threshold around the 72h mark, where REE starts to fall. But most people fast 36 hours or less.

But isnt fasting going to eat my muscles?

The rise in plasma beta-hydroxybutyrate (bOHB) concentration during fasting has anti-proteolytic properties. There is also a lowering of free T3 which reduces muscle proteolysis. Accordingly, 40h of fasting has failed to show any marked increase in atrogenic genes (myostatin, atrogin-1, MuRF-1).

A very nice summary of short term fasting metabolism is done by MacDonald and Webber:

As stated in their paper:

The primary metabolic responses to fasting-starvation are summarized in Fig. 1 , and comprise a fall in peripheral (i.e. non-neuronal tissue) glucose utilization, an increase in fatty acid and ketone body utilization and decrease in proteolysis. These changes are mediated in part by reductions in plasma insulin and free triiodothyronine (T3) concentrations and by increases in plasma glucagon and adrenaline.
There is also evidence that 20h of fasting elicits a marked activation and an increase in mRNA content of the PDK4, LPL, UCP3 and CPTI genes in skeletal muscle, changes that are associated with an increased use of free fatty acids (UCP3, LPL, CPTI) and glucose sparing (PDK4). Interestingly, changes in these markers were affected differently by the type of meal eaten after the fast: transcription of PDK4 and LPL were further increased 1h after the ingestion of both a high carb (HC) or a high fat meal (HF) and transcription of CPTI remained elevated, while UCP3 remained elevated only after HF.

In Summary

During a short fasting period your body tries to spare glucose reducing the peripheral tissue glucose uptake and oxidation, with a concomitant rise in lipolyisis and ketogenesis. This means that your body uses less glucose and more free fatty acids* for energy. Because of the short duration of the fast, muscle protein is barely affected. On top of it, your energy expenditure goes up and your adrenoceptor sensitivity is through the roof, so any stimulant will work much better.

So next time someone ask you which is the best fat loss breakfast, you can be sure to answer “a big hot cup of strong coffee”.

* Although there is a degree of re-esterification occurring.