The Truth About Intermittent Fasting

Ranjan Sinha

June 08, 2020

If you've been on social media lately, chances are, you've been exposed to intermittent fasting (IF) "facts," in posts, videos, and even celebrity social media feeds. Park all that information for a minute, and check whether everything you've encountered matches up to these actual facts, backed by science.

Common methods of intermittent fasting

Basically, IF means taking periodic breaks from eating. The common variants include:

Periodic Prolonged Fasting (PF)/ Intermittent calorie restriction (ICR)

This involves fasting for up to 24 hours once or twice a week, with ad libitum (ad lib or impromptu) food intake on the remaining days.

Time-restricted feeding (TRF)

This involves eating only for 8 hours, then fasting for the other 16 hours of the day

Alternate-day fasting (ADF)

As the name suggests, the practitioners alternate between feast (ad lib intake) and fast days (serving 25% or less of energy needs, with some protocols allowing no caloric intake on fast days). In the case of ADF, the amount of intake and fasting is variable depending on the specific protocol being followed.

The effect of intermittent fasting on gut microbiota

Interest in modulators of gut microbial balance is on the rise. Preliminary animal models suggest that IF may be one of these modulators.

The microbiome is believed to modulate adiposity and protect against obesity-associated metabolic dysfunction; and animal studies have shown that intermittent fasting promotes the conversion of white adipose tissue to brown, a beneficial reaction in fighting obesity. In addition, gut microbiota fermentation products such as acetate and lactate were seen to increase during fasting.

These products, together with the upregulation of monocarboxylate transporter 1 expression in beige cells, ameliorate obesity and insulin resistance.

In addition, the length of the daily fasting intervals may influence the outcome, as would the exact protocol being followed. While the variations in intermittent fasting brought about different alterations in the gut microbial compositions, the changes were seen to disappear after cessation of fasting.

In human studies, it was shown that water-only fasting also changed the gut microbial composition, making it more homogenous, and some effects persisted after food was reintroduced.

A recent study reported that a brain-gut pathway activated in the brain during fasting enhances gut epithelial integrity and promotes better energy balance.

Intermittent fasting can have caveats if not done right

Studies have shown intermittent fasting to be effective for short-term weight loss, but limited data is available regarding the consistency, tolerability, and safety of ADF among the general population when used as an intervention for weight loss. In addition, it is not established how intermittent fasting will affect older adults.

Experts at Harvard Medical School also speculate as to the dangers posed by IF to people who take heart or blood pressure medications, as they are more likely to experience imbalance in potassium and sodium when they are fasting, which could be dangerous. IF may also be best avoided if you have diabetes, must eat at specific times, or take medication to modulate your blood glucose levels.

More human trials on intermittent fasting with follow-up periods designed for the long-term are needed and the data they yield will be valuable to understanding mechanisms of weight loss maintenance, as well as the long-term impact of IF on our metabolic pathways. Future studies might also include specific subgroups such as individuals with cardiovascular risk factors and type 2 diabetes mellitus as these patient populations stand to benefit more from weight loss-driven modification of the disease process.

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