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Fasting: Not Just A New Fad


Fasting: Not Just A New Fad

August 26, 2021

There’s been a lot of talk about fasting lately, and it seems like a new fad. Yet fasting is anything but new. Humans have been doing it for thousands of years, as part of religious ceremony, and even as part of basic existence. Our caveman ancestors were not likely to have had abundant food sources, particularly in winter months, which meant they probably went for days without eating on a regular basis. And think about this—the word “breakfast” literally means “to break a fast.” We are fasting every night while we sleep.

There is a great deal of research on the benefits of fasting, including decreasing the risk of diabetes and obesity, helping prevent certain cancers and autoimmunity, and prolonging the lifespan. And indeed, the old “wisdom” that we need to eat breakfast to jump-start our metabolism in the morning is turning out to be untrue, as is the notion that people who graze all day will burn more calories. Constant eating and snacking throughout the day is more likely to create insulin resistance which leads to diabetes and obesity. This is because when you eat all day, your insulin stays high, and insulin actually stops the body from burning fat. So if you don’t give your body a long break between meals, the body does not have time to burn fat, and instead continues to store it.

How long do you need to fast to see improvements in blood sugar and weight loss?

Intermittent fasting, where you only eat for a certain number of hours every day (6-8) and fast the rest (16-18), can lower blood sugar and insulin and help reverse diabetes. Fasting in the evening is more powerful than morning fasting, as the insulin response to meals in the evening is usually higher. But you don’t need to fast every single day to see benefits, nor do you have to do the same thing every day. You will still get benefits if you skip breakfast one day, skip dinner the next, and then have a day where you only eat 3 meals and no snacks. Some studies showed decreases in insulin and blood pressure with only 12 hours of fasting. In order to achieve weight loss and reverse diabetes, however, you need to fast at least 16 hours on a consistent basis. And the best benefits are seen when you drink only water for days at a time.


If you fast for more than four days, you get the added benefit of something called “autophagy,” which was described in 2016 by Yoshinori Ohsumi, who won the Nobel Prize in Medicine that year. Autophagy is the phenomenon by which the body breaks down old worn-out cells for energy, thus removing damaged and dysfunctional cells from your body. When you start eating again, you regenerate new and healthier cells. Cell regeneration is especially benefician for the immune system and indeed may help prevent autoimmunity and some cancers.

Will fasting slow my metabolism?

Many worry that days of fasting will lead to a slower metabolism and muscle loss, but studies show clearly that this does not happen. Water-only fasting for days can be challenging, and for some people it can be dangerous- leading to dehydration and low blood pressure.

Luckily there are easier ways to “fast” for several days. Dr. Valter Longo, a pioneer in research on the benefits of fasting, developed a “fasting mimicking diet” (FMD), where you eat a limited amount of very specific nutrients over the course of 5 days. This diet has the same benefits of fasting (including autophagy) without the feeling of starvation. In his book, The Longevity Diet, Dr. Longo describes how a FMD can help starve cancers, improve the effectiveness of chemotherapy, and promote healthy aging.

The longevity diet

Dr. Longo recommends that healthy people do this type of fast several times a year, and those with diabetes or obesity can benefit from doing it more often. Talk with your provider at AIM if you have questions about this fasting mimicking option to optimize your health.

Aside: What about Ketogenic diets??

Ketogenic diets, which emphasize large amounts of fat and limited carbohydrates, can also mimic fasting to some extent. These diets cause the body (and especially the brain) to get used to using a different fuel source (ketones instead of glucose), which can lead to resilience and helps prevent seizures in those with epilepsy. These keto diets do NOT give the benefit of autophagy, however, and long-term use of the diets can lead to constipation, gallstones, kidney stones, high cholesterol, and increased risk of heart disease. Keto diets are also detrimental for the microbiome, which relies on high fiber to feed the good bacteria. So, keto diets are best reserved for short-term use or specific health concerns such as epilepsy.

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