Dangers of Intermittent Fasting – and Four Possible Solutions


by Nils Osmar. February 15, 2022

Fasting (including daily intermittent fasting, also known as time-restricted eating) has many. benefits. But it can also have drawbacks, and sometimes move people into a danger zone without their being aware of it.

This 2020 article describes a study in which the participants doing 16:8 intermittent fasting lost significant muscle over the duration of the study.

Loss of muscle isn’t just cosmetic; it can cause serious problems. Poor muscle tone can have a major impace on our mobility, increasing the risk of falls (the leading cause of injury-related death in the elderly) and undermining our metabolic health.

Other problems related to fasting can include building up nutrient deficiencies, including deficiencies in micronutrients such as vitamins and minerals, over time. The effect of these may be hidden initially but become apparent as years go by.

Let’s look at the study more closely:

From the Article:

“… a new randomized clinical trial on the 16:8 intermittent fasting approach raises questions about its safety and effectiveness.

‘The researchers were surprised to find that weight loss was moderate, at best, and was accompanied by a larger than expected loss of lean mass, which includes muscles. “Participants who engaged in intermittent fasting only lost an average of 2 to 3.5 pounds, just slightly more than the control group. They also didn’t display any significant improvements in fat mass, fasting insulin, blood sugar or blood lipids.

Participants in the fasting groups lost 65% of their lean mass instead of the 20% to 30% normally expected with weight loss.”

Option 1: Do More Resistance Training

One possible approach would be to stop doing time restricted eating and go back to eating three meals a day. This might be the right solution for some people. But for those who want to continue fasting, there are some other options.

In this short video clip, Peter Attia describes a patient of his who had a similar problem: the patient went from 18 percent to 30 percent body fat over a year and a half of doing a similar kind of IF.

  • His weight hadn’t changed but he’d lost 12 pounds of muscle and gained the same amount of fat. He was the same weight but his health had gone downhill. A dexa scan revealed what was really going on inside his body.
  • Attia mentions that one way to prevent this is to do regular resistance training. If we’re not doing so, we may be damaging our health by fasting.
  • Attia does resistance training, for example, during his monthly three-day fasts. He always loses some muscle mass during these fasts but has found that he can minimize it by doing weight training.

Option 2: Cut Back on Fasting

Another approach would be to fast (or do time restricted eating) every other day instead of every day. Eat as much as you want on day 1, and do some fasting or time restricted eating on day 2.

Option 3: Doing Fasting Mimicking Diets instead of Fasting

FMDs were developed partly as a way of preventing the loss of macronutrients that can occur during fasting. If you’re in danger of losing muscle mass, doing a ProLon-style FMD that provides a lot of nutrients (but restricts some specific ones) is an alternative to be aware of. More information

Option 4: Increase Your Nutrient Intake (Including Protein and Leucine)

A third possible approach would be to significantly increase your nutrient intake, including your intake of protein, on your non-fasting days (or during your eating window if you do daily IF).

Both the study and Attia’s video, are a reminder of the importance of making sure that our protein is high enough when we’re not fasting to compensate for the loss of nutrients that occurs during fasting, particularly if we’re doing a lot of fasting.

Some people are lowering their protein, or avoiding proteins with leucine (the amino acid that triggers muscle growth) in order to activate AMPK (the longevity pathway). This could turn out to be a particular problem in the anti-aging community, where there’s a tendency to want to push ourselves deeper and deeper into hormesis by combining things like fasting every day; eating low calorie; and eating low protein. Each of these has been shown to have benefits, so why not do them all at once? But there could be a danger. mTOR activation is as important as AMPK activation. We need both to have a shot at achieving both optimal health and optimal longevity.

What I’m Doing

  • I do a lot of fasting. I’ve written about its possible health benefits elsewhere on this website.
  • I do resistance training. (I’m currently doing heavy workouts twice a week.)
  • I eat a high nutrient diet when I’m not fasting, particularly on my workout days (after my workout). I used to try to push AMPK activation all of the time; I don’t anymore. I push mTOR activation to fuel muscle growth on my workout days. On these days in particular, I try to “eat my height in centimeters” in high quality, animal-based proteins. (If you’re vegan, just choose plant based sources.) I’m 180 centimeters, so I aim to eat 180 grams of protein from sources like fish, meat or whey on these days.
  • Am I in danger of overactivating mTOR and underactivating AMPK? Some might say so, but at my age (69) the greater danger is probably underactivating mTOR. And remember, I’m fasting two or three days a week. On those days I eat very little protein (under 16 grams) and very low carbs (under 20 grams). So it’s key, in my estimation, to make sure I’m getting enough protein and other nutrients on my feasting days.

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