Is fasting safe?

By Dr. Stephen Phinney and the Virta Team

In general, there are no major concerns with either time-restricted feeding or intermittent fasting for durations less than 24 hrs as long as:

  • There are adequate protein and vegetable intakes within the ‘feeding window’ to support lean tissue preservation (where the vegetables are a source of potassium and magnesium)
  • The person is not on fixed doses of diabetic medications such as insulin or sulfonylureas that could cause hypoglycemia on the feeding-restricted day

The goal of achieving metabolic health and improved body weight depends upon optimizing fat oxidation and preserving lean tissue, and in our experience the best first step is keeping protein intake moderate and dietary carbohydrates low. In addition to that, if one can intermittently skip one or two meals per day without stimulating hunger and cravings, this may benefit some individuals, but time-restricted feeding or intermittent fasting are not required for success at achieving weight loss or metabolic health.

However, any regimen involving fasting beyond 24-hours has not been proven effective in sustaining weight loss long term. Even though fasting can temporarily speed up weight loss, the price of this is lean tissue loss. After just one day of fasting, you begin to lose body protein from lean tissue – including from muscle, the heart, the liver, and the kidneys. We do not recommend fasting for longer than 24 hours.

Read more about the risks of fasting.

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  1. Avatar

    Dr Phinney’s video on fasing claims that “lean muscle loss” (as a result of sustained fasting) has substantial scientific proof. Since Virta’s reputation for questioning long-standing medical “proof”, I’d like to see links to those studies that he claims are there. Saying that there is no “proof” that fasting helps long term weight loss – ignores the fact that there is also no economic incentive for those kinds of studies. But if knows of studies that dispute the “fact” he should also share that information. You can’t pick and choose which medical “misinformation” you choose to oppose.


    1. Virta Health

      This post may provide the details that you are looking for:


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