Whether you follow 18:6 intermittent fasting or buckle down for a full 48-hour fast, studies show fasting can increase stress resistance and longevity, and decrease your risk of chronic disease—including cancer and obesity (1). Exactly why longevity doctor, Peter Attia, M.D., once fully subscribed to the practice.
In his fasting era, Attia was known to do a seven-day water-only fast once a quarter, and a three-day water-only fast once a month. He was even featured in National Geographic’s docuseries, Limitless, where he coached Chris Hemsworth through a four-day fast in the name of a longer lifespan and better healthspan.
But in a recent interview with celebrity trainer and nutrition influencer Thomas DeLauer, Attia shared that he’s changed his stance on fasting.
“While there are clearly some benefits of [fasting], it’s very difficult to measure what’s happening cellularly,” says Attia. Those benefits also come with a huge downside, decreasing muscle mass, which is why Attia hasn’t done a multi-day fast since 2020. “Today, I just don’t feel that that trade-off is worthwhile, at least at the extreme level that I was doing.”
Fasting Can Reduce Muscle Mass
While religiously participating in monthly and quarterly multi-day fasts, Attia was losing muscle mass. “Over a period of about three years, I probably lost about ten pounds of lean mass,” he said.
One review concluded that a major downside of fasting is the accompanying loss of muscle (2). If there is a way to make up for this loss, Attia hasn’t found it. “As much as you might exercise during those periods of fasting, which I tried to, you’re just not going to be able to maintain lean mass,” he said.
Holding onto muscle is essential as you age. Even if you don’t fast, starting at age 30, you can lose as much as three to eight percent of your muscle mass per decade (3). Muscle loss slows down your metabolism, and decreases your strength and functional ability to complete daily tasks with ease.
Should You Fast?
“One not uncommon scenario that we see with [fasting] is that a person loses weight on the scale, but their body composition alters for worse: they lose lean mass (muscle) while their body fat stays the same or even increases,” Attia wrote in his book Outlive.
That’s why Attia believes fasting is a “tool” that should only be used for the ideal candidate. That person is someone who either has a good deal of muscle to spare or someone who has over 35 percent body fat and can handle losing a little muscle because they have so much fat to lose.
How to Fast Without Losing Muscle
If you want to snag the longevity benefits of fasting without losing precious muscle, Attia told Delauer an occasional 20 to 24-hour fast is the sweet spot.
Why? Fasting for longer periods comes at a price: “You are virtually guaranteed to miss your protein intake with this approach,” Attia wrote in Outlive. Alternatively, “sixteen hours without food simply isn’t long enough to activate autophagy, inhibit chronic mTOR elevation (mTOR is a gene that regulates protein production and helps you build muscle, but too much is linked to diseases like diabetes and cancer (4, 5), or engage any other longer-term benefits of fasting we would want to obtain,” Attia said.
Sandwich your fast with dialed-in protein intake. “My rule of thumb for any eating pattern is that you must eat enough to maintain lean mass (muscle) and long-term activity patterns,” he says. Protein needs vary from person to person, but if you’re in a caloric deficit you’ll want to shoot for one gram of protein per pound of body weight per day or more to maintain muscle (6).