In nutrition, as in climbing, diet trends and buzzwords run amok. One of those terms is “intermittent fasting” (IF), aka “time-restricted eating”: the practice of limiting your caloric consumption to a particular time window. Typically, the process involves eating within certain hours—for example, between 10 a.m. and 6 p.m. Evidence has shown IF’s potential benefits in sleep, weight management, and generating a faster metabolism, as well as better liver health and a reduction in inflammatory and metabolic disorders. It is not, however, for everyone, especially those at risk of disordered eating.

Among climbers, IF has become an increasingly common way to control and/or lose weight. Numerous friends, as well as some clients, have dabbled with their own versions to hit “sending weight.” But does IF work? The anecdotal accounts I was hearing varied, running the gamut from weight loss, perceived improved recovery, and perceived blood-sugar regulation, to overeating, extreme energy crashes, brain fog, hormone imbalances, and increased stress injuries.

As a nutritionist, I know that there really is no one-size-fits-all with nutrition, so these results weren’t unusual. But there are fundamental protocols to any health plan, and I wondered if the climbers out there following IF had done so correctly.

Upon further discussion with 11 climbers trying IF, I discovered that those who’d experienced positive effects seemed to have a good grasp on an appropriate window of feeding, with a clear vision of their goals. However, most folks experiencing negative effects were engaging in “extreme fasting,” with only a foggy understanding of their goals.

Trying IF Safely

Time-restricted eating (TRE) is the safest and most sustainable method, and is the one we’ll discuss here.

1. TRE is not about caloric restriction; it’s about limiting your caloric intake to a certain time window (8–10 hours). You should still eat your daily allotment of approx. 2,000 calories. (The total will be bio-individual and depend on activity level; for protein, eat roughly 0.75–1 gram of protein per pound of body weight daily.) Overall, be consistent in your timeframe and find a sustainable way to work TRE into your work, social, and climbing life.
2. Abstain from caloric consumption within the first 60 minutes after waking and at least two to three hours prior to bed. This helps prolong the sleep-related fast, since it generally takes about five hours post-meal for the fasting process to begin. (Tea and coffee without cream/milk or sweetener will not break your fast.)
3. Balance your macronutrients throughout your feeding window to roughly follow the 40/30/30 model, in which your calories are comprised of real, whole foods in the ratio of 40 percent (complex) carbohydrates, 30 percent fat, and 30 percent protein. This model provides the most nutrient variety and density.
4. Have clear goals, and track them: Are you trying to lose fat? Maintain weight? Improve sleep? Increase performance? Decrease inflammatory markers? Improve metabolic issues?

Determine Your Ideal Window

Finding the “correct” IF window varies by person, with more than one way to structure it.

• Time-restricted eating (TRE): Numerous studies have shown that the best method for athletes is eating your normal caloric intake within an eight- to 10-hour window—e.g., between 10 a.m. and 6 p.m.—and then fasting. TRE has also been shown to be the most sustainable both for long-term improved metabolic health and over shorter interims. (For more, listen to the Huberman Lab podcast, episode 41.)

• The 5:2 method: Here, you eat your metabolically appropriate caloric intake with no time constraints five days a week, and then on the other two, nonconsecutive days consume 25 percent of your normal calories.

• Alternate-day fasting (ADF): This involves completely fasting (no food) every other day, or taking a modified, somewhat gentler approach in which you eat approximately 500 to 800 calories, or around 25 to 35 percent of your normal energy requirements, on fasting days.

For the 5:2 method and ADF, it’s important to take a slow approach and eat reasonable portions of a wide range of foods, without overindulging on non-fasting days. Both methods have shown benefits in reduced insulin resistance, improvement in blood lipids (the fatty substances found in the blood, including cholesterol and triglycerides), and decreased inflammation. However, they may not be suitable for people with metabolic or eating-disorder issues.

The Benefits of IF

Done correctly, IF can have numerous health benefits, perhaps chief among them giving our digestive system a break. It’s also been shown to improve liver health and fat metabolism, reduce inflammation, increase longevity, and positively affect circadian rhythms—potentially allowing us to sleep better, recover better, and better handle the stresses of climbing.

Liver Health

When we eat, a period during which we continue to digest, absorb, and go through gastric emptying follows. When we eat continuously over a prolonged window (like 14–16 hours) or in an unregulated manner over 24 hours, cellular processes involved in digestion may start to express certain pro-inflammatory markers associated with glucose intolerance, obesity, hypertension, and myriad liver issues.

The liver is involved in the regulation of sex hormones, thyroid hormones, cortisol and other adrenal hormones, and metabolism; when we have disease or toxicity of the liver, we can experience dysfunction with any of the above. However, there is evidence that IF can turn this around. A September 2021 study in Oxford Academic found that IF may either help reverse the damage done to the liver from unregulated eating or instigate healthier liver conditions.

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Fat Metabolism

Insulin, created by the pancreas, allows our cells to use glucose for energy. During a fasting state, however, our insulin levels drop; the body thus has to make its own sugar, via the liver turning stored glucose into glycogen. When glucose is depleted, the liver will use amino acids, waste products, and fat byproducts in a process called gluconeogenesis. The glucose it produces is prioritized for the brain, while the organs and muscles rely on ketones—chemicals your liver produces when it breaks down fats. The cascading effect of reduced insulin levels can thus improve fat metabolism and promote weight loss.

Inflammation & Longevity

The low levels of insulin fostered by IF can also help maintain heathy cells. While the digestive system takes a break, the body can focus on cellular repair. Every day, we undergo the cellular processes of division, repair, and remodeling. But, as we get older, we decline in our ability to undergo cell division. This is called cellular senescence, aka cell death—in simple terms, it’s aging. Cellular senescence has been connected to cancer, diabetes, and myriad age-related conditions, including osteoporosis, cardiovascular disease, stroke, Alzheimer’s and related dementias, and osteoarthritis. A lot of these are diseases of inflammation.

However, IF is believed to promote autophagy—basically, the cellular clean-up of junk materials in order to maintain energy homeostasis—that mitigates cellular senescence. Autophagy typically happens while we sleep, since we’re in a fasting state, but it can occur any time—as during IF, which extends our sleep-related fast. A 2019 review from Duke University showed that an increase in autophagy promotes both healthspan—the span of years in which we are in good health—and lifespan. And the longer we’re healthy, the more we get to climb!

Circadian Rhythms

Circadian rhythms manage several biological functions, from our sleep-wake cycle to metabolism and appetite. Sunlight is a driving force, but food plays a strong role as well. Following set meal times, as in IF, helps reinforce your circadian rhythms.

In a 2018 study, participants were found to improve sleep quality on several levels after a week of IF—their sleep was more restful, they moved less, and they were less likely to wake during the night. As a result, they spent more time in REM sleep, a stage responsible for mental processing and recovery. Additionally, they were found to have higher levels of human growth hormone, which aids in muscle restoration, cellular repair, and fat burning. Better sleep also allows for more energy and focus—and thus better performance on the rock.

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Caveats with IF

Two friends who had negative experiences with IF were eating within an eight-hour window five days a week and abstaining from all food for two 24-hour periods, essentially combining TRE with the 5:2 method. They’d shed what little fat they had and were performing well during a monthlong visit to Bishop, where I live. However, they also suffered constant hunger, aches and pains, poor sleep, and climbing burnout. They were also regularly bumping into post-workout hypercaloric compensation, in which extreme hunger drove them to stuff their faces, exceeding their caloric needs. A habit like this, especially if prolonged, will promote weight gain and systemic inflammation.

Fortunately, back home, my friends resumed normal eating. Had they continued, they’d have run into escalating problems, such as a slowed resting metabolic rate (RMR). RMR is the amount of energy needed to power key functions like heartbeat, respiration, brain activity, etc., and requires about 60–70 percent of our caloric intake. When we deprive ourselves via unregulated fasting, our RMR slows and we signal the body that it’s in a survival state; it then holds onto calories in response to the stress.

Additionally, extreme fasting can lead to hormonal imbalance: The depletion alters our cortisol levels, which can lead to anxiety, insomnia, and sugar cravings, and can negatively impact thyroid hormones, growth hormone levels, and the production of gonadotropin-releasing hormone (GnRH). These imbalances in turn cause symptoms like depression, anxiety, weight gain, thinning hair, dry skin, and irregular or absent periods. IF can also, if not done carefully, lead to disordered eating. Hence those in recovery are advised to avoid IF—the very topic can be triggering.

If you try IF, have a dietitian or nutritionist help structure a protocol. If IF works well for you, and you have energy to climb and train, wonderful. If it doesn’t, and you’re strung out and hungry all the time, ditch it. Again, IF is not for everyone—it’s simply one approach among many.

Katie Lambert is a pro rock climber based in Bishop, California. In 2016, she received her master’s in nutrition. She helps clients use food as fuel and medicine through her business High Sierra Nutritional Wellness.