When Hugh Jackman was training for his role as Wolverine, the actor had to consume 6,000 calories a day to fuel his grueling body-building regimen.

You might imagine that Jackman ate constantly, from breakfast to bedtime, to take in that many calories and build such massive muscles. But surprisingly, he fasted for 16 hours a day, and consumed all of his food within an eight-hour window.

And the guy got seriously ripped.

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Jackman was practicing a form of intermittent fasting called time-restricted eating, or TRE. Unlike forms of intermittent fasting that prohibit food on certain days (one day a week, alternate days, etc.) TRE allows you to eat every day, restricts nothing — not even calories — and requires only that you eat your food within a certain window of time.

Most people choose a TRE window between eight and 12 hours; the shorter, the better, as health benefits increase dramatically with every hour the eating window is condensed. Regardless of the option chosen, all eating and drinking (except water) should stop by 8 p.m. at the latest — preferably earlier.

You may cringe at having to give up your late-night snacks or evening pinot, but most people adjust quickly and experience welcome benefits, including weight loss, better sleep, improved digestive woes, better blood sugar, decreased hunger, and — a plus for Tahoe athletes — a boost in physical endurance.

Clearly, our bodies thrive on TRE, which begs the question: If food gives us energy, why do our bodies love it when we don’t eat all the time? It’s because we’re not designed to eat all the time.

Humans evolved over millennia to be active — hunting, gathering, eating — during the day, and asleep at night. As a result, our built-in rhythms are circadian, driven by a master clock in our brain and clock genes in every organ that turn certain biological functions, like digestion, on during the day when we need them, and off at night, so organs can go into repair mode.

We may be 21st-century beings, but our biology — down to the deepest cellular level — is still very much that of a hunter-gatherer, and functions best when we eat on a schedule similar to that of our ancestors.

When we chow down for most of our waking hours, especially toward bedtime, our digestion is less efficient. In the evening, clock genes tell our gut, liver, pancreas, and kidneys to ratchet down activity. Insulin production slows to a trickle, which causes blood sugar to stay elevated. Gut motility slows, so food takes longer to digest. The stomach produces more acid in the evening, and saliva production — normally an acid neutralizer — drops 10-fold at night. The result? Acid reflux.

Also, the longer and more continuously we eat, the more time we spend in fat-making mode, and the less time in fat-burning mode, with obvious consequences when we try to button our pants.

TRE, however, aligns our innate rhythms. In human studies, when people changed nothing about their diets except to practice TRE, they lost weight, slept better, had more energy, and were less hungry during the day — the result of balanced hunger and satiety hormones.

In another study, when mice were given high-fat, high-sugar food and an identical number of calories, those on an eight-hour TRE lost weight, had better blood sugar and cholesterol, less inflammation, a healthier liver, and better motor control than mice who self-fed at any time.

These benefits were due to alignment of clock gene expression in nutrient and metabolic pathways. Put simply, TRE makes you run like a well-oiled machine. TRE also allows our bodies adequate time in vital repair mode. The very act of eating and metabolizing creates oxidative stress and cell damage throughout the body. Every night, up to 14% of our gut lining is replaced, hundreds of miles of blood vessels are repaired, and cells in our skin, blood, and in every organ die and need replacing. These repair processes are enhanced when your organs aren’t still trying to digest food.

Autophagy — the critical recycling of cell parts damaged by oxidation — is significantly boosted the longer we fast.

Repair and autophagy mechanisms keep inflammation and cell damage in check, which lowers the chance of cancerous mutations. In a study of women with breast cancer, those who practiced an 11-hour TRE had a stunning 36% reduction in breast cancer recurrence.

If you’re worried that you’ll wither away on TRE, research shows that muscle growth and repair are actually boosted. Mice on TRE gained 10 to 15% muscle mass and loss only fat. Human studies are needed, but anecdotal evidence shows similar results in people. If you saw the last Wolverine movie, Hugh Jackman is a testament to that.

Tips for Time-Restricted Eating

• Start with a 12-hour window and gradually go shorter. The best results come from an eight- or nine-hour TRE. Anything less than eight hours is hard to maintain and tends to interfere with social and family life.
• If you’re on medication, keep taking it as prescribed. Consult your doctor before making any changes.
• Toothpaste doesn’t break the fast, as long as you don’t swallow it.
• TRE accommodates keto and other diets as long as all food is eaten within the desired window.
• Most fat burning occurs six to eight hours after finishing your last meal, and increases dramatically after 12 hours of fasting. So any TRE under 12 hours promotes weight loss. After you reach your target weight, you can lengthen your TRE.

What about coffee and tea?

The effects of black coffee, black tea, and herbal tea on fasting are largely unknown, but any caloric additive — milk, soy milk, sweetener, etc. — triggers insulin and definitely breaks the fast. Caffeine obviously affects the wake/sleep cycle, and coffee and tea contain biologically active compounds that may, or may not, affect various aspects of fasting. More research is needed.

Join a research study

If you’d like to be part of ongoing crowd-sourced TRE research at the Salk Institute, go to mycircadianclock.org to sign up. The app tracks your food, and the input from people worldwide has contributed greatly to our knowledge of fasting and circadian biology.