Intermittent Fasting, Decoded
What you need to know about this increasingly popular diet plan, including what it is, why it works, and how to do it safely.
Heading out the door? Read this article on the new Outside+ app available now on iOS devices for members! Download the app.
You’ve heard about intermittent fasting for weight loss, blood sugar control, and other health benefits. And it’s not just the latest in a long line of fad diets. Research shows that periodic fasting encourages weight loss (especially reductions in belly fat) and, done the right way, protects against disease, improves overall health, and enhances longevity. When done poorly, however, intermittent fasting can leave you dehydrated, fatigued, and lacking in vital nutrients. Here’s what you need to know, and how to do it safely.
The Basics of Intermittent Fasting
Intermittent fasting is an eating routine that restricts or eliminates daily food consumption to a specific window of time in a very specific pattern. It’s nothing new. Fasting has been used by various cultures throughout history for therapeutic and spiritual purposes. And in primitive times, humans didn’t have 24/7 access to food and were forced to fast, sometimes for days. As a result, our bodies evolved to survive—and thrive—without eating for extended periods of time. So in spite of our obsession with food, we humans can function well, and benefit greatly, from periodic fasting.
Because it naturally restricts calories (unless you binge during your eating periods), intermittent fasting encourages weight loss. Abstaining from food also prompts physiological changes that boost metabolism, making stored body fat more accessible and promoting weight loss—especially in the abdominal area—while retaining lean muscle. Studies show that periodic fasting can often lead to significant decreases in weight and body fat, with less muscle loss than standard calorie-restricted diets, and, in some studies, as much as a 7 percent reduction in waist circumference (belly fat).
But it’s not just for weight loss. Fasting also initiates physiological processes associated with protection from disease and enhanced lifespan. Research shows that fasting impacts the expression of genes associated with longevity and triggers cellular repair processes, including autophagy, in which damaged cells are cleaned out, and new, healthier cells are regenerated. Abstaining from food also inhibits the release of pro-inflammatory cells, lowering inflammation and improving chronic inflammatory diseases.
The result: studies show that intermittent fasting improves metabolic health, lowers the risk of diabetes, enhances cognitive health, and protects against heart disease, cancer, dementia, and other chronic illnesses. And some research suggests that it may even extend lifespan, helping you live longer and better.
How to Do It Right
Because intermittent fasting is less a diet and more an ongoing way of eating, there aren’t a lot of rules. It’s super-flexible, allowing a wide variety of foods without counting calories. It can easily accommodate specific eating plans—vegan, gluten-free, low-carb, low-FODMAP, Keto, or any diet designed for food sensitivities. And the “eat, don’t eat” pattern can be tailored to fit your lifestyle. The most common approach: daily, time-restricted eating, in which you limit food consumption to a set number of hours within a 24-hour period.
For example, with the 16/8 plan, you might finish dinner at 7 p.m. and then have your next meal at 11 a.m. the next day—so you’re consuming food for 8 hours, fasting for 16. But you’ll find plenty of variations on that basic cycle. The Circadian Rhythm diet, designed to align periods of eating with the body’s internal clock, involves fasting from 7 p.m. to 7 a.m. The “every other day” routine involves alternate-day eating, in which you abstain from food every other day, and eat normally on non-fasting days. And other regimens limit, rather than eliminate, food. On the 5:2 plan, for example, you’d restrict food to around 500 calories for two, non-consecutive days of the week, and eat normally the rest of the time.
And that’s the tricky part: eating “normally.” Because there are no restrictions on what or how much you eat, you may be tempted to binge on sugary, processed, high-calorie treats—especially if you haven’t eaten for 16 hours or longer. So, if your intermittent fasting plan looks like starving yourself, then stuffing your face with ice cream and potato chips, you probably won’t lose weight. And worse, you’ll come up short on vital nutrients.
If you do it right, intermittent fasting can be a lifelong regimen. But it’s not for everyone. If you’re prone to bingeing, pregnant, or have an eating disorder or certain other conditions, it may not be right for you. Check with your health care provider before you start.
- To do intermittent fasting safely, follow these simple tips:
- Make the most of your eating periods—boost nutrient intake with clean, unprocessed foods, and steer clear of empty calories.
- Before you start, purge your kitchen of refined, packaged, processed foods and restock with clean, nutrient-dense foods.
- Emphasize vegetables, fruits, and lean proteins.
- If you eat grains, choose only whole, unprocessed versions.
- Include nutritious fats such as nuts, seeds, olives, and coconut oil.
- Stay hydrated. Stock up on sparkling water, green tea, and unsweetened flavored waters to encourage you to drink more.
- Start slowly. Begin by restricting food for 8–10 hours, then gradually increase fasting time by an hour or two a week.