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Time-Restricted Eating: Is It When, Not What, You Eat That Really Matters?


Time-restricted eating - Dr. Axe
When you hear the word diet, you probably think of something that’s hard and confusing to follow, leaves you feeling deprived, and involves a list of foods to avoid, right?

Well, what if there’s another way to lose weight fast that’s dramatically different from most standard “diet” plans? And what if this alternative essentially eliminates the need for calorie-counting, sacrifice, deprivation and even willpower?

What Is Time-Restricted Eating?

Time-restricted eating (TRE) is a type of intermittent fasting, which in one way or another has been done by humans across the world for thousands of years. While most initial studies on TRE have been done using animals, humans are believed to likely react the same way. It makes sense to researchers, considering daily fasting was likely something that was practiced unintentionally by our ancestors who didn’t have 24/7 access to food like we do today.

And without even knowing about the scientifically proven health benefits of fasting, nearly every religious group throughout history has practiced some sort of variation of fasting rituals.

According to researchers at the Regulatory Biology Laboratory at the Salk Institute for Biological Studies, when we eat may be as important – or even more important – than what we eat. The Salk Institute in California has been at the forefront of the time-restricted eating phenomenon, working to understand the impacts of fasting for health and how the body reacts when its forced to fast for the majority of the day.

Researchers first stumbled upon this breakthrough in animal studies when mice were allowed to eat whatever they wanted, but only during a set time of day. The mice on the time-restricted eating plan ate what was considered a “poor diet” high in calories, sugar and fat, yet they still didn’t gain the weight that they were expected to.

However, once they had access to the same food any time they wanted, the mice’s weight gain doubled despite eating the same number of calories.

  • 9-hour of access to food caused 26 percent weight gain in the mice
  • 15-hour access to food caused 43 percent weight gain
  • 24-hour access to food caused 65 percent weight gain

Their stunning conclusion was that periods of regularly fasting for 12–16 hours a day might dramatically impact body weight. Time-restricted feeding caused less weight gain than all-hour access for mice eating a high-fat, high-sugar diet over 12 to 26 weeks, and it also led to weight loss of up to 12 percent when applied to mice that were already obese.

What does this mean for the dieting industry? You might be able to eat whatever you want and still lose weight, simply by limiting the period of time in which you consume food.

And this might even be true if you increase calories — especially calories from fat, because your body seems to burn these best during the remainder of the day, your “fasting” period.

Guide to time-restricted eating - Dr. Axe

How It Works

As you can see, time-restricted eating is truly different from standard diet approaches, which usually fall into one of two categories: either calorie-restricted diets or food-restrictive diets. Time-restricted eating, on the other hand, simply allows you to choose the foods that work best for you and eat them in any eight- or nine-hour window that you choose.

Fasting for roughly 15 or 16 hours a day — possibly even as little as 12 hours — while keeping the times that you eat to a shorter period appears to have significant effects on hormone levels that determine your metabolism, blood sugar, and whether or not you burn fat.

The idea of eating as much as you want — and of any foods that you want — plus that calories don’t actually matter in the long run for weight maintenance goes against pretty much everything we’ve ever been told about losing and gaining weight. But yes, this is what results from several clinical trials using animals suggest.

These recent eye-opening studies show that by only eating during a shortened eight- to nine-hour window each day, your body is more likely to burn fat and keep your weight at a healthy level. And this even seems to be the case without the need to cut calories, avoid entire food groups, or count macronutrients like carbs and fat. How is this possible?

It appears that our bodily functions operate best when they act something like clockwork — preferring to schedule our repair, maintenance and “system backup” during planned downtimes. This means that when the body follows a predictable schedule of eating and fasting, our hormones might respond by producing fat burn and weight loss — potentially even rapidly.

Maybe you’re convinced that TRE can work for weight loss, but you wonder if fasting is healthy.

And aside from manageable weight loss, other benefits of fasting and practicing time-restricted eating might include:

  • lower levels of inflammation
  • better management over blood sugar levels and a lower risk for developing diabetes
  • enhanced detoxification
  • better control over appetite hormones, including leptin and ghrelin
  • improved heart health
  • better immunity
  • lower risk of cancer
  • improved brain function and lower risk for dementia
  • better muscle recovery from workouts
  • and less harmful effects from aging or stress

A study conducted by the University of California San Diego involving 2,200 overweight women found that time-restricted eating also has positive effects on immunity and blood sugar control, which are closely tied to weight gain as well. Poor blood sugar control is a risk factor for diabetes, obesity and cancer, among other things.

When someone is overly sensitive to insulin, the “fat-storage hormone” that signals cells to take in calories from food, more is produced by the pancreas, and this promotes the growth of cells, even mutated cancer cells.

After comparing women not eating or drinking anything for at least 12 hours with those who fasted less than 12 hours, researchers found that women who fasted for longer nightly intervals had better blood sugar control than those who didn’t fast as long. And this was independent of other eating behaviors such as how many calories women were eating.

Related: The Warrior Diet: Reviews, Meal Plan, Pros & Cons

Beginner Tips

All of this might seem too good to be true, but the early research results are undeniable. So how can you implement time-restricted eating in a realistic way?

More research is still needed to determine the ideal meal schedule, period of fasting and period of restricted eating. For example, we still aren’t totally sure if it matters when someone starts her fast each day or how many days a week of time-restricted eating is best. As of now, the recommendation is to go between 12–16 hours without eating several times per week, but some people see even better results from further restricting their eating windows to only 5–6 hours daily.

Of course, when you are eating, what you choose to eat is still important for overall health. You don’t need to count calories, but aim to make the vast majority of your diet whole foods, including quality protein foods, healthy fats and plenty of different vegetables.

Luckily, it also appears that you don’t need to follow time-restricted eating every single day to see results. Eating within an eight- or nine-hour window most days of the week — about four to five — seems to still do the trick. In fact, Dave Zinczenko, author of the top-selling book “The 8-Hour Diet,” recommends following time-restricted eating only three or four days a week.

For many people, going against the norm and skipping breakfast seems to be the easiest way to practice time-restricted eating. Although for decades we’ve been told that breakfast is “the most important meal of the day” and that we’re doomed to gain weight without it, this doesn’t seem to necessarily be true for everybody.

While time-restricted eating really relies on eating within a shortened window, and doesn’t mean you need to skip breakfast necessarily, it might be the most manageable way to follow a fasting program.

Related: 5:2 Diet: Guide to How It Works, Meal Plan, Benefits & More

Risks and Side Effects

TRE might not be for everyone, and some people appear to do better with practicing various types of fasting in general than others. Fasting has an impact on blood sugar, so anyone dealing with low blood sugar (hypoglycemia) should steer clear of fasting until glucose and insulin levels are well managed.

Fasting can also impact hormones, including cortisol and adrenaline. Women might be more impacted by these effects than men, although that isn’t necessarily a bad thing. Fasting isn’t meant to become a source of stress, but in some people with compromised or high cortisol and adrenaline levels, a further increase in these hormones from fasting can result in some unwanted side effects.

If you have existing adrenal or hormonal issues, or you’re healthy and try TRE but notice you’re experiencing fatigue, anxiety and irregular periods due to the hormone disruption, then TRE might not be for you — it might be better to eat more often throughout the course of the day.

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