Intermittent fasting for weight loss: what does the science say?

November 9, 2020

Intermittent fasting seems to be one of the most popular approaches to eating at the moment - but does it improve metabolic health and aid weight loss? 

Unlike diets that tell you what to eat, intermittent fasting focuses on when you should eat. There are a few different types of intermittent fasting, including one of the most common forms known as time-restricted eating (TRE).

We speak to our nutrition expert Dr. Sarah Berry about the science behind intermittent fasting, who might benefit most from it, and how you can pick the best eating pattern that works with your unique biology.

How do intermittent fasting and other forms of time-restricted eating work?

The vast majority of us eat three meals and two or three snacks per day. But in recent years, diets with alternative eating patterns, such as intermittent fasting, have been increasing in popularity.

Intermittent fasting is a broad term that includes multiple eating patterns that share similar characteristics, most notably a ‘fasting’ aspect, where your eating window (the time between when you start and end eating each day) is limited. 

Time-restricted eating is an attractive way to eat because it is simple. There’s no calorie counting or complicated diet rules to follow. You just eat during a specific time window (such as 12-8 pm or 7 am-1 pm) and then stop once your time is up.  

“We spend the vast majority of the day responding to our food and processing our meals, which is stressful for our bodies,” says Sarah. “The idea with time-restricted eating is to eat all our food within a shorter space of time, reducing the amount of time in a day your body is responding to food and giving it more time to recover.”

We can’t eat when we’re asleep, which is obvious enough, so we naturally fast during this period. Time-restricted eating usually involves extending that window from the average 8-12 hours a day to 16-18 hours so you eat your meals closer together and spend more time in a fasted state. 

In practice, this often means that people simply choose to skip breakfast and eat their first meal later in the day.

Is fasting good for you?

In a fasted state, your insulin and blood sugar levels are not elevated, so your body mobilizes fat for energy. 

The thinking behind time-restricted eating is that you spend more time each day in a fat-burning state, improving insulin sensitivity and metabolic flexibility. 

In 2014, studies in mice showed that time-restricted eating improved blood pressure, cholesterol, blood sugar control, and insulin sensitivity.

Since then, intermittent fasting and other time-restricted eating patterns have exploded in popularity and several trials have suggested that this approach may offer health benefits, including weight loss, in humans too.  

But other studies looking at the effects of time-restricted eating on cholesterol levels, blood pressure, insulin sensitivity, and blood sugar control have shown mixed results. 

Overall, while animal studies show that there may be advantages for metabolic health, the evidence for the benefits of time-restricted eating in humans is inconclusive

Will time-restricted eating help me lose weight?

Recently, results came out from a randomized clinical trial called TREAT looking at whether time-restricted eating offered any benefits in terms of weight loss, body composition, and overall health. 

The study asked one group of people to eat within a specific window between 12 pm and 8 pm -  essentially skipping breakfast - while the other group ate three regular meals spaced throughout the day. 

By the end of the study, there were no statistically significant health benefits or weight loss for either approach when averaged out across all the participants, with both groups losing less than a kilo.

However, the variation in responses to time-restricted eating varied significantly between individuals, with some people losing over 5% of their body weight while others gained nearly 5%. 

So although there was no benefit to time-restricted eating on average, some individuals did seem to benefit from the technique. This may explain why previous trials, which just looked at the average weight lost or gained using time-restricted eating, gave conflicting results. 

“The takeaway is that for the average person there's no clear advantage or disadvantage of having breakfast or skipping it,” says Sarah “It works for some, but it doesn’t work for all, and that’s why we have to always look beyond the average one-size-fits-all diet advice. We're all individuals and we all respond differently.” 

Who could benefit from time-restricted eating?

So should you eat your meals close together in a shorter eating window?

The answer is that it depends on you, how you respond to food, and how your responses change throughout the day. 

Data from our PREDICT studies show that people respond differently to foods depending on the time of day, with most people responding best in the morning, but there is a huge variation between individuals. 

“If your time-restricted eating means you are eating more of your food when you respond best, which is usually in the morning, that may be better for your metabolic health,” says Sarah. “But if your time-restricted eating window is towards the end of the day and you usually respond better in the morning, it may not give you much benefit.” 

To find out the best meal timing for you, and whether time-restricted eating may be good for your health, you need to understand your unique biology.

Our at-home test kit has everything you need to understand your unique responses to food and get personalized recommendations for thousands of foods that will work with your body to help you optimize your metabolism and be your healthiest weight.

Fast facts on intermittent fasting 

  • Time-restricted eating refers to eating all your meals within a restricted time window
  • Trials have shown that, on average, time-restricted eating is not better for weight loss than other approaches
  • Everyone responds to food differently, and some people may respond well to time-restricted eating
  • The best time for you to eat depends on your unique metabolism 
  • Understanding your unique biology may help you decide if time-restricted eating is for you, and when is the best time for you to eat

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