When You Eat Can Affect Your Weight

What you eat is important, but did you know that when you eat is important too? This article will explain the research on how the timing of meals can make a difference for those trying to lose weight. Please note that the research specifically about weight loss described below only studied overweight and obese individuals.  

The problem with late meals 

The circadian rhythm is like the body’s internal “clock” that regulates when we are sleepy and when we are awake during the 24 hour day. For example, you may have heard that your body releases the hormone melatonin at night, which is what makes us sleepy. Our metabolism is also affected by the circadian rhythm. Eating a meal late at night leads the body to produce more glucose than eating the same exact meal in the morning.1 People who eat most of their calories earlier in the day are less likely to be overweight than those who eat them later in the day.2

Research has looked at how the timing of meals can affect weight loss in obese or overweight individuals. For example, researchers compared obese men and women who were undergoing a 20-week weight loss treatment. Those who tended to eat their lunch before 3pm were compared with those who ate lunch after 3pm, and those who ate late lunch lost less weight and lost weight more slowly than those who ate before 3pm, even when the two groups were eating a similar diet and amount of calories, and both groups were exercising and sleeping a similar amount.3

Another study measured the results of a 12-week weight loss program where obese women were randomly assigned to eat either a high calorie breakfast (700 calories) paired with a small dinner (200 calories), and the other group ate a small calorie breakfast (200 calories) and a large calorie dinner (700 calories). Both groups ate a 500 calorie lunch. The women were instructed to eat their meals within specific time windows, with breakfast between 6:00-9:00 am, lunch between 12:00-3:00 pm, and dinner between 6:00-9:00 pm. The researchers found that a high-calorie breakfast and low calorie dinner improved weight loss, insulin sensitivity, and hunger suppression, regardless of when the women ate their meals.4

Does skipping breakfast matter?

Many people believe that “breakfast is the most important meal of the day” and that people who eat breakfast are healthier than those who do not. However, people who report eating breakfast tend to eat more regular meals and/or end their eating earlier in the day, whereas many people who skip breakfast eat a bigger meal late at night.1 For that reason, it is not known whether breakfast itself is related to better health, or if people who eat breakfast have other healthier eating habits, such as not eating late in the evening. 

What is intermittent fasting?

Intermittent fasting is an umbrella term for a type of diet that includes fasting or restricting eating during some times of day or some days. Research has found that intermittent fasting is safe, and it can help lower blood pressure in addition to helping people lose weight.5

These diets are all considered types of intermittent fasting. 

Alternate-day fasting 

Zero-calorie alternate-day fasting is when people alternate back and forth between days of eating no food at all vs eating as they would normally. Modified alternate-day fasting is when people alternate between days where they eat normally and days when they eat no more than 600 calories. A 2016 study assigned obese participants to either 8 weeks of zero-calorie alternate-day fasting or 8 weeks of a restricted calorie diet and found that there was no difference in how much weight people were able to lose between the two diets.6 A 2017 study that assigned obese participants to either a modified alternate-day fast where they ate no more than 500 calories on fast days, a restricted calorie diet, or no diet (control) and followed them for up to 1 year. The study found that both types of diet helped people lose weight, and there was no difference in how much weight was lost between either modified alternate-day fasting or a more traditional calorie restricted diet.7

5:2 diet

On the 5:2 diet, people eat no more than 600 calories per day for 1 or 2 days per week (they can be either consecutive or nonconsecutive days) and eat normally the other days of the week. Research has found that the 5:2 diet is effective at helping overweight and obese people lose weight. A 2021 study that analyzed the results of many studies found that a 5:2 diet is associated with statistically significant weight loss.8 Another 2021 study on the 5:2 diet compared overweight women assigned to follow either a 5:2 diet or a restricted calorie diet (1500 calories per day) for 12 months. Both groups of women lost weight, and there was no difference in the amount of weight loss between the two types of diet.9

Time-restricted eating 

Time-restricted eating is when people eat as they normally would each day of the week, however, they eat all of their meals within an 8-10 hour span, rather than throughout the whole day. A 2018 study found that 12 weeks of time-restricted eating could help obese adults lose weight.10 Time-restricted eating may help reduce hunger as well, which could help with weight loss. Research has also found that people using a time-restricted eating diet naturally started to eat fewer calories per day, even when not instructed to change how much they eat.1 However, studies have shown that it is important to have the time span for eating earlier in the day rather than later because a later time span for having meals could leads to more blood glucose and more hunger.1 And a 2021 study that combined the results of 45 smaller studies reported that time-restricted eating did not result in significant weight loss.8

A word of caution about intermittent fasting

Many of the studies on intermittent fasting are small, and larger studies are needed to learn more about what works for which types of patients. Most research on weight loss has focused on people who are overweight or obese, rather than people who are considered to have a healthy weight but would like to lose a few pounds. The 2021 study that combined 45 studies suggests that modified alternate day fasting and the 5:2 diet are the only types of intermittent fasting that help people lose a statistically significant amount of weight.8 However, there are some limitations to the studies that have been conducted thus far.  And as is usually true with weight loss diets, the number of calories consumed and the amount of physical activity is key.  For example, if a person eats much more than usual on non-fast days, or during non-fasting times of day, they are unlikely to lose weight. Or if fasting results in less physical activity, that would also limit the effectiveness of the diet for weight loss.

Researchers who study intermittent fasting diets also note that people need to be cautious while on these diets. Those who are fasting should drink more water to help prevent dehydration. If someone is doing an alternate day fast, they should exercise on days when they are not fasting.  Intermittent fasting can cause constipation, and eating more fruits and vegetables can help relieve this problem. Also, experts recommend that people who are doing an alternate-day fast or a 5:2 fast not drink alcohol on the days when they are not eating or are restricting their calories.5

Intermittent fasting is not suitable for everyone. For example, it is not recommended for pregnant or lactating women, children under 12, those who have eating disorders, shift workers, and people who take medication that should be taken with food.5 It is also not recommended for those who are underweight (with a Body Mass Index under 18.5).

The bottom line

Due to your circadian rhythm, your body processes food differently later compared with earlier in the day. While it is commonly believed that eating breakfast makes people healthier, the more important strategy may be to stop eating earlier in the day. Intermittent fasting could be a useful alternative for some people trying to lose weight who have the willpower to eat “normally” on some days or some times of day, even if they are fasting or dramatically restricting their calories at other times or other days. If you are interested in trying intermittent fasting, talk with your healthcare provider, as it is not recommended for everyone. 

All articles are reviewed and approved by Dr. Diana Zuckerman and other senior staff.

The National Center for Health Research is a nonprofit, nonpartisan research, education and advocacy organization that analyzes and explains the latest medical research and speaks out on policies and programs. We do not accept funding from pharmaceutical companies or medical device manufacturers. Find out how you can support us here.


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