There’s evidence to suggest that intermittent fasting can help with short-term weight loss and may reduce the risk of heart disease and type 2 diabetes. But there are also potential drawbacks.
Intermittent fasting, also called time-restricted eating or time-restricted feeding, is a way of eating that has become increasingly popular over the last few years. As the name suggests, it focuses on when you eat, not what you eat.
There are different types of intermittent fasting. They involve not eating for a certain number of hours each day, followed by a specific window where you’re free to eat without restricting calories or specific food groups.
At ZOE, we run the largest nutritional science study in the world, with over 10,000 participants so far. Our research has found that the way your body responds to food is unique to you.
Eating the best foods for your body starts with an understanding of your unique biology.
You can take our free quiz to find out how the ZOE program can help you achieve your long-term goals.
Read on to learn about the pros and cons of intermittent fasting.
Join our mailing list
Get occasional updates on our latest developments and scientific discoveries. No spam. We promise.
What is intermittent fasting?
Some types of intermittent fasting restrict the time you can eat to a specific period each day. Some people call this time-restricted eating.
The most common types of time-restricted eating are:
16/8: This involves fasting for 16 hours and eating within an 8-hour window. For example, if you have dinner at 8 p.m., you would fast until noon the next day.
18/6: This involves fasting for 18 hours and eating within a 6-hour window.
20/4: This version reduces your eating window to 4 hours, and you fast for the remainder of the day.
While you’re fasting, you can consume water and calorie-free drinks like black coffee and tea.
Other types of intermittent fasting involve reducing the amount you eat on certain days. One way of doing this is to fast for 24 hours once or twice a week.
Another approach is to follow the 5:2 fasting method. With 5:2, you eat your regular diet 5 days a week and reduce your daily calorie intake to 500–800 calories on the 2 fasting days.
Some people may find that certain forms of intermittent fasting fit in with their lifestyles better than traditional diets. For instance, if you’re someone who usually skips breakfast, you could find that 16/8 intermittent fasting works well for you.
Potential benefits of intermittent fasting
Most research that looks at intermittent fasting is very recent, and many studies have included only small groups of people. Scientists need to do more work to look into the long-term effects of intermittent fasting.
So far, studies have found that intermittent fasting can help people lose weight and reduce their risk of type 2 diabetes and heart disease.
Because you can eat what you like during non-fasting periods, some people may find intermittent fasting more appealing than counting calories throughout the day.
However, it doesn’t seem to be any more effective than a standard calorie-controlled diet or other diets for weight management, and more research is needed to say whether it leads to sustained weight loss in the longer term.
Losing weight can be very challenging, and different weight loss strategies work for different people. There is no one-size-fits-all solution to weight management.
Unpublished research has found that people who closely followed their personalized gut-friendly ZOE nutrition program for 3 months lost an average of 9.4 pounds, while around 80% said they had more energy and didn’t feel hungry.
The ZOE at-home test analyzes your blood sugar and blood fat responses to food, in combination with your gut microbiome, and uses the latest science to find the best foods for your unique metabolism.
Type 2 diabetes
There is some evidence that intermittent fasting could help with risk factors for type 2 diabetes, including insulin resistance.
A recent study involving men with a high risk of diabetes looked at the effects of intermittent fasting with a 6-hour eating window early in the day, with the last meal at 3 p.m.
The researchers saw that this improved both insulin sensitivity and the function of the pancreas, which produces insulin. The study also showed that these benefits happened for the participants, even without weight loss.
This could be because intermittent fasting syncs your food intake with your circadian rhythm, or body clock. Eating according to your body clock is helpful for processing glucose and reducing your risk of diabetes.
However, more studies are needed to understand how intermittent fasting influences diabetes risk in the long term.
Heart health and cholesterol
Intermittent fasting may help improve your heart health. Studies have shown that fasting can reduce risk factors related to heart disease, including:
low-density lipoprotein (LDL) cholesterol, often referred to as “bad” cholesterol
triglycerides, a type of blood fat that can raise the risk of heart disease
high blood pressure, which can lead to heart attack and stroke
inflammation, which causes the development of plaque that can block the arteries to the heart
Possible downsides to intermittent fasting
We’ve discussed the potential benefits of intermittent fasting, but there are also some possible downsides.
Because intermittent fasting can involve a prolonged period where you’re not eating, your body may be running on an empty tank for longer than it’s used to.
Studies show that intermittent fasting can cause fatigue, feelings of low energy, irritability, and can make it harder to concentrate — although there’s some evidence that these symptoms may improve with time.
It can be hard to sustain
Many people find it hard to stick to intermittent fasting in the long term, just like many people find it difficult to stick to other diets in the long term.
In one study, 38% of participants in the alternate-day fasting group left the study within the year, compared with 29% of those on the calorie-restricted diet group. However, other studies have found the opposite — people in the intermittent fasting group continued the study longer than those in other diet groups.
If you enjoy eating as a social activity, it’s important to plan your fasting windows or days around occasions like birthday parties, family meals, or dinner with friends.
It’s also important to choose high quality foods when you’re eating and to make sure you drink enough water during fasting periods.
Although intermittent fasting doesn’t dictate what you can and can’t eat in terms of calories or particular food groups, it can promote disordered eating for those at risk.
Research shows that fasting is linked to both bulimia and binge eating.
Many people will be able to follow intermittent fasting without it negatively impacting their relationship with food, but others could find it triggering.
Who should not try intermittent fasting?
Although intermittent fasting may be safe for some people, those who should avoid it include:
Pregnant or breastfeeding women. Although there are no specific studies on intermittent fasting during pregnancy or breastfeeding, women generally need more calories during these periods so should not go on restrictive diets.
People with a history of eating disorders. Fasting has been linked to an increased risk of bulimia and binge eating, so it’s not recommended for anyone who has experienced disordered eating in the past.
People with type 1 diabetes. People with diabetes or blood sugar problems are at risk of experiencing hypoglycemia with fasting, since the amount of insulin they take needs to be adjusted with fasting.
People on medication that needs to be taken with food. If your medication needs to be taken several times a day at mealtimes, cutting out meals could make this impossible.
Older adults. Fluctuations in energy intake and glucose concentration can cause falls and fractures in older adults, who may have problems with their balance.
If you have a preexisting health condition, speak to your doctor before changing your eating patterns.
There are a number of different types of intermittent fasting. With some, you eat within specific time windows. Others involve significantly reducing your calorie intake on certain days of the week.
There is evidence that intermittent fasting can lead to weight loss in the short term. While it may not be any more effective than a traditional calorie-controlled or weight loss diet, it could suit some people better, as it doesn’t limit what you can eat during non-fasting periods.
However, like most diets, intermittent fasting can reduce your energy, could lead to disordered eating, and may be hard to sustain in the long term.
There is evidence that intermittent fasting may help to improve risk factors for type 2 diabetes and heart disease.
Our research shows that one-size-fits-all nutrition advice doesn’t work. What is best for your body, health, and weight is unique to you. With the ZOE program, you get personalized recommendations to find the best foods for you.
Take our free quiz to find out how you can reach your long-term health goals.
Beneficial effects of time-restricted eating on metabolic diseases: A systemic review and meta-analysis. Nutrients. (2020). https://www.mdpi.com/2072-6643/12/5/1267
Early time-restricted feeding improves insulin sensitivity, blood pressure, and oxidative stress even without weight loss in men with prediabetes. Cell Metabolism. (2018). https://www.cell.com/cell-metabolism/fulltext/S1550-4131(18)30253-5
Effectiveness of intermittent fasting and time-restricted feeding compared to continuous energy restriction for weight loss. Nutrients. (2019). https://pubmed.ncbi.nlm.nih.gov/31614992/
Effect of alternate-day fasting on weight loss, weight maintenance, and cardioprotection among metabolically healthy obese adults: a randomized clinical trial. JAMA Internal Medicine. (2017). https://jamanetwork.com/journals/jamainternalmedicine/fullarticle/2623528
Fasting increases risk for onset of binge eating and bulimic pathology: a 5-year prospective study. Journal of Abnormal Psychology. (2008). https://pubmed.ncbi.nlm.nih.gov/19025239/
Insulin resistance and diabetes. (2021). https://www.cdc.gov/diabetes/basics/insulin-resistance.html
Intermittent energy restriction and weight loss: a systematic review. European Journal of Clinical Nutrition. (2016). https://pubmed.ncbi.nlm.nih.gov/26603882/
Intermittent fasting and weight loss: systematic review. Canadian Family Physician. (2020). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7021351/
Intermittent fasting in cardiovascular disorders — an overview. Nutrients. (2019). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6471315/
Intermittent fasting, Paleolithic, or Mediterranean diets in the real world: exploratory secondary analyses of a weight-loss trial that included choice of diet and exercise. The American Journal of Clinical Nutrition. (2020). https://academic.oup.com/ajcn/article/111/3/503/5687899
Intermittent fasting: the choice for a healthier lifestyle. Cureus. (2018). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6128599/
Intermittent versus continuous energy restriction on weight loss and cardiometabolic outcomes: a systematic review and meta-analysis of randomized controlled trials. Journal of Translational Medicine. (2018). https://translational-medicine.biomedcentral.com/articles/10.1186/s12967-018-1748-4
Potential benefits and harms of intermittent energy restriction and intermittent fasting amongst obese, overweight and normal weight subjects — a narrative review of human and animal evidence. Behavioral Sciences. (2017). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5371748/
Therapeutic use of intermittent fasting for people with type 2 diabetes as an alternative to insulin. BMJ Case Reports. (2018). https://casereports.bmj.com/content/2018/bcr-2017-221854
The safety and efficacy of intermittent fasting for weight loss. Nutrition Today. (2020). https://journals.lww.com/nutritiontodayonline/Abstract/2020/11000/The_Safety_and_Efficacy_of_Intermittent_Fasting.5.aspx
Time-restricted eating to prevent and manage chronic metabolic diseases. Annual Review of Nutrition. (2019). https://www.annualreviews.org/doi/abs/10.1146/annurev-nutr-082018-124320