Intermittent fasting for weight loss: What the science says
Here's what evidence says about the efficacy – and safety – of different styles of intermittent fasting for weight loss
Intermittent fasting for weight loss can be an effective way to lose unwanted fat, particularly in those with obesity and metabolic syndrome. Multiple studies have found that the metabolic impact of intermittent fasting, coupled with the calorie deficit that it tends to generate, can help to create a downward trend on the scales.
There are different styles of intermittent fasting, so if you're looking to embark on this approach it's important find the right eating pattern for you. Whether it’s a more extreme form like the 5:2 diet, or a gentler approach such as 16: 8 intermittent fasting, intermittent fasting should be sustainable and fit with your lifestyle.
In this article, we'll break down the science behind intermittent fasting for weight loss, and which approaches are most effective. Plus, we've spoken to experts in cardiometabolic health and oncology about the best ways to lose weight sustainably and safely through intermittent fasting. Remember, it's best to consult a qualified medical professional or registered dietitian if you are considering making significant changes to your diet.
How can intermittent fasting help with weight loss?
Intermittent fasting can be effective in reducing body weight in people of every size, according to a report in Nutrition Reviews. Most studies focused on alternate day fasting or whole-day fasting trials, but there is also evidence to support the success of time-restricted eating. As well as weight loss, intermittent fasting can have cardiometabolic benefits for those with obesity, as found in another review in the 2021 Annual Review of Nutrition.
Similar to the way the keto diet helps with weight loss, intermittent fasting can put you into a state called ketosis where you are using ketones as fuel. A study in the journal of Cell Metabolism indicates that in the state of ketosis, the body metabolizes fat and produces ketones, which the body can use as an alternative energy source.
- Related: Is breakfast important?
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“After a meal, you pass through four different metabolic states as your body digests the food,” explains Dr Deborah Lee, MD, from Dr Fox Online Pharmacy. “These are: the fed state, the post-absorptive state/early fasting state, the late fasting state, and starvation.
Having worked for many years in the U.K's National Health Service, initially as a GP, and then as Lead Clinician for an integrated Community Sexual Health Service, Dr Deborah Lee now works as a health and medical writer, with an emphasis on women’s health. She is a menopause specialist.
“Most of the time your body uses glucose for energy. After a meal, glucose levels rise, and fat is stored as triglycerides in adipose tissue. In the normal situation, the body uses glycogen stored in the liver for energy. However, after 12 hours of fasting, all the liver glycogen has been used up. The body is now forced to take triglycerides from adipose tissue and break these down into fatty acids and glycerol. Fatty acids are converted into ketone bodies, which are then used for fuel, as an alternative to glucose.”
She adds that when the body passes from the post-absorptive to the fasting state, this is referred to as ‘the metabolic switch.’
“It is at this point, when the switch is turned on, that the body starts to use ketones for energy,” she says. “In the fed and post-absorptive state, the primary hormone is insulin, whereas in the fasting state, the primary hormone is now glucagon.”
Intermittent fasting also works by restricting your overall calorie intake and putting you into a calorie deficit (where you are using more calories than you are consuming) . Even if you eat more after your fast, you are unlikely to eat as much as you would in a normal eating pattern. A calorie deficit is one of the most effective ways to lose weight, according to a report in the Journal of Obesity and Metabolic Syndrome, so this aspect of intermittent fasting may also help you to lose weight.
What is the best intermittent fasting plan for weight loss?
Dr Sam Watts, founder of Mind Body Medical, tells Live Science that there are two clear leaders when it comes to intermittent fasting styles. “Lots of clinical evidence exists around the potential weight loss benefits of different intermittent fasting plans,” he says. “For quicker and more profound weight loss, the alternate day fasting plan is arguably the most effective. A less-extreme and thus easier-to-adopt approach is the 16:8 model. This is a time-restricted eating version of intermittent fasting that involves consuming all of your calories in an eight-hour window and fasting for the remaining 16 hours.
“This approach has consistently been shown to induce significant and progressive weight loss in an arguably more sustainable model.”
Twenty-four-hour fasting is one of the toughest fasting styles, where you fast completely, or heavily restrict calorie intake for 24 hours, often for one or two days a week. This may not be the best method of fasting for a lot of us, as it isn’t compatible with many lifestyles, and longer periods of fasting (48-72 hours) can trigger a starvation response, which can encourage your body to store fat when you do eat.
This style of fasting is a flexible fasting style where you fast two days a week and eat normally the rest of the time. On fasting days you restrict your calorie intake to 500-600 calories, generally consuming this in one meal at the start of the day and then fasting until the next morning.
Alternate day fasting
With alternate day fasting, you fast every other day, eating to satiety on alternate days. On fasting days you will typically consume up to 800 calories a day, although some people choose to fast completely.
“Using this approach, you fast every other day while eating a healthy, well-balanced diet on the alternate non-fasting days,” says Watts. “This approach has been shown to facilitate significant and progressive weight loss.”
Time-restricted eating: 20:4, 16:8, 14:10, 12:12
Time-restricted eating is a fasting style where you fast for a certain amount of time and eat in the remaining window. Common types of time-restricted fasting include 20:4, 16:8, 14:10 or 12:12, with the first number being the fasting window and the second number the eating window. There isn’t evidence to suggest any particular benefit to one over the other, and it is worth experimenting to find what works best for you.
How to succeed on an intermittent fasting plan
Stay well hydrated
A study in the Annals of Nutrition and Metabolism journal found that proper hydration can help to keep your energy levels consistent and your mood stable, which may indirectly keep cravings at bay and your motivation levels steady.
Watts advocates drinking lots of water and calorie-free drinks to help curb hunger. “When the symptoms of hunger set in during the fasting periods, sipping water, herbal teas and other non-caloric drinks can help fill up the tummy, reduce hunger and generally make life more comfortable.”
Investing in one of the best water bottles can help to keep you hydrated too.
Choose the right fasting style for you
An Obesity Society study found alternate day fasting can cause an impaired glucose response in women with a healthy body weight, so this may not the best fasting style for women. Mor agrees that women may need to be more careful with intermittent fasting than men. “More studies are needed to understand the impact of intermittent fasting on women in particular. There has been some evidence that men can benefit from intermittent fasting, while women may be negatively affected.”
“Intermittent fasting may be easier in terms of fitting around a typical working day, where you are busy during the day which takes your mind off not eating," says Dr Lee. "With any diet, but perhaps especially calorie restriction, you need to plan your meals and prepare them in advance. You can also work intermittent fasting around your social life more easily perhaps, making sure you only accept invitations on non-fasting days.”
But intermittent fasting can have side effects, she says. As blood glucose levels fall, this can spark headaches, and result in feelings of dizziness and weakness.
"Studies of those fasting during Ramadan have shown that during fasting, severe hypoglycemia can occur. Intermittent fasting may not be suitable for the elderly, as low blood glucose levels can increase the risk of falls.”
Our feature on intermittent fasting for beginners: expert tips to get you started has more information for intermittent fasting novices.
Eat filling foods
Eating foods that are high in fiber can help to limit feelings of hunger, as fiber serves as bulk that takes time to pass through your digestive system. A study in the Journal of Nutrition concluded that dietary fiber intake promotes weight loss and dietary adherence in adults consuming a calorie-restricted diet.
Additionally, protein has been shown to help with satiety, as indicated by a review in the British Journal of Nutrition. Feeling satisfied after a meal can help you stick to your fast, as you are less likely to experience hunger or food cravings.
“During your non-fasting period, it is essential to eat enough healthy calories to allow you to thrive during the fasting period,” says Watts. “To facilitate this, prepare meals you enjoy and eat enough to feel fully satisfied and full. It is also important to ensure an adequate intake of lean protein (animal or plant-based), a good portion of complex carbohydrates, plenty of vegetables and fruit and, most importantly, some healthy fat, such as extra-virgin coconut oil or olive oil, omega-3 fatty acids, nuts and seeds. These help to stabilize blood sugar, sustain energy and prevent extreme states of hunger.”
This article is for informational purposes only and is not meant to offer medical advice.
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Lou Mudge is a health writer based in Bath, United Kingdom for Future PLC. She holds an undergraduate degree in creative writing from Bath Spa University, and her work has appeared in Live Science, Tom's Guide, Fit & Well, Coach, T3, and Tech Radar, among others. She regularly writes about health and fitness-related topics such as air quality, gut health, diet and nutrition and the impacts these things have on our lives.
She has worked for the University of Bath on a chemistry research project and produced a short book in collaboration with the department of education at Bath Spa University.