New research published this week challenges a popular belief that intermittent fasting diets such as alternate day fasting or the ‘5:2’ are the most effective ways to lose weight.
Over recent years, diets that see people fast on a few days each week have increased in popularity, reinforced by images of people’s miraculous weight transformations, and backed by celebrity endorsements.
However, evidence to date about the effectiveness of fasting compared with more traditional diets which aim to reduce calorie intake over the course of a full week has been limited.
Published in the prestigious journal Science Translational Medicine, the new study from a team of physiologists at the University of Bath builds this evidence and indicates that there is ‘nothing special’ about fasting.
Three Distinct Dieting Strategies
Participants in their randomized control trial lost less weight when fasting in comparison with those following a traditional diet — even when their calorie intake was the same overall.
The trial, organized by a team from the University’s Centre for Nutrition, Exercise & Metabolism (CNEM), saw participants allocated into one of three groups:
- Group 1 fasted on alternate days with their fast day followed by a day of eating 50% more than usual.
- Group 2 reduced calories across all meals every day by 25%.
- Group 3 fasted on alternate days (in the same way as Group 1) but followed their fast day with one day of eating 100% more than usual.
Participants across all three groups were consuming a typical diet of around 2000-2500 kcal per day on average at the start of the study. Over the course of the three-week monitoring period, the two energy-restricted groups reduced this to between 1500-2000 kcal on average. Whereas groups 1 and 2 reduced their calorie intake by the same amount in different ways, group 3’s diet saw them fast without reducing overall calories.
Their results found that the non-fasting dieting group (Group 2) lost 1.9 kg (4.2 lb) in just three weeks, and DEXA body scans revealed this weight loss was almost entirely due to a reduction in body fat content.
By contrast, the first fasting group (Group 1) who experienced the same reduced calorie intake by fasting on alternate days and eating 50% more on non-fasting days, lost almost as much body weight (1.6 kg or 3.5 lb) but only half this weight loss was from reduced body fat with the remainder from muscle mass.
Group 3, who fasted but increased their energy intake by 100% on non-fasting days, did not need to draw on their body’s fat stores for energy, and therefore weight loss was negligible.
Fasting Diets Not Superior to Traditional Calorie Reduction
Professor James Betts, Director of the Centre for Nutrition, Exercise & Metabolism at the University of Bath who led the research explains: “Many people believe that diets based on fasting are especially effective for weight loss or that these diets have particular metabolic health benefits even if you don’t lose weight.
“But intermittent fasting is no magic bullet and the findings of our experiment suggest that there is nothing special about fasting when compared with more traditional, standard diets people might follow.
“Most significantly, if you are following a fasting diet it is worth thinking about whether prolonged fasting periods is actually making it harder to maintain muscle mass and physical activity levels, which are known to be very important factors for long-term health.”
These results focused on participants who were defined as ‘lean’ (i.e. body mass index 20-25 kg/m²). 36 people participated in the study which was conducted between 2018 – 2020 and funded by the University of Bath.
Reference: “A randomized controlled trial to isolate the effects of fasting and energy restriction on weight loss and metabolic health in lean adults” by Iain Templeman, Harry Alex Smith, Enhad Chowdhury, Yung-Chih Chen, Harriet Carroll, Drusus Johnson-Bonson, Aaron Hengist, Rowan Smith, Jade Creighton, David Clayton, Ian Varley, Leonidas Georgios Karagounis, Andrew Wilhelmsen, Kostas Tsintzas, Sue Reeves, Jean-Philippe Walhin, Javier Thomas Gonzalez, Dylan Thompson and James Alexander Betts, 16 June 2021, Science Translational Medicine.
DOI: 10.1126/scitranslmed.abd8034
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12 Comments
The study merely confirms that reduced calorific intake results in weight loss, and its methodology is unsuited to provide information about which method is best.
My experience is that it is much easier to stick with a regime of intermittent fasting than maintaining a uniformly calorie-reduced diet, and there is plenty of similar opinion around.
As for the metabolic benefits of intermittent fasting, there is much in the medical literature to support it.
Not a refutation of people who fast on a daily basis, for example: don’t eat anything before noon and after 10PM. The fast in the study has an entire fasting day followed by an entire eating day. My takeaway from reading this article is that you should keep a consistent schedule every day. The jury is still out on limiting the hours during a day that you eat food.
Group 3 are so far from reality they offer zero insight. Who in the real world would eat double the calories each second day. Did they have to force feed these people crap to make their minimum calorie target. I fast daily and do so limiting myself to a 4 hour eating window, results I have lost 30kg in 15 months. Fasting does not mean your free to consume extra calories without consequence and that’s the premise of this article. Justa
Fasting one day doesn’t naturally lead to eating more / more calories the next day. As a regular faster, I find the premise of this study utterly ridiculous.
Why choose to focus on BMIs of 20-25? Not the typical patient struggling with a healthy bodyweight.
The time period of those study was too short! Benefits of intermittent fasting go beyond weight loss. My dentist reports are stellar because inflammation in my body is so improved on fasting. My energy levels area higher and I feel much more hope about keeping this health and a lower weight for the rest of my life compared to calorie-reduction. I have been fasting for 18-24 hours daily for over a year now and keep seeing good results.
The last paragraph says the results focused on “lean” people. Why us this even published? Lean folks don’t generally try and lose a bunch of weight. Ridiculous article on a ludicrous study. The only thing this study shows is how poorly constructed studies and journalism on those studies has become. Group 4: fasting daily 18-6 with 1,000Kcal deficit every day. Great results, all centered in belly fat, chest fat, and thigh fat. But why try that? It is just a scientifically viable option. SMH bad “science.”
So….a study conducted by individuals (who are funded by or work directly for the food industry e.g. PepsiCo, Danone, Nestlé, Kellogg’s) conduct a short-term (3 week) study on a small (12) group of healthy (Lean, low BMI) individuals who are not suffering from metabolic syndrome or type 2 diabetes only to learn that they won’t lose her weight while fasting. Gee, maybe once the millions of obese Amricans get to that point in their own health, we can worry about such things.
It would seem that the reason to choose lean individuals for a weight loss experiment was primarily to achieve a result of muscle loss in the fasting individuals. That would also explain the short duration of the experiment, as such an outcome would be potentially unethical if carried out on a longer term to a more drastic effect.
In overweight individuals, losing significant amounts of lean tissue along with body fat is entirely expected–no one argues that the lean tissue loss has potentially harmful long-term health implications because it is understood that those tissues exist as a function of maintaining a higher bodyweight and that any potential downsides are completely overshadowed by the health benefits of no longer being overweight or obese. Replicating this experiment with individuals who would actually benefit from a reduction in body weight should show lean muscle loss along with fat loss in both fasting and caloric restriction groups.
On lean people?????????
The first month I did alternate dieting I found the same. I measured my waist and it did not improve ( I was group 3 eating double calories on the eating days). I fasted 3 days a week. I increased calories by adding ice cream to breakfast oats for example. Terrible idea. A month later I tried again and this time I made sure that I always break my fast with high fat low carbs food with some protein added to all 3 or 4 meals and avoiding all sugars and fast burning carbs for the break – fast and the last meal of the day. Then my waist improved without loosing body mass. Study of 1 but I can say that for breaking my fast with a traditional breakfast ( cereals, breads , rice , some protein some sugar in the form of fruits etc. ) does not work for me. For me it needs to be low GI breakfast with no sugar and almost no fruit or high carbs. I do alternate day fasting for reducing inflammation , recycling bad cells, improving skin and not for optimizing body composition.
IMO this looks like manipulation and I have no idea whos interest it might be. Popular fasting is intermittent fasting, when you eat daily, for limited hour window, like 18h fasting and 2 meals during 6h. No one fasts like this (in this studies) on regular basis…