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A Duel of Diets: Is Alternate Day Fasting Inherently Better Than Regular Calorie Cutting?

Posted on 11 October 2021

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We know from animal studies that dietary restriction (DR) – defined as a reduction in nutrient intake without causing malnutrition – can have drastic effects on health and longevity, sometimes extending lifespan significantly. Unfortunately, the effects of DR on human lifespan isn’t currently known, and most studies have focussed on measuring weight loss.

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Benefits of dietary restriction and mutations/drugs, from yeast to humans.

While it’s pretty indisputable at this point that DR has health benefits, there are still many unanswered questions, including ‘which forms of DR are most effective?’. As someone who has tried several types of fasting for its general health benefits, this is a question that I find particularly interesting, and about which I wrote a full article comparing intermittent fasting with periodic fasting. Earlier this year, a new study was published in the journal Science that made a direct comparison between three types of DR. Let’s take a look at what they found.

What was the goal of the study?

There is no question that diets like alternate day fasting (ADF) help people lose weight by cutting down on calories. What the researchers were interested in here was whether ADF was any more or less effective than regular calorie restriction that achieved the same overall calorie reduction. In other words, if two people cut down on their average calorie intake by 25%, is the person who does it by fasting every other day better-off than the person who uses simple, daily calorie restriction?

What did the researchers do?

In an effort to answer this question, researchers recruited 36 lean (BMI 20.5-24.9) healthy (free of diabetes and other metabolic diseases) adults aged 18 to 65. For four weeks, data was collected about participants habitual energy intake and physical activity. They were then randomised to follow one of three different food intake patterns for a period of 20 days:

  • Continuous calorie restriction (CR only): Participants reduced their calorie intake each day by 25%
  • Calorie restriction via alternate day fasting (CR + ADF): Participants alternated between days on which they ate nothing, and days on which they ate 150% of their habitual daily energy intake. This amounts to a net calorie reduction of 25% – the same as the CR only group.
  • Alternate day fasting without calorie restriction (ADF only): Participants alternated between days on which they ate nothing, and days on which they ate 200% of their habitual daily calorie intake, meaning that their net calorie intake was unchanged.

At the end of the study, participants’ body weight and fat mass were measured. Participants’ physical activity was recorded throughout the study using wearable activity recorders.

What were their findings?

All three groups lost weight by the ends of their respective dietary protocols. Not surprisingly, the group who fasted but didn’t reduce their overall calorie intake (ADF only) lost the least weight – an average of 0.52 kilos (1.15lb). Of the two groups whose overall intake was reduced, the CR only group lost more weight on average than the group who practiced ADF: 1.91kg (4.21lb) vs 1.60kg (3.53lb) respectively. When the researchers used dual-energy x-ray absorptiometry (DEXA) to measure the percentage of this weight loss that was attributable to fat, it came out as (on average) 91.6% for CR only, 46.3% for CR + ADF, and 23.1% for ADF only. So the CR group not only experienced the greatest average weight loss, but almost all of that weight loss was due to reduced fat mass. In contrast, the other two groups lost more fat-free mass than they did fat.

Changes in body weight and composition following the diet period.

The researchers observed significantly reduced physical activity in the ADF+CR group, mainly due to ‘reduced spontaneous light- and moderate-intensity movements’, which could explain why CR+ADF ended up being less effective than plain calorie restriction. Essentially, participants practicing ADF had less energy and subconsciously or consciously reduced their physical activity levels.

The researchers didn’t find significant differences in metabolic markers like post-meal glucose, insulin or fatty acids.

What were their conclusions?

According to lead investigator James Betts, these findings suggest that intermittent fasting is in no way ‘special’ when compared to calorie restriction when it comes to weight loss, and people should think carefully about whether fasting techniques might be harming their health in other ways.

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.

Professor James Betts, Director of the Centre for Nutrition, Exercise & Metabolism at the University of Bath

So this research challenges the commonly held view that alternate-day fasting results in greater weight loss than more consistent calorie restriction. Lean people who want to use alternate day fasting should make a conscious effort to ensure their levels of physical activity don’t drop as a result.

My thoughts

This study is a good reminder that the effectiveness of a diet is about more than just how much weight you can lose, but there are some important considerations when applying these results to real-world scenarios. This study compared CR only and CR+ADF in people who had the same net overall calorie intake. This was achieved by having the CR+ADF group eat 50% more than usual during non-fasting days. However, many people who use intermittent fasting techniques find that they barely eat more, if at all, during feeding days than they otherwise would. So while there might not be anything inherently better about restricting calories through ADF, that doesn’t mean some people won’t find it easier to achieve a given level of CR using this method.

It’s also important to remember that participants were of normal weight. This is particularly important when considering the findings regarding loss of fat mass vs fat-free mass. When operating at a calorie deficit, where those missing calories are drawn from depends greatly on the size of fat stores – the less adipose tissue you have remaining, the greater the percentage of energy that will instead come from lean mass. So if you are overweight, you are unlikely to lose that much muscle mass regardless of which form of dietary restriction you use – at least so long as you don’t greatly reduce your levels of physical activity.

We need larger studies of this kind, and also studies that last much longer – might results have been different if participants had maintained their diets for months or even years? This study might be enough to give lean people who are fasting pause for thought, but for those struggling to maintain a healthy weight, I’m not sure how much it matters. For most of us, the main challenge when it comes to staying healthy is simply making the lifestyle changes we already know we need to make. I still think that the best weight loss strategy for any given person is simply whichever one they find easiest to adopt and maintain.

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