When Dr. Michael Mosley produced the BBC documentary Eat, Fast, and Live Longer in 2012, intermittent fasting exploded in popularity like never before.
At that point, fasting was a somewhat fringe practice that seemed too extreme for the mainstream and in the documentary, Mosley himself struggled considerably with the hunger produced from a day without calories. But he was too convinced by his research into its purported health benefits to simply not continue with it, so he invented a compromise: the 5:2 diet.
As probably the most common form of non-religious fasting, 5:2 deserves a close look. In this article we’ve looked at several studies and spoken to a physician who specializes in weight loss to address the following concerns.
- What Is Intermittent Fasting?
- What Is the 5:2 Diet?
- Pros and Cons of the 5:2 Diet
– Weight Loss
– Insulin Sensitivity
What Is Intermittent Fasting?
The 5:2 diet is considered a variant of intermittent fasting, the practice of abstaining from calories for periods of time anywhere from 12 hours to days at a time.
The more popular methods of fasting usually involve daily fasts of 16 hours (including the time you spend sleeping) or weekly fasts of 24 hours. You’re always allowed water and almost always allowed plain tea or coffee; check out our complete guide to intermittent fasting to read more on the topic.
What Is the 5:2 Diet?
Fasting can be tough. After some practice, many realize that the difficulty was more mental — you’ve eaten food every day of your life thus far, and disrupting one of your most ingrained routines can produce discomfort that’s more linked to your mind than to the body actually needing food.
But we’re certainly not saying that fasting is for everyone. If it makes you unhappy, remember that the majority of healthy people have probably never gone a day without food.
With that said, the 5:2 diet is Mosley’s attempt to reap the benefits of fasting without struggling so much with hunger. For that goal, he came up with a pattern of eating that looks like this:
- Eat normally for five days of the week
- For two days, only consume a quarter of your daily calories
This is often shortened to something like, “eat 500 calories on two days of the week,” given the average person needs about 2,000 calories to maintain their weight. You’re meant to make sure there’s at least one non-fasting day between the two “fast” days.
That’s it. You can structure the calories however you like; Mosley likes to have a couple of soups throughout the day and a small meal at night, or you can “save” all your calories for a 500-calorie dinner. You could also have them all at breakfast, but most people find that it’s easier to skip breakfast and lunch than lunch and dinner. Again, this is all based on individual preference.
[Learn more: Does Fasting Affect Women Differently to Men?]
Pros and Cons of the 5:2 Diet
Let’s start with the benefits of fasting in general. At the very least, there’s a relatively broad consensus that it doesn’t result in the body consuming its own muscle and increasing your body fat stores. A lot of research has found that whether you’re taking a day or two off of eating or eating throughout the day, if your total calories are the same at the end of the week then you’ll lose the same amount of fat and retain the same amount of muscle.(1)
This means that if you find it more convenient or enjoyable to fast all day and have a big meal at night as opposed to having smaller meals throughout the day, there doesn’t seem to be any downsides to it provided your calories are in check, you’ve spoken to your doctor, you don’t have a history of disordered eating, and you’re not living with diabetes or other conditions that might require more consistent intake of nutrients.
“So it comes down to calorie cutting and the obvious question is does calorie cutting work? Technically, yes, it does,” says Dr. Aastha Kalra, a New York-based physician who specializes in weight loss. “Whenever you go on a low calorie diet you will lose some weight, and then once you stop restricting calories it comes back. So this diet is better than going on crazy yo-yo diets because there’s some structure to it. If it’s sustainable and it works for somebody, then it’s certainly viable.”
There’s a good amount of research on other forms of intermittent fasting, but very little on the 5:2 diet specifically. However, one of the studies that has been published found that it was as effective as a regular calorie-controlled diet at producing weight loss and producing favorable changes in hemoglobin A1c levels among people with diabetes.(2)
Most people agree on that much: it’s as good as regular calorie restriction. There’s also a solid amount of research to suggest that fasting can be useful for insulin sensitivity.(3)(4) A lot of factors contribute to insulin resistance, which is when you produce more insulin than you ideally would in order to accomplish the job of moving nutrients out of the bloodstream and into the muscles and organs where they’re needed.
This is an oversimplification, but it’s a little like how consuming a ton of caffeine makes you less sensitive to it than a person who drinks coffee once a week. Eating a lot of refined carbs spikes your insulin a lot, making you less sensitive to it. Fasting means you’re not eating as much, so insulin is spiking less often, so it helps to improve your insulin sensitivity, which is also linked to improved body composition and nutrient absorption.
Insulin resistance is something of a scale, the end point of which is the body being unable to produce enough insulin to move sugars out of the blood: Type 2 diabetes. Note that a lot of factors, like stress and poor sleep, are also linked to insulin resistance, so combating it as effectively as possible requires a healthy lifestyle overall.
Dr. Kalra is a little less sold on the 5:2 diet for insulin sensitivity than she is on longer fasts that actually eliminate calories, though. (Here we should emphasize that many don’t consider 5:2 “fasting” at all, given you’re consuming calories on your fast days.)
“That’s very questionable for me,” she says. “In my patients, I prescribe sixteen to twenty-four hour fasts or longer when they have Type 2 diabetes. That’s where I see more effects on insulin and greater weight loss, as it’s ideal to really deplete your glycogen stores.”
(While Dr. Kalra does sometimes recommend her diabetic patients try fasting, it’s very important to speak to your doctor if you do have diabetes and are considering it; not every specialist recommends this approach.)
That said, remember that restricting calories, no matter when you eat, will result in weight loss even if you’re not truly fasting and depleting your glycogen. So long as your diet is helping you lose weight (and you’re in a healthy weight range), it’s helping with insulin resistance. “Truly” fasting isn’t one hundred percent necessary.
Mosley spends a lot of his documentary (and subsequent book) describing the longevity benefits of fasting. It’s not just a way to consume fewer calories — it may lead to, as the title of his show suggests, a longer life.
More research is needed but a lot of experts put this down to autophagy, a phenomenon that occurs when the body “eats” its own diseased and damaged cells and recycles them for new parts. It’s about as close to a “cleanse” as the body gets and while it’s stimulated by exercise, there’s also decent evidence that fasting increases autophagy as well, particularly in the brain.(5)(6)(7)(8) Note that most of this research has been performed on rodents.
“Autophagy is a process involving cellular cleanup and it’s regulated by protein in the body called m-TOR,” says Dr. Kalra. “But in order for autophagy to happen, although it does vary from person to person and most studies are in vitro, but we’ve seen that it doesn’t usually start until 16 or 18 hours of fasting. Autophagy happens once you’ve depleted your glycogen stores, so the autophagy benefits of the 5:2 diet are questionable if you’re consuming small amounts of protein and carbs throughout the day.”
To maximize autophagy from the diet she recommends saving your calories until as late as you can on your “fast days,” or to consume only fats during the day — say, with bulletproof coffee — as this will keep your carbs and insulin secretion low.
Again, significant longevity benefits have also been ascribed to simply following low-calorie diets regardless of whether or not you fast and autophagy is increased with exercise, so don’t think the benefits are off limits if you don’t enjoy skipping meals.
There are very few studies on 5:2 specifically, and there’s no doubt that it can be a useful way to lose weight, provided you don’t overeat on your feeding days. If your main goal is weight loss, 5:2 appears to be useful. If your main goal is longevity, the very act of calorie restriction does indeed appear to help with insulin sensitivity and countless other areas of your health even if it may not be maximizing your autophagy.
Since you’re able to follow the diet while snacking throughout your “fast” days, it’s debatable as to whether or not this is indeed a form of intermittent fasting or if you’ll enjoy the precise same benefits as true fasts. That said, it may be an easier method of attaining many of these benefits — just make sure you speak to your doctor before embarking on any new weight loss regimen.
Featured image by Timolina/Shutterstock
1. Catenacci VA, et al. A randomized pilot study comparing zero-calorie alternate-day fasting to daily caloric restriction in adults with obesity. Obesity (Silver Spring). 2016 Sep;24(9):1874-83.
2. Carter S, et al. Effect of Intermittent Compared With Continuous Energy Restricted Diet on Glycemic Control in Patients With Type 2 Diabetes: A Randomized Noninferiority Trial. JAMA Netw Open. 2018 Jul 6;1(3):e180756.
3. Halberg N, et al. Effect of intermittent fasting and refeeding on insulin action in healthy men. J Appl Physiol (1985). 2005 Dec;99(6):2128-36.
4. Horne BD, et al. Usefulness of routine periodic fasting to lower risk of coronary artery disease in patients undergoing coronary angiography. Am J Cardiol. 2008 Oct 1;102(7):814-819.
5. Alirezaei M, et al. Short-term fasting induces profound neuronal autophagy. Autophagy. 2010 Aug;6(6):702-10.
6. Li L, et al. Chronic intermittent fasting improves cognitive functions and brain structures in mice. PLoS One. 2013 Jun 3;8(6):e66069.
7. Singh R, et al. Late-onset intermittent fasting dietary restriction as a potential intervention to retard age-associated brain function impairments in male rats. Age (Dordr). 2012 Aug;34(4):917-33.
8. Uchiyama Y, et al. Autophagic neuron death. Methods Enzymol. 2009;453:33-51.