Last Tuesday, I ate some green beans, a Clif bar, and one homemade sous vide egg bite. That’s 2000kj. I swear I don’t have an eating disorder — it’s just how you do things on this diet I’ve been trying. On Wednesday I was back to my regular 8400-ish kj and feeling fine.
Illustration by Elena Scotti/Lifehacker/GMG, photos via Shutterstock
On the 5:2 diet, you “fast” for two days a week, say Monday and Thursday. You eat 25 per cent of your usual kilojoules on those days. The other days, you eat normally. By the end of the week, you’ve eaten a similar number of kilojoules those suckers on 6300 calorie regimens, but you only had to spend two days dieting.
The diet has been wildly popular in the UK for a few years, starting with a 2012 BBC documentary. When I first heard about it, I almost liked the idea. I’d previously done a form of intermittent fasting that basically amounts to skipping breakfast and delaying lunch. I focus better in the mornings this way, and I seem to eat healthier overall, but it does take some getting used to.
Fasting for a whole day sounded too hard, but then again, it seems like half of England does it (including Benedict Cumberbatch). I had a few kilograms to lose, and I get frustrated and hungry with traditional kilojoule-counting diets — so I thought I’d give the 5:2 a try.
Why Would Somebody Do This?
No matter what a diet claims as its raison d’être, let’s be real: Most people are interested in weight loss. That’s true here too. This diet, as well as others that fall under the intermittent fasting umbrella, is also supposed to improve the function of your brain, heart and metabolism.
The appeal is that you don’t have to stick to the 5:2 diet for more than a day at a time. Sure, it’s a rough day, but it’s nothing like the Whole30 diet, where you commit to a strict set of rules that forbid bread and sugar for a month. And it isn’t the endless slog of a typical kilojoules-in, kilojoules-out diet for long term weight loss, where you need to watch what you eat for months or years.
The first day I tried fasting, I was ravenous by the end of the day. I pulled up Facebook on my phone to distract myself, and my friends were sharing their favourite curry recipes with full-fat coconut milk. I had a pang of jealousy but then thought, “Hey, I can make that tomorrow.” How many diets let you say that on the first day?
Does It Work?
The 5:2 diet itself hasn’t been rigorously studied, but a close relative, alternate-day fasting, has some data to back it up. Alternate-day fasting works as well for weight loss as traditional diets, according to studies like this one published in Obesity. But it works better for some people than others, and researchers are trying to figure that out, too. One trial published in Obesity Research and Clinical Practice found that white people and older people were more successful on the diet, but there were a ton of factors they didn’t account for, like whether some groups had better access to healthy foods. Men and women had equal luck on the diet.
The other health benefits, besides weight loss, are not as easy to pin down. We’ve long known that mice live longer when they’re underfed. There have been a ton of studies trying to figure out what other benefits come from kilojoule restriction, and whether they can translate to humans in safe and practical ways.
For example, fasting should help your body learn to manage blood sugar better. If you overeat, your cells can become resistant to insulin, eventually leading to type 2 diabetes. Dieting and exercise both seem to reverse this effect, and from what we know about the way the body manages blood sugar, fasting should help even more. Unfortunately, we don’t have enough evidence to say if that’s actually what happens.
The book explaining the 5:2 diet, The Fast Diet, is more honest about this than I expected a diet book to be. Author Michael Mosley, a doctor turned BBC presenter, tried several fasting-based diets for his Eat, Fast, and Live Longer documentary. He explains that he settled on the 5:2 pattern as a compromise between the different methods. Essentially, it’s based on his experience and gut feelings. I’m OK with that. We don’t have enough evidence to say that fasting (or even dieting) must be done a certain way, so if 5:2 is tolerable and has a decent chance of working, I figure it’s worth a try.
What Is a Fasting Day Like?
They’re tasty, I swear.
I wake up thinking about breakfast. That’s not off the menu, but I only have a 2000kj budget for the day, and I’d rather save it for later. So I fill my belly instead with some kind of beverage: Coffee, water, carbonated water, diet coke. The craving usually subsides.
If it doesn’t, I’ll go for a snack of veggies, since they are nearly kilojoule-free. I go for a microwave-steamable bag of fresh green beans. With plenty of salt and pepper, it’s flavorful and almost filling, and the entire bag is just 520kj. I split it into a morning and evening snack. [Ed note: Ew. Microwaveable green beans for breakfast.]
Either way, I’m usually fine until 2PM. If I’m having trouble concentrating at work, then I know it’s time to have some real food. This could be a carefully measured portion of pretty much any food, but I don’t see the point in researching recipes, shopping and cooking up a snack-sized meal. (The Fast Diet has plenty of recipes, though, if you prefer this approach.) More often than not, I choose a Clif bar: Around 1000kj, depending on the flavour. I find it about as satisfying as you can expect a 1000kj meal to be.
I don’t get ravenous until evening, and that’s the hardest part for me. I’ll prowl around the kitchen, debating how to spend my last 400 or 800kj, making myself even hungrier in the process. I like to have something dense and filling, like an egg bite, but it’s never enough. Tomorrow, I have to tell myself. I can have more tomorrow.
In fact, it’s easiest to just stay out of the kitchen and keep your mind off food. Fasting is easiest on the days I’m busy at work, and hardest on a weekend when I might have time to lounge around or find myself at a party.
Can You Exercise While You’re Fasting?
I never tried it, until recently. I just scheduled my workouts and my fast days so they didn’t coincide. But yesterday I thought, hey, why not try going for a run and write about how terrible it feels?
It was not terrible. I was shocked: It was afternoon and I hadn’t had anything besides Diet Coke and carbonated water all day. I brought a Clif bar just in case, but ended up jogging for the better part of an hour without feeling any hungrier than when I was sitting at my desk. I ran at my usual speed, and even ran longer than I had originally planned. (Bad judgement induced by hunger? I won’t rule it out.) And afterwards, I set the Clif bar aside and didn’t have a bite until evening, when I realised I could treat myself to a 1700kj super burrito.
Some people say they have more energy when they exercise on an empty stomach. I always figured that was something you could get used to, but I didn’t expect to experience it on the first try. Maybe it was a fluke. Maybe not.
What Do You Eat When You’re Not Fasting?
It’s like the see-food diet. You see food, you eat it.
When it isn’t a fast day, you’re supposed to eat “normally”. Nothing is off-limits, but whether you’re using this to lose weight or just a healthy lifestyle, you’d be smart to eat your veggies and protein and not too many cupcakes.
I do get a little hungrier the day after a fast (although it’s easy to skip breakfast, even if I was hungry at bedtime). But there’s no guarantee you’ll end up pigging out. “A calorie slash of 75 per cent on a fast day generally gives rise to a little more than a 15 per cent increase on the following feed day,” Dr Mosley and his coauthor, Mimi Spencer, write in The Fast Diet. They cite this study, which also noted that people feel less hungry on fast days by the second week. (Again, this research is on alternate-day fasting rather than the 5:2 diet).
I find that, mentally, I can’t use a diet tracker like LoseIt on my non-fasting days. The tracker has decided I should eat 5900kj a day, so on fast days, it tells me that I’m doing a good job but also displays a warning that this doesn’t seem like enough food. Fair enough. But then the rest of the week, I’m tempted to stick to that 5900kj goal, which is not how the 5:2 diet works. I never felt ready for another fast day, because I always felt a little starved. So I ditched the tracker, and easily got back into the 5:2 rhythm.
MyFitnessPal can do different calorie goals for each day, if you pay for a premium subscription. prefer to go without a tracker and let the calories fall where they may.
Should I Try It?
This definitely isn’t the diet for everyone. If you’re happy with a plan that asks you to undereat just a little every day, stick with that plan. You’re not missing out on much.
But if you have a hard time sticking to a typical kilojoule-restriction diet, you might find intermittent fasting easier to take. The 5:2 diet was designed to be a form of fasting that is easy to stick to: You get some food rather than none, and you can schedule your fast days to always fall on whichever days work for you. You can even put them back-to-back if you want, but even Dr Mosley admits that’s too difficult for most people to handle.
The authors note that there’s nothing magic about 2000kj (they even allow 2500kj for men, or just 25 per cent of what you would normally eat). Likewise the schedule: They suggest you cut down to just one fast day a week if you’re maintaining your weight, or you can do three fast days if you’re having fun and want to speed up weight loss.
They suggest another tweak that I found really helps: Doing a 24-hour fast instead of trying to make it through a night, a full day, and then another night. This scheme is so easy I’ve found myself doing it by accident. Have a big, late lunch, say around 2PM, and then skip dinner. When you wake up, if you can skip breakfast, then all you have to do is push off lunch until 2PM again, and bingo — you’ve done a 24 hour fast.
I did lose a few kilograms while trying the 5:2 diet, although I didn’t do it consistently, and I also also changed some other things in my life around the same time, like exercising more. I’m not ready to give the diet full credit, but so far I’m happy with it.
Whether this diet works for you will probably depend on how you spend your time (do you have a busy work schedule?) as well as how you handle hunger and willpower. It’s definitely doable, and even though it’s trendy, it’s refreshingly free of pseudoscientific claims. If you’re ready to give it a try, read the ground rules on the Fast Diet website and snoop the forums there to pick up some tips, then pick a busy day and stay out of the kitchen.