Fad Diets Ripped Apart: The 5:2 Diet

Fad Diets Ripped Apart: The 5:2 Diet

The 5:2 Diet

You may have heard of a fad diet sweeping across the country called the 5:2 diet. You may even know several people who have done, or are doing, this diet and have lost weight with it.

Yes, this diet can work, but in my opinion it is one of the stupidest diets you can do.

What the 5:2 Diet Is

If you go on this diet, you will have to eat only 500 calories for 2 out of the 7 days of the week. On the other 5 days, you do not track calories .

Why the 5:2 Diet is Stupid

  • The 5:2 diet kind of does the right thing by putting you into a calorie deficit over the long term (if you follow it correctly). However, it also makes the process miserable by allocating your calories in a stupid way that makes you feel like shit for 2 days of the week.
  • It won’t work for everyone because some people will manage to eat enough on the untracked days to take themselves out of the deficit they’ve created on the two 500 calorie days. If they just eat “normally”, they’ll eat in the way that got them overweight in the first place, potentially undoing what they’ve done on the two days of misery.
  • Some people see “untracked” as “unlimited” and go crazy on 5 days of the week, easily erasing the calorie deficit and actually going into a calorie surplus that makes them gain weight.
  • 500 is an arbitrary number that does not take into account the different maintenance calories of different people.

Example 1: Jim (tall male with considerable muscle mass but also some fat to lose)

  • Weight: 100kg
  • Activity level: Heavy weight training 3-5 days per week, plus regularly walks 10,000+ steps per day.
  • Maintenance calories: 3,200
  • Size of calorie deficit on the 500 calorie days: 2,700 (x2 = 5,400)
  • How long it will take to lose 10% of Jim’s starting weight IF he eats at maintenance on the other 5 days: 21 weeks.
  • Hunger level on the two 500 calorie days: A million out of 10.
  • How quickly Jim could lose 10% of his starting weight by eating 2,200 calories EVERY day, instead of 500 on 2 days: 16 and a half weeks.

If Jim follows the diet strictly, he would lose 10kg in around 21 weeks. This is because he is eating at maintenance for 5 days of the week, and creating a weekly deficit of 5,400 calories on the other two days. As it takes a deficit of roughly 3,500 calories to burn off a pound of body fat, simple maths tells us that it will take Jim roughly 21 weeks to lose 10kg.

As Jim is a big, active guy with a lot of lean mass (muscle), he has a higher amount of maintenance calories. His body simply requires more energy to sustain itself and fuel his activity over the week. For this reason, eating 500 calories will be absolutely miserable for Jim on the days that he does it because it is such a huge deficit (85% under his maintenance!)

What’s more, he could eat a much more manageable 2,200 calories every day, even fitting in some tasty treats that others would call “bad” food, and create a BIGGER weekly deficit of 7,000 calories vs. 5,400. This would result in Jim losing 10kg in about 16.5 weeks.

And because he’s not starving himself for 2 days of the week, he won’t have the temptation to binge on his “untracked” days like some people on this diet do.

Example 2: Anne (short female, with very little muscle mass and considerable fat to lose)

  • Weight: 85kg
  • Activity level: Anne walks the dog a few times a week
  • Maintenance calories: 1,700
  • Size of calorie deficit on the 500 calorie days: 1,200 (x2 = 2,400)
  • How long it will take to lose 10% of Anne’s starting weight IF she eats at maintenance on the other 5 days: Roughly 27 weeks.
  • Hunger level on the two 500 calorie days: Hungry, but nowhere near as hungry as Jim who NEEDS more food.
  • How quickly Anne could lose 10% of her starting weight by eating 1,200 calories EVERY day, instead of 500 on 2 days: Roughly 18.5 weeks.

The 5:2 diet will also work for Anne, but she can lose the weight much faster and without restricting herself on 2 days of the week, if she just eats 1,200 calories every day. This will be a far more pleasant experience!

1,200 calories will be manageable for Anne. I got to this figure by taking the same % deficit as Jim (2,200/3,200) and then rounding up.

I wouldn’t recommend 1,200 calories for Jim because he has a much higher activity level and much more lean mass (muscle), therefore requiring more calories to sustain this. Men usually have more lean mass naturally, as do taller people. People experienced with strength and weight training obviously have more lean mass too.

Both these examples assume that while doing the 5:2 diet, the subjects do not overshoot their maintenance calories on the 5 “untracked” days.

Unfortunately, people less in tune with fitness and less knowledgeable about the macro and calorie contents of foods will not be able to accurately estimate or “eyeball” their calorie intake on the off days. This makes them more likely to shoot past the maintenance figure, further slowing their weight loss.

Others take “untracked” to mean “unlimited” and go all out on these 5 days (especially right after the two 500 calorie days) completely erasing all progress and even resulting in a calories SURPLUS over the week, meaning gaining weight despite doing the 5:2 diet!

Note: It’s important to point out that I’m not picking on women (or anyone called Anne) with this example. I have only used a woman as an example to demonstrate the important point that the vast majority of women need to eat fewer calories than men to sustain their weight. I could also use the example of a 5’5 man who is very inactive and will need fewer calories than a taller, more active man.


The 5:2 diet MAY work for you but it only works the same way ALL weight loss works: By putting you into a calorie deficit.

You can lose weight much more effectively and without punishing yourself by just figuring out your maintenance calories, and eating at a moderate deficit.

How to Work Out Your Maintenance Calories?

You can use an online calculator. Do a Google search for “TDEE calculator” (TDEE stands for “total daily energy expenditure”).

I find this one to be fairly good.

Important: Results from these calculators are just ballpark figures. You may need to revise the numbers up or down slightly. The calculators don’t really take into account things like if you walk an hour to and from your workplace every day vs. someone who gives the same answers as you but drives to work and sits down all day. This will effect maintenance calories.

Let Me Work It Out Your Maintenance Calories for You And Tell You How Much to Eat to Lose Weight

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