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Can one bad day of food REALLY ruin your whole diet? Let’s look at what science says…

can one bad day ruin your diet

This article:

6 min read

Sometimes, despire our best intentions of getting leaner and fitter, our diet goes out of the window and we smash through our calorie target.

Heck, I’ve done it myself.

But how damaging can these days be?

I mean, it’s only one day. Is it really possible that one bad day can ruin your whole diet?

I’m going to dive into the science on this and answer that question for you, and I’ll also show you a few neat tricks you can use to ramp up your results even if you’re not 100% on track.

Here’s Why > Ruin < Is The Wrong Word

am I not eating enough to lose weight

Your diet is ultimately a numbers game.

Sure, we can dress it up with more complex terminology like carb-cycling, intermittent fasting, ketogenic, etc, but the net result is that you’ll gain weight if you eat more food than your body needs.

So let’s look at how a typical bad day can throw your numbers out of sync.

These unplanned days of over-eating can throw your numbers our of sync, and that’s what causes weight gain instead of weight loss.

For example, if you currently eat 1900 calories per day but Tuesday goes to hell and you eat 3000 calories, your numbers may look like this:

  • Monday: 1900 cals
  • Tuesday: 3000 cals
  • Wednesday: 1900 cals
  • Thursday: 1900 cals
  • Friday: 1900 cals
  • Saturday: 1900 cals
  • Sunday: 1900 cals

See the red line above?

That spike caused your average daily calories to rise to 2057, which is higher than your target of 1900. This pushes you above your maintenance level and might prevent weight loss (could even cause a little bit of weight gain).

The good news is these days happen to everyone.

I can recall many times when I’ve woken in the middle of the night with an empty “family-sized” (lie) chocolate wrapper next to me while Shaun T yelled through the television screen about “I-N-S-A-N-I-T-Y!!!”.

But here’s the thing…

You haven’t “ruined” anything. If you really wanted to ruin your diet, you’d need to do this over a prolonged period of weeks and months.

In fact, it’s often this false belief that we’ve ruined things which causes people to hit the “f**k it button” for the rest of the week, and that’s where the real damage is done.

Honestly, if you have an off-day and want to keep things on track you have some pretty straightforward options to do that. My clients have used all of these over the years. See below.

You Can Still Average It Out Over The Whole Week


Your body doesn’t care what day of the week it is.

As long as you hit your numbers for the week, that’s all that counts. This gives you a bunch of options you can use if you need to counter-balance your bad days in the rest of the week.

Some people like a more agressive approach; i.e. they ate 1100 extra calories so they’ll go 550 under target for the next couple of days and get their calories back up to maintenance for the weekend…

… and some people like to make the deficit as small as possible; i.e. they’ll go 220 calories lower for the rest of the week.

It doesn’t matter which approach we use. In both situations we’ve created a weekly average of 1900 calories, bringing you right back on track despite a higher day.

See how it works:

can cheat day ruin your diet

Or You Can Add Extra Cardio To Burn Through It

does one bad day ruin your diet

Burning extra calories in the gym gives you an alternative way to balance your calories – this time without impacting the food you eat.

However, I’m not a fan of this approach and I recommend using it sparingly.

You see, there’s an old fitness saying:

“If you create a big enough fire, you can throw anything on it.”

This is true, but I would also add that the bigger the fire, the more chance you’ll get burned. In this case I’m talking about the risk of injury, which would rise significantly depending on how big the surplus was.

Also, a 2021 study pubslished in Current Biology shows that more exercise doesn’t always mean better results. Interestingly, resecarchers from the University of Ottawa discovered that our basal metabolic rate (BMR) slows by an average of 28% when we markedly ramp up physical activity. They also suggested that people with more fat tissue could get even greater compensation, underlining the important role nutrition plays in reducing body fat. (1)

Or You Can Use The Clean Slate Method

how to get back on track after a cheat day

I began implementing this simple technique with clients about ten years ago, and it’s had a great success rate.

You simply wake up tomorrow and start afresh!

Don’t feel guily about it… don’t beat yourself up… don’t judge yourself for it… just move forward and re-focus your efforts from now.

This strategy really comes into its own when trying to build a healthier relationship with food (us fitness folks tend to be way too hard on ourselves in this regard), and it’s also great in situations where your calorie surplus is just too difficult to balance out.

For instance; if your target was 1900 calories per day but you ate a massive 5000 calories in one day this means you’d need to go down to 1280 for the rest of the week in order to balance it out. It could be done, but it would take a f**kload of determination and would leave you feeling snappier than an extra from The Walking Dead, which defeats the whole point of trying to eat healthier in the first place (because you want to feel great, right?!).

So in those situations I recommend just getting back to your maintenance calories the following day and chalking it up to experience.

This technique is also useful in the beginning stages of a diet plan, because there’s nothing worse than beating yourself up. Provided it isn’t a regular occurrence it will not hold you back (other than perhaps slowing your progess for that week). If those crazy days do become a regular thing, I’d take it as a sign from your body that your daily calorie target has been set too low and is simply not sustainable.

Which Strategy Do You Like The Most?

can one bad day ruin your whole diet

I’ve personally used all three of these over the years, and I’ve passed them on to thousands of clients, too.

They all work.

But the main thing is that we create a diet which gives us flexibility and sustainability. That’s how real transformations are achieved in the long-term. (2, 3)

So pick your favourite, go with it, and see how it works for you.


  1. Halsey L. G., et al. Energy compensation and adiposity in humans. Curr Biol (2021).
  2. Hall K. D., et al. Maintenance of lost weight and long-term management of obesity. Med Clin North Am (2018)
  3. Stewart T. M., et al. Rigid vs. flexible dieting: association with eating disorder symptoms in nonobese women. Appetite (2002).

Who Is Russ Howe PTI?

russ howe pti

Featured in Men’s Fitness magazine and voted in the world’s top 50 fat loss coaches by HuffPost, Russ is among the UK’s most subscribed personal trainers with 104,357 people getting his free weekly fitness tips e-mail.

In the gym, clients range from busy parents, to models, to athletes and actresses. Russ also worked alongside the UK government for 8 years in a venture combating childhood obesity in England.

Outside of the gym, he’s a proud Dad to three young boys.

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