The Science Behind Why You Shouldn’t Worry About Weight Gain

Being home-bound and isolated can lead us to stray away from the diet we’ve been working so hard on — one chocolate bar here and there can’t hurt, right?

This can lead many to think; “I’m going to regain all the weight I’ve lost”.

The prospect of this might be quite anxiety-inducing and even cause some people to lose the confidence to be sociable, put a strain on relationships, and potentially miss out on enjoying new food experiences.

Understanding how weight change works and how rapidly you can regain weight can help to put your mind at ease and free you from these worries.


How weight change works

Outside of changing body composition at a steady weight, almost all contributions to body fat gain can be explained by changes in body weight. 

If we gain weight over time, some of that is likely to be fat gain.

However, short term changes in weight can actually be down to a whole host of other reasons, including:

  • Changes in levels of sex hormones1 
  • Increased fibre and food volume intake2 
  • Needing to go to the bathroom 
  • Changes in body water3
  • Inflammation 

That’s why a more gradual change in body weight is a greater indicator of weight gain, i.e. increases in lean muscle mass, fat mass, bone density etc.

This is because it relates to lean muscle mass and fat mass changes, which involve a very basic principle of  energy in vs. energy out.  Or, calories in vs calories out.

Quite simply, if your goal is to lose weight, then you need to consume fewer calories than the number required to maintain your weight — a bit like when you’re trying to save money you spend less than what comes in on your paycheck. 

If your goal is to gain weight then you need to consume more calories than required to maintain — this is not like saving money, and if you take the same approach you will be in debt very quickly!

If you aim to maintain your weight and focus more on body re-composition, then you need to eat at a calorie intake around your maintenance, and focus more on macro splits and nutrient timing. 

That’s how weight change works in a nutshell. It’s relatively straightforward and not as scary as you’ve been made to believe. However, sometimes weight isn’t the issue — it’s your body fat. 


How quickly can you regain lost body fat? 

How fast you regain any body fat depends on whether you consumed more energy than you spent over that day.

Not every single calorie consumed over your maintenance amount will be stored as fat.

If the body doesn’t burn it off through increased exercise it may also use it for muscle building or some other beneficial function. 

To get to grips with how much body fat you could gain in a day you must understand some of the following factors that are at play:

1. Your TDEE (Total Daily Energy Expenditure)

Here’s how to calculate your TDEE.


2. Your existing levels of glycogen storage 

Glycogen stores are the body’s internal stores of carbohydrate found in muscle tissue and the liver. The more glycogen we deplete, the more food that we eat will be taken in and stored as glycogen as opposed to fat.4


3. Your dietary composition that day 

Studies have shown that protein overfeeding, or the consumption of a high protein diet, may not result in a gain in body weight or fat mass,5 both short and long-term, despite consuming calories that exceed your perceived maintenance intake. 

When investigating differences in fat storage following overeating between carbohydrates and fats with a 50% calorie surplus, it was shown that after one day of overfeeding, those who overfed on carbohydrates stored only 10% of that as fat. Those who overfed on fat, however, stored 60% of that as fat.


4. Increases in NEAT (Non-Exercise Activity Thermogenesis) variation 

NEAT is simply all the calories we burn thanks to the activity which isn’t programmed exercise. This can be things we overlook, such as fidgeting and walking around the house.

This is important as NEAT varies greatly between person to person and increases in response to overfeeding.7


5. Your current body composition 

Those with greater levels of body fat have more fat cells to store fat. Those who are leaner are likelier to have less. So, leaner individuals may have a reduced capacity to store fat. 


Don’t stress about the slip-ups

Another factor that has an effect on the fat gain is the frequency that you overeat/overfeed the body.

A one-off splurge isn’t going to do much in the grand scheme of things, it’s when it happens repeatedly that you will see an eventual, notable gain in body fat. 

Studies have shown that, even in groups who revert to old habits after a diet intervention, the rate of total body weight regain is only a mere 0.04kg a month.8

This number decreases even further when the groups are informed about, and taught the skills to adequately track calorie intake.


On top of that, the following things can help to prevent gaining weight:

  • Doing exercise
  • Eating a higher protein diet
  • Having a diet of greater nutritional density, quality and consistency

So it’s more than likely that you’re already doing the right things to combat regaining any significant amount of body weight.


Take-Home Message

Weight or fat regain is fairly simplistic in how it works, it’s a straight-forward equation — the balance of calories in vs. calories out.

Just to be clear, we definitely aren’t recommending you stop eating because you’re not going to the gym — that’s dangerous. Just make sure you balance the equation to suit your needs and activity level. Everyone is different.

Stick to what you’ve been practising — exercise, a high-protein diet, more fruits and vegetables, and being kind to yourself. The odd chocolate bar here and there won’t kill you or ruin your diet.

Keep active, mind your mental health, and just keep doing what you’ve been doing. Chances are you’ll come out of this leaner, fitter, and healthier than ever. But if you don’t, it’s nothing to punish yourself for. 

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Our articles should be used for informational and educational purposes only and are not intended to be taken as medical advice. If you’re concerned, consult a health professional before taking dietary supplements or introducing any major changes to your diet.

1. Watson, P. E., & Robinson, M. F. (1965). Variations in body-weight of young women during the menstrual cycle. British Journal of Nutrition19(1), 237-248.

2. Reale, R. (2018). Acute weight management in combat sports: pre-weight-in weight loss, post weight-in recovery and competition nutrition strategies. Sports. Sci. Exch29(183), 1-6.

3. Benton, D., Braun, H., Cobo, J. C., Edmonds, C., Elmadfa, I., El-Sharkawy, A., … & Kenney, W. L. (2015). Executive summary and conclusions from the European Hydration Institute expert conference on human hydration, health, and performance. Nutrition reviews73(suppl_2), 148-150.

4. Flatt, J. P. (1995). Use and storage of carbohydrate and fat. The American journal of clinical nutrition61(4), 952S-959S.

5. Leaf, A., & Antonio, J. (2017). The effects of overfeeding on body composition: the role of macronutrient composition–a narrative review. International journal of exercise science10(8), 1275.

6. Altiındağ, E., & Baykan, B. (2017). Discover the world’s research. Turk J Neurol23, 88-89.

7. Levine, J. A. (2002). Non-exercise activity thermogenesis (NEAT). Best Practice & Research Clinical Endocrinology & Metabolism16(4), 679-702.

8. Johns, D., Hartmann-Boyce, J., Aveyard, P., Onakpoya, I., Jebb, S., Phillips, D., … & Perera, R. (2014). Weight regain after behavioural weight management programmes. PH53 Overweight and obese adults-lifestyle weight management: supporting evidence: Review 1c. NICE.

Jamie Wright

Jamie Wright

Writer and expert

Jamie Wright holds an MSc Degree in Human Nutrition and a BSc (Hons) in Sports and Exercise Science, and now works with multiple organisations as well as running his own private nutritionist coaching services company, Balance, along with his team of qualified experts, to help individuals with their nutritional goals. He is accredited with the Association for Nutrition and has helped hundreds of clients; from those with eating disorders to internationally competing athletes. Jamie supports his clients with evidence-based, holistic nutrition programming to reach their health and fitness goals. In addition to running his practice, Jamie regularly contributes to the field of nutrition presenting and writing on its many facets. He has had his research presented at the UK Obesity Congress as well as overseas conferences and has authored several e-books whilst contributing to others (including charitable sporting organisations). His research has centred around weight management as well as sports / exercise performance and supplementation. A massive sport nut, avid gym goer and lover of all things dog related, Jamie’s goal in sharing the experience and knowledge he has gained academically and professionally is to provide a source of clarity in the vast amount of “misinformation and noise” that exists within the health and fitness industry. You can check his work out further at Balance, @balance_ie or @jamiesdietguide on social media.

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