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Why eating 3500 calories less a week is bad advice

Posted on | Last updated 06-09-2016

“Bro, you’ve gotta go low carb to get lean”

“If you cut out gluten you’ll shed fat”

“Eat fat, burn fat and get shredded”

Regardless of how you spin it, we know that when stripped back to basics, fat loss comes down to consuming fewer Calories than you expend.

You give the body less energy than it needs and it uses its own energy stores such as body fat to compensate. Simple.

Whilst this is true, the idea that it means a daily Calorie deficit of 500 Calories will equate to 1lb of weight loss per week isn’t quite so much.

It stems from the fact that 1lb of body fat equates to roughly 3500 Calories. Based on this, simple maths will tell you that reducing your daily intake by 500 Calories below your maintenance requirements (when intake = expenditure) will add up to a deficit of 3500 Calories over a week and a subsequent loss of 1lb.

The Maths

-500 kcal x 7 days

= -3500 kcal per week

= -1lb weight loss per week.

The body is far from simple though and a basic math equation cannot predict what goes on under the hood.

We don’t JUST lose body fat

The first problem with this line of thinking is that it assumes all the weight lost is body fat. Whilst it would be great if this were the case, in humans, it rarely is and typically some combination of stored energy in the form of body fat, glycogen (stored carbs) and muscle is used to compensate for the 3500 Calorie deficit. Where our bodies end up taking this energy from will depend on a number of factors.

For example, when you first start dieting you’ll lose more fat free mass (stored carbs, water, muscle) (1) and this is why weight loss is often pretty rapid when starting out as Calorie for Calorie, fat free mass weighs more than fat. You’ll also lose more fat free mass if you don’t eat enough protein (2, 3) and as you get to low body fat percentages (4).

On the other hand, the more body fat you have, the greater proportion of fat you’ll lose (4). Likewise, a high(ish) protein intake may lead to more fat loss (5, 6).

So ultimately the energy stores that contribute to weight loss from dieting are typically some mix of body fat, stored carbs and muscle

And because the energy density of these vary, the weight you lose will differ depending on where the net loss of energy is from. For example, 100 Calories lost from stored carbohydrates or muscle weighs more than 100 Calories lost from body fat.

For arguments sake though, let’s say all the weight lost is body fat. Does the 3500 Calorie rule hold true?

Again, the answer is no.

This is because it assumes the Calories in vs Calories out rule is a static one, when in reality the goalposts are constantly shifting. That is, our Calorie intake is never 100% consistent and nor is our expenditure. They change in response to dieting and reduce the size of our Calorie deficit as a result.

Calories in vs Calories out is NOT static

Let me explain:

Energy Intake

When considering energy balance (Calories in vs out), the intake side of the equation is the Calories that are actually available to us from the food/drink we consume after the digestion and processing of food.

In other words, just because an apple contains 70 Calories, you won’t necessarily get the whole 70 Calories from it as digestion of food is not a perfect process. Gut bacteria, stress and a foods level of processing can all impact the Calories available to us from food so restricting your food/drink consumption by 3500 Calories doesn’t necessarily lead to a 3500 Calorie deficit.

Energy Expenditure

Total daily energy expenditure (aka metabolic rate or energy out) can be split into three components, which may all change as a result of dieting and losing weight/fat.

1. Basal metabolic rate (BMR)

This is the energy used at complete rest. e.g. sack of potato state/lay in bed all day not moving, eating or drinking.

BMR is largely dictated by bodyweight and therefore it’s not surprising that it will decrease with weight loss. Also what many people don’t realise is that body fat is metabolically active tissue, meaning it uses up energy. In actual fact, the energy body fat uses when we’re at rest isn’t a lot less than muscle (2kcal/lb vs 6kcal/lb per day) (7).

In addition to losing metabolically active tissue, hormonal changes may also occur when we diet for long periods of time and we may also become more efficient at using energy (8). Again, these changes mean that the ‘energy out’ side of the energy balance equation will decrease as you diet and lose body fat. I will point out that these changes in BMR aren’t as dramatic as many people make out once weight loss is accounted for though (9). More on this in a minute.

2. Thermic effect of food (TEF)

Similar to notion that you’ve got to spend money to make money, you have to spend energy to get energy from the food/drink you consume. This is the thermic effect of food and is defined as the energy used for the digestion, absorption, and storage of food (10). The amount of energy used will depend on what you eat as well as how much you eat, however, typically TEF accounts for around 10% of total Calories out/metabolic rate (8, 10). So seeing as decreasing daily Calorie intake by 500 kcal means decreasing food intake, it’s no surprise that TEF is less when dieting, as there will be less food to process.

For example, say someone was eating 3000 kcal/day at maintenance. 

10% of this to account for the TEF is 300kcal. 

If this person were to drop down their Calorie intake by 500 kcal to 2500 kcal/day in an effort to reveal their six pack, then the thermic effect of food would decrease to around 250kcal (10% of 2500kcal). So from the TEF alone that’s an instant 50kcal drop in metabolic rate/Calories out and their 500kcal daily deficit would change to a 450kcal deficit. 

3. Activity thermogenesis

Activity thermogenesis can be further divided into exercise activity thermogenesis (EAT), which is energy used during purposeful exercise like playing sport or lifting weights

And non-exercise activity thermogenesis (NEAT), which accounts for the energy used for all other activities like walking, fidgeting and even maintaining posture.

First off, if you’re eating fewer Calories your performance in the gym may suffer over time. This will most likely decrease EAT to some extent.

Secondly if you’re carrying less weight as a result of dieting you won’t have to expend as much energy moving about during exercise and whilst going about your day.

Thirdly, our bodies get lazier when we diet and we subconsciously decrease our NEAT. This will result from behavioural changes such as less standing and fidgeting compounded by more sitting and slouching. The extent that NEAT changes when dieting may vary a lot between individuals. That is, some people may decrease their activity levels and therefore metabolism more dramatically than others (10, 11, 12).

But Don’t Lose Hope

Whilst I’ve gone on a lot about some of the factors that decrease metabolic rate as a result of dieting, this is just me splitting hairs and is merely to prove a point. It’s important to note that these metabolic changes are a natural consequence of dieting and the best way to minimise any potentially negative adaptations is to take a sensible and gradual approach to dieting (which I will outline in a minute).

Also the extent that these adaptations decrease metabolism is often massively exaggerated with claims of fat gain whilst on sub 1000 Calories and tonnes of cardio. In these cases, some form of misreporting (intentional or unintentional) or subconscious eating is most likely going on. In reality, at the higher end, decreases in metabolic rate that aren’t explained by weight reduction or changes in NEAT might add up to around 15-20% (9). This might equate to 300-400kcal for a large person; certainly not enough to gain fat on sub 1000kcal alongside lots of cardio.

So, as you can see, all these changes decrease the size of the Calorie deficit you create and will therefore have a knock on effect on your rate of weight loss. Ultimately this means the 3500 Calorie rule overestimates weight loss (13).

But what can you take away from all of this?

First off, whilst the whole ‘3500 Calories = 1lb of fat’ isn’t necessarily correct, that’s not to say decreasing your intake by 500kcal/day isn’t an effective way to lose fat; it is and provided you’re consistent with this you’ll see great results over time. I will say that a better way to estimate your Calorie needs for fat loss is to work with percentages though. So the first take home is:

Take Home 1. Use a percentage based approach to work out your calorie deficit

This is because it will scale proportionally based on your size/current intake.

Say, for example, someone with a maintenance requirement of 2200kcal/day were to apply the 500 Calorie rule and reduced their intake to 1700kcal. That’s a ~23% drop in Calories which would be a much more challenging change to make compared to a larger and/or more active person dropping their maintenance intake of 3500kcal to 3000 (~15% change).

So as a general rule go for a 10-25% Calorie deficit, aiming for a 0.5 to 1% loss of bodyweight per week. This rate of weight loss will help maximise the proportion of weight lost as fat and therefore maintain your gains.

Another way to lessen the potential negative physiological and psychological impacts of dieting is having some form of diet break.

Take Home 2. Have periodical diet breaks

As the term suggests, a diet break is a period of time where you’re no longer in a deficit. This might seem counterintuitive when you’re trying to shed fat, however, it plays a number of important roles.

  • Notably, the reduction in energy expenditure/metabolic rate I’ve outlined above is a natural consequence of sustaining a Calorie deficit for a prolonged period of time and coming out of a deficit for a week or so may reverse some of this metabolic ‘slowdown’ as well as boost performance and energy levels
  • Also, having a break from dieting will make your fat loss journey suck less in general by giving yourself a psychological break from restricting your intake as strictly

To implement diet breaks, simply bump up your calorie intake (mostly from carbs) to maintenance requirements for 1-2 weeks every 8-12 weeks or so (the appropriate frequency will depend on how lean you are. Ie: less body fat = greater frequency). This approach to dieting will help preserve your gains whilst losing fat, maintain your performance and will also make your diet suck less. Win, win, win.

The final take home point is probably the most important as it revolves around your mind-set when it comes to dieting.

Hopefully by digesting what I’ve outlined above (excuse the pun) you now understand that the body is hugely complex and whilst the calories in vs out rule is inarguably true, it is too simplistic to assume that we can work out the Calories going in and out of our system with 100% precision using food scales, phone apps and online calculators. There are just too many variables that we cannot account for and therefore these tools ultimately provide us with estimates of our actual intake and expenditure.

That doesn’t mean that working out your needs using a decent Calorie/macro calculator, portioning out your food and tracking it on myfitnesspal is an ineffective way to shed fat, far from it in fact. It just means that you don’t have to stress out about that grain of rice that escaped the food scale or the 5 or so grams of carbs you went over on your daily target.

So the final take home is:

Take Home 3: Don’t get too hung up on hitting your Calories/macros bang on the whole time.

A practical example of how to go about this is to define ranges for your Calorie/macro targets, personalizing the precision you aim for to your goals and then aiming to consistently hit these ranges the majority of the time.

For example, if you’re already pretty shredded and looking for that last half a pound of fat loss before a photo shoot, you might aim to be within +/- 5g of your macro targets and +/- 100 Calories. This narrow range is more suitable at low levels of body fat when you’ve got less room for error.

On the other hand, if you’re just looking to cut down a few pounds for the beach you may aim to be within +/- 10g for your macros and a little more lenient with Calories.

Or you may even decide to use an entirely different approach altogether.

Either way, this quote from Alan Aragon is a good one to end on as it puts things into perspective:

"You should spend 10% of your energy on diet, 10% on training and 80% on having a f'ing life”.

If learning more about the science behind nutrition and the practical application of that science appeals to you, if you want to improve your health and performance or that of your clients, team mates or family the BTN Practical Academy opens it's doors for another 12 months of interactive learning soon.

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  1. Heymsfield, S. B., Gonzalez, M. C., Shen, W., Redman, L. and Thomas, D. (2014). Weight loss composition is one‐fourth fat‐free mass: a critical review and critique of this widely cited rule. Obesity Reviews15(4), 310-321
  2. Pasiakos, S. M., Cao, J. J., Margolis, L. M., Sauter, E. R., Whigham, L. D., McClung, J. P. and Young, A. J. (2013). Effects of high-protein diets on fat-free mass and muscle protein synthesis following weight loss: a randomized controlled trial. The  Federation of American Societies for Experimental Biology Journal27(9), 3837-3847
  3. Mettler, S., Mitchell, N. and Tipton, K. D. (2010). Increased protein intake reduces lean body mass loss during weight loss in athletes. Medicine & Science in Sports & Exercise,42(2), 326-37
  4. Hall, K. D. (2007). Body fat and fat-free mass inter-relationships: Forbes's theory revisited.British journal of nutrition, 97(06), 1059-1063
  5. Layman, D. K., Boileau, R. A., Erickson, D. J., Painter, J. E., Shiue, H., Sather, C. and Christou, D. D. (2003). A reduced ratio of dietary carbohydrate to protein improves body composition and blood lipid profiles during weight loss in adult women. The Journal of nutrition133(2), 411-417
  6. Longland, T. M., Oikawa, S. Y., Mitchell, C. J., Devries, M. C. and Phillips, S. M. (2016). Higher compared with lower dietary protein during an energy deficit combined with intense exercise promotes greater lean mass gain and fat mass loss: a randomized trial. The American journal of clinical nutrition,103(3), 738-746
  7. Wang, Z., Heshka, S., Zhang, K., Boozer, C. N., & Heymsfield, S. B. (2001). Resting energy expenditure: systematic organization and critique of prediction methods.Obesity research, 9(5), 331-336
  8. Trexler, E. T., Smith-Ryan, A. E., & Norton, L. E. (2014). Metabolic adaptation to weight loss: implications for the athlete.Journal of the International Society of Sports Nutrition, 11(1), 1
  9. Johannsen, D. L., Knuth, N. D., Huizenga, R., Rood, J. C., Ravussin, E., & Hall, K. D. (2012). Metabolic slowing with massive weight loss despite preservation of fat-free mass.The Journal of Clinical Endocrinology & Metabolism, 97(7), 2489-2496
  10. Levine, J. A. (2005). Nonexercise activity thermogenesis (NEAT): environment and biology (vol 49, pg E675, 2004).AMERICAN JOURNAL OF PHYSIOLOGY-ENDOCRINOLOGY AND METABOLISM, 288(1), E285-E285
  11. Levine, J. A. (2004). Non-exercise activity thermogenesis (NEAT).Nutrition reviews, 62(suppl 2), S82-S97
  12. Levine, J. A. (2007). Nonexercise activity thermogenesis–liberating the life‐Journal of internal medicine, 262(3), 273-287
  13. Hall, K. D., & Chow, C. C. (2013). Why is the 3500 kcal per pound weight loss rule wrong?.International journal of obesity (2005), 37(12)

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