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How Many Extra Calories Do You Need To Become Obese?


One of the most common obesity myths (propagated in countless publications and statements) is that it only takes a small daily caloric excess to put on vast amounts of weight over a lifetime. Thus, it is often said that eating as few as 20 extra calories a day, resulting in around 7000 extra calories per year, will add up to approximately one kilo annual weight gain, which, if continued over decades, can add up to tens of kilos of extra weight.

However, this calculation is far too simplistic, because as the body gains weight, its energy requirements also increase. As soon as the body’s daily energy requirements increase by 20 calories, there is no longer any caloric excess and so weight gain stops. To continue gaining weight, you would need to now add another 20 extra calories which will in turn increase body weight till those too are just enough to meet increased demands, at which point you would need to add another 20 calories to keep gaining weight.

In other words to continue gaining weight, it is not enough to simply eat 20 extra calories per day and hope for the best – rather, the amount of extra calories has to keep increasing to sustain weight gain over time. This means that you would soon need to be eating 100s of extra calories per day more than you were eating before you started gaining weight just to maintain the same rate of weight gain.

This simple physiological truth is now once again explained by Martijn Katan and David Ludwig in a brief but enlightening article published in JAMA.

Using simple examples, Katan and Ludwig illustrate that for most people to move from a BMI of say 25 to a BMI of 35 actually requires 100s of extra calories per day (in the 300-500 cal range). Because of the natural growth and ever increasing caloric demands in children, the daily energy excess required for kids to become obese is even greater (in fact 500 to 1000 extra calories per day for young children).

Sadly, for exactly the same reasons that weight gain for a given caloric excess is “self-limiting”, so is weight loss for a given caloric deficit.

This means that a given reduction in caloric intake (let’s say eating 500 calories less per day) will only result in weight loss for as long as it takes the body to decrease its daily caloric needs by that amount. Unfortunately, this generally occurs within a few weeks of starting your diet (as the body loses weight and hormonal changes kick in to reduce energy metabolism). This is when you stop losing weight, despite still eating 500 calories less than before. To start losing weight again, you would now need to reduce your calories even further, but even then weight loss would once again stop as the body adjusts to even fewer calories. (This is why people who have bariatric surgery, despite consuming far fewer calories than before, don’t simply continue losing weight till they disappear).

Thus, in the same way that continued weight gain requires relatively large net daily increases in calories (albeit in progressive steps), so does continued weight loss require huge caloric deficits to be maintained over time.

The commonly promoted myth that small changes in energy intake will eventually lead to large gains or losses in weight over time is simply wrong.

Indeed, it is exactly because gaining or losing large amounts of weight takes a considerable caloric excess or deficit that has to maintained over time, that weight management is so difficult.

If small changes could make huge differences, we would not have an obesity epidemic.

AMS
Edmonton, Alberta

Hat tip to my colleague Geoff Ball for pointing out this article

14 Comments

  1. I totally agree with your assessment, particularly with regards to weight loss. As the metabolism slows it becomes increasingly difficult to lose weight at the same rate (or even at all).

    I think this highlights the need for resistance exercise to maintain muscle mass during weight loss. Recently I encountered one woman who lost 50lbs and approximately 20lbs of that was measured to be muscle loss. Her metabolism had slowed dramatically and she had reached a weight loss plateau. She did no exercise.

    Another woman on the same nutrition plan lost the same amount of weight, but only 3lbs of muscle were measured to be lost. She continued to lose weight at a steady rate and felt fantastic. Her routine included 3 days per week of cardiovascular exercise and two weight training bouts per week.

    Obviously, this is not a large sample, but certainly an example of how exercise can work to prevent slowing of the metabolism.

    Also, as I’ve mentioned before, bodybuilders often use full day high calorie and high carbohydrate “refeeds” to elevate leptin and maintain their high metabolisms. Whether this truly works has yet to be measured by science, but there is no doubting their success. Just an idea to ponder.

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  2. I don’t quite understand. I get why adding 100 calories won’t cause someone to continually gain weight. Eventually, the body gains mass until those once extra calories are needed to maintain the new size? And then someone would need to consume additional calories (more than the original 100) to gain any more weight. So why couldn’t a small but steady over the long-term increase in intake cause weight gain?

    As for the weight loss, again I understand that there is a metabolic effect where people who lose weight have metabolic slowing even greater than can be accounted for with mass loss. However, from what I have read I understood it to be in the range of maybe a few hundred calories. Is this not correct? And is the amount of this slowing related to the severity of the restriction? If someone who normally might need 2600 calories to maintain their weight, starts eating 2100 calories per day will it really only take a few weeks for the body to drop their metabolic rate by 500 calories each day? Has this been found in metabolic ward studies?

    I don’t disagree with the main point that assertions that cutting out or adding a small amount of calories each day will lead to long-lasting body changes are wrong. Nor with the idea that weight-loss is made difficult by metabolic shenanigans. I’m just trying to sift through everything I hear from all sides. Also, I’ll admit to totally non-scientific interest in that I’m someone who’s lost a little over 80lbs by employing small changes over time. But I’m still eating over 2000 calories every day. I could just be an outlier though.

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  3. While I do understand this blog is meant mainly for doctors I really find this post one of the most depressing things I have read in a very long time. Is there no hope but surgery?

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  4. Your caloric density for weight gain/loss does not match the records. The typical value is 4.0 gm/C, not 7.0 gm/C for long term changes.

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  5. make that C/gm

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  6. Philisophically speaking, adding five small “100 calorie deficit changes” could cumulatively add up to that significant 500 calorie deficit. So if one were not looking for immediate weight change, but started applying these strategies (in combination, perhaps, with an increase in calorie expenditure through intentional exercise or incidental movement, i.e. an active commute), over time, these could lead to weight loss. I think that if the goal is people whose overall health is improved, not only temporary reduction in weight, that finding those areas where the calories are not missed (for some, this might be switching from drinking regular soda to drinking iced tea with artificial sweetener, for example) and areas where adding in movement actually improves quality of life (dancing, active commuting, participating in sports) could make a difference in weight, over time. These changes could be cumulative.
    This is in part why I think a “Health at Every Size” approach can lead ultimately to a reduction in weight. Not for weight loss as a goal, but as people are no longer in a resistant or oppositional stance (a very human response to discrimination). When approached with compassion and benificence, changes to eating and movement habits that have merit for their own sake, not only for weight loss, can then be adopted.
    As a woman, I have a higher risk of developing breast cancer than a man does. Would gender reassignment surgery be an appropriate strategy for combatting breast cancer? I think becoming someone who is thin would require just as dramatic a change — and would not benefit my overall long-term health. But incorporating activities and action into my life that have meaning and value beyond weight loss are worth doing, not because they make me thinner, but because they make me “more me.”

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  7. The basic physics of fat loss is “calories in – calories out = fat.” The assumption here is that excess calories become fat. That assumption has yet to be overturned by evidence..Anyhow, Nice piece of information.

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  8. In my clinical practice with obese patient or normal patient who don’t want developed in obese, I simply use the negotiation methods, more calories for more physical activity daily at least two hours for losing weight. Alerting the obese person the exercise must be progressive, 30 minutes at the beginning.

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  9. I believe the 3500 calories equals 1 lb originally came from the work of Ancel Keys’ starvation studies. It’s been about 20 years since I read these studies in which a group of young men underwent calorie deprivation as a way to contribute to the war effort while at the same time being conscientous objectors. I remember being struck by the fact that 3500 calories was the average calculation for these men but there was large variability in how much of a calorie deficit was required for a one pound weight loss. Later, during the refeeding part of the experiment, weight regain, was also quite variable. Like all things, we cannot apply statistics to individuals.

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  10. This is kind of bunk, because the problem with over eating is that it contributes to problems with proper metabolic functions. Yes, if you over-eat 20 calories a day, you should reach a weight that has those requirements, but the chronic over-glycolisizing and overeating changes your body chemistry to a point where you are now on the road to constantly feeding a demand to over-compensate your energy needs. Thus, the problem with Western obesity…. Normal people don’t suddenly overeat by 500 calories to become obese. They have a change in lifestyle (being active in high school or college, while downing colas and occasional pizza binges while staying a healthy weight) then move to sedentary lifestyle only minimally changing their eating styles to a point where there is a chronic over eating of calories. That’s a road-path toward metabolic syndrome.

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  11. ALSO, we all understand the caloric deficit issue, which is why it’s better for obese people to learn to maintain their weight or have a SMALL caloric deficit to begin their diets. They ARE on the path toward gaining, and they are most likely already eating 20 to 100 or more calories more than they need to maintain their current weight. The only way to combat the problem is to restore the body back to maintaining vs. gaining.

    I don’t like these types of studies because it implies that obese people were always gluttonous huge eaters, but most only started out with a small caloric surplus that increased over time–not because they were such gluttons, but because as they gained weight, their bodies required an additional surplus from years of chronically overeating minimally.

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  12. I totally agree with the study. It is very well explained. Physiologically it makes sense. That is why people hit plateaus. Weight loss has to be strictly connected with behavior modification. If people are not able to solve their emotional problems, most likely the weight gain will be on an off during their life time. Sedentary lifestyle is the most important problem we have in this modern society. We tend to work long hours + telecommuting, coming back home tired with our brain totally dead. Living a lonely life with no opportunities to develop true friendships. It is the sum of many many aspects of our lives, not just only if we eat 20-100-500 more calories per day.

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  13. As a professional working in weight management using mainly lifestyle modification i totally agree that if patients do not change their eating habits and continue to eat healthier pick less saturated energy dense fat choices they will surely start to gain as this change in my opinion will help maintain achieved weight and if they aim towards more fat loss i always have to be a bit stricter in dietary counselling given for the 2nd 10Kgs loss than i was w the first 10Kgs
    But from observations i always notice that if lifestyle modification strategies included increased dependence on exercise this will facilitate new weight maintenance as it at least prevents wt gain

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  14. Ok so here’s my problem, due to genetics i’m what you call a “hard gainer” This to me is awful, try being 6’2 and down to 148 pounds from 167 all my life which is still too light. I was in a car accident and couldn’t cook much since i lived on my own, finally i got better to start eating properly BUT that being said i’m back working my physical job and it’s hard enough to simply maintain, let alone gain.

    If the said statement isn’t true of 3,500 calories to gain then what? How do i gain the weight i lost and gain more then before? Not only genetics play a part in it but i’ve always have led a active lifestyle i.e. lots of exercise and physical labour.

    What suggestions can you “bring to the table” so to speak since not working as hard as i do is not a option at this point, nor activity. I’m also not the only one in this situation, there are more of us then you may realize. feel free to comment or send a email so i can get the message out there to people like me.

    Thanks, Bill

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