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.

Edmonton, Alberta

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