According to the laws of physics when [calories in] exceed [calories out] people gain weight.
Unfortunately, when you actually deal with people (read: biological systems), this simple law is anything but simple. This is because, thanks to complex biological feedback mechanisms, designed by nature to keep us alive and thriving, changing caloric intake in turn affects caloric expenditure and vice-versa.
Let me illustrate this important difference between physics and biology with a few analogies:
Consistently putting less fuel in your car will never make your car “learn” to run the same distance on less fuel, but our bodies actually quite readily and effectively adapt to eating less food by simply burning fewer calories (ultimately resulting in the most frustrating “weight-loss plateau”).
Regularly overfilling your gas tank will never make your car “learn” to burn more fuel, whereas eating too much actually does increase energy expenditure (we sweat, we get restless, we fidget).
Over or underfueling your car will never eventually lead to a bigger or smaller car, whereas this is exactly what happens to us, a fact that leads to a further complication: larger bodies (like bigger cars) need more fuel.
When you now consider that all of these biological reponses to changes in [calories in] and [calories out] vary considerably between individuals (due to genetics, age, sex, hormones, medications, etc.), you end up with a situation where trying to precisely predict weight changes by simply calculating [calories in] and [calories out] becomes more or less like forecasting next week’s weather – sometimes surprisingly accurate but, perhaps as often, completely off the mark.
This is probably why population studies have such a hard time figuring out why exactly we are all getting bigger.
As recently blogged, the recent Canadian Health Measurements Survey certainly does not convince me that overweight and obese people are any less physically active than non-obese Canadians – ALL Canadians seem to be inactive, period!
Now it turns out that overweight and obese people may actually not be simply “overeating” either.
At least not according to a study just published in the American Journal of Nutrition that reports on changes in energy intake in normal-weight, overweight, and obese individuals between 1971-2006.
In this study, Gregory Austin and colleagues from the University of Colorado Denver, compared carbohydrate, fat, protein, and total energy intake in adults aged 20-74 y from the first NHANES (NHANES I, 1971-1975; n = 13,106) to data from NHANES 2005-2006 (n = 4381).
While the prevalence of obesity increased from 11.9% to 33.4% in men and from 16.6% to 36.5% in women, there were rather modest changes in percentage of energy from carbohydrates (44.0% vs. 48.7%), fat (36.6% vs. 33.7%), or protein (16.5% vs. 15.7%). Importantly, these trends were identical across normal-weight, overweight, and obese groups, therefore it is hard to really blame the massive change in obesity rates simply on these rather modest changes in relative macronutrient intake.
But now comes the surprise: compared with NHANES I, normal-weight men consumed an additional 247 kcal per day, overweight men consumed an additional 165 kcal, and obese men consumed an additional 225 kcal in NHANES 2005–2006. In other words, ALL men were eating more – the normal-weight men in fact increased their caloric intake slightly more than the obese men!
In women, the results were not quite the same, in that normal weight women consumed an additional 183 kcal, overweight women consumed an additional 304 kcal, and obese women consumed an additional 341 kcal.
But once again, let us remember that larger people actually need more calories because they tend to have higher caloric expenditures (like bigger cars needing more fuel), so really it is not fair to simply compare the number of calories without adjusting these for higher requirements. Once you do such adjustments, the difference in increased calorie intake in obese women does not appear all that much greater anymore.
In their paper the authors go on to discuss the potential importance of the relative changes in carbohydrate, fat, and protein intake between the two surveys, but as these trends did not differ between the normal, overweight, and obese groups, I will not get into their arguments here.
The whole surprising point here is that it is actually by no means straightforward to deduce from these data that obese people are obese because they “overeat” compared to their non-obese peers. If anything, the thin guys are the ones who are actually “overeating” (and getting away with it).
So if obesity is not because of overeating or undermoving, why the heck are so many people so much heavier today than ever before?
Either our assessments of caloric intake (and expenditure) are simply completely off because of “underreporting” or other methodological problems – but in that case, this entire exercise of trying to figure out caloric intake and expenditure as a cause of the obesity epidemic is worthless.
What I believe this data actually shows is that we are seeing the population effect of the same changes in energy intake (and expenditure) affecting some people more than others.
Thus, readers will recall the recent post that heavier kids appear to be far more “sensitive” to obesogenic factors than thin kids – the “naturally” thin kids are just lucky and get away with their crappy lifestyles, while the poor “naturally” heavier kids pack on the pounds by doing exactly what the thin kids are doing (namely eating too much and sitting around all day).
Similarly, it seems that while EVERYONE is eating more calories now than they were back in 1971, “naturally” thin people somehow manage to stay thin, while “calorie-sensitive” folks get heavier.
Thus, the laws of physics which would tell us that obese people gain weight because they simply eat more and move less don’t quite tell us why thin people can eat more and move less and still stay thin.
So if the obesity epidemic is not simply due to people becoming obese because they’re eating more and moving less (than thin people), then the solution is probably not in simply having them eat less and move more – which, incidentally, is probably why “eat-less-move-more” (ELMM) so seldom works.
Somedays I wish life were simpler.
Austin GL, Ogden LG, & Hill JO (2011). Trends in carbohydrate, fat, and protein intakes and association with energy intake in normal-weight, overweight, and obese individuals: 1971-2006. The American journal of clinical nutrition PMID: 21310830
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