Dr. Katz is the Director of Yale University’s Prevention Research Center
Understanding the Physics of Weight Loss
I recently addressed the reason why weight loss drugs have historically fared quite badly, and in my view are likely to do so for the foreseeable future. Weight gain is normal when calories in exceed calories out on a daily basis. You cannot medicate away normal human physiology — any more than you give a fish a pill to let it breathe out of water.
Now, I would like to pick up the tale where I left off. Because there is more to the story of human weight gain than calories in versus calories out, as not a few of you have stressed in your comments and correspondence. Fundamentally, we are all a lot alike, but also quite a bit different.
The ways we are alike are what I have emphasized so far. It is, indeed, normal for the human body — any human body — to turn surplus calories first into glycogen (a small carbohydrate energy reserve) and then into fat (a much larger potential energy reserve).
And we are all governed by the prevailing laws of physics that relate matter and energy. Calories are a measure of energy, and matter cannot be created without energy input. Arguments against the fundamental role of energy balance in weight regulation — against calories in versus calories out — are arguments with Isaac Newton. Folks, nobody wins an argument with Isaac Newton!
And yet, I believe the innumerable patients I’ve had over the years who tell me something doesn’t quite add up for them. I believe them when they tell me they eat and exercise the same amount as someone else — but they get fat, while the other person stays slim. Or, worse still, that happens even though they eat less, and exercise more. If they are telling the truth- and I am thoroughly convinced they are- doesn’t it mean Newton had it wrong?
Nothing of the sort. All it means is that we are different, as well as alike.
While it is true for all of us that weight control is overwhelmingly, if not entirely, about calories in versus calories out, it is equally true that the number of calories required for weight loss, weight maintenance and weight gain vary drastically among us. And we even know why, for the most part.
There are, once we are done growing up, three ways we burn calories: physical activity, the generation of heat and just existing. There are technical terms for the second and third: thermogenesis, and resting energy expenditure (sometimes referred to as basal metabolic rate). What should be noteworthy right away is that you are not in charge of two out of the three!
You can choose how much exercise to do. But you don’t get to choose how thermogenic you tend to be, and that can matter quite a lot. Like exercise, thermogenesis accounts for roughly 15 percent of total energy expenditure on average, but there is lots of variation on the theme of average. People who generate more heat from calories have fewer available with which to make fat. They tend to be people who can eat a bit more, and stay thin anyway.
But that’s a drop in the bucket compared to resting energy expenditure. Roughly 65 percent of calories are burned to support the fundamental workings of cells and organs that keep us alive. The number of calories burned at rest, and the actual percentage of total calories burned this way, also vary substantially around the average. People with a high resting energy expenditure are, in our modern world of epidemic obesity, the fortunate few most people love to hate: the folks who cannot seem to get enough to eat, and can’t put weight on when they try.
If you don’t control your thermogenic tendencies, nor your resting energy expenditure — who does? The idiosyncrasies of the genetic hand you were dealt, which are not necessarily idiosyncratic at all.
Take the case of the Pima Indians, for instance.
When the Pimas live a traditional lifestyle and eat traditional Pima foods — mesquite and tepary beans, for example — they have unremarkable health. When they live and eat like everyone else in America, they develop almost universal, severe obesity and diabetes. For a time, the Pimas had the highest rates of obesity and diabetes on the planet, and they are still, alas, contenders for those laurels.
The dire plight of the Pimas resulted in intensive study of them, and it led to both revelations, and the obvious. The Pimas have, uniformly, a very low resting energy expenditure. They are, in other words, highly fuel efficient — even for a fuel efficient species — and it makes perfect sense. The Pimas lived for generations in a harsh desert climate where food was unusually scarce, and physical activity demands unusually high. Pimas who were not highly fuel efficient simply didn’t survive long enough to pass on their disadvantage to any future generation of Pimas. (People who don’t live to make babies make very poor ancestors.)
Presumably, the variations in metabolic efficiency to which we are all subject can also be traced to variations in the experiences of our ancestors. But for most of us, the magnitude of genetic mixing that has gone on makes those pathways impossible to follow.
So we wind up feeling — those among us who gain weight easily and lose it with extreme difficulty, if at all — that we are enigmas, outcasts, anomalies. We feel that the claim that we can eat less, exercise more, and weigh more nonetheless falls on unsympathetic ears.
Mine are not such ears. I hear you, and I believe you. It is about calories in versus calories out, but the calories required to reach or exceed a personal threshold vary widely, and under influences we neither choose nor control. That isn’t fair, but who ever promised it would be?
As a practical response to this, I am considering the establishment of a national exchange for the weight loss resistant, so these stories come together and reach a critical volume. I would also like to use it as a means of troubleshooting the problem; many heads will be better than any one. Let me know via email if you would like to join us.
In the interim, I remain unwilling to refute Sir Isaac. But that we are, despite being much the same, quite a bit different — I fully appreciate. I bet Newton would agree; I hope everyone does. Back to top
Dr. David L. Katz; www.davidkatzmd.com