Let us start with nothing. A naive human has come to stay in your home. Perhaps it's a Tarzan type of character, just arrived from the wild, or, if you like a modern twist, it's a government agent who has been hit over the head and can't even remember his name. In any case, this naive human will, within a few hours of arrival, make his way to your kitchen and begin poking around, because he's hungry.

So let's start by recognizing that: Hunger is universal. How we modern,complicated, guilty, preoccupied people talk about hunger may sometimes confuse the point ("I just eat mindlessly") but let us start with the fact that hunger exists.

He's hungry, but, for what? Let's hypothesize that the food is extremely unrecognizable to him. He's a particularly primitive Tarzan or the blow to the head was quite heavy. He takes the olive oil out of the cupboard and begins to drink it down, straight from the bottle. Within a swallow or two, he's gagging and spitting out the recommended healthy oil. Next, he spies the sugar bowl, sniffs and finds it pleasing. He tilts the container to his face and tries a mouthful. His mouth dries, he can barely swallow, but manages. It's an appealing flavor, but it's not food. He couldn't repeat that swallow again without feeling the gag reflex return. He tries again with butter - gag. Saltshaker - gag. Maple syrup -gag. What's going on?

It is a commonly stated notion that we are trained to seek salt, sugar and fat by some evolutionary process. We sometimes consider ourselves to be actually "addicted" to these three flavors. But as we've just demonstrated with our naive human, we need these flavors in a certain balance, in particular amounts, with particular textures and densities. Our preference, actually more than preference, our primitive survival-oriented,  instinctive reaction to food is to want a very highly designated type of nutrition. We may think that our eating behavior is driven by marketing and what our mother has taught us, but there is a more fundamental regulator than those: our own body.

The body not only helps us distinguish "food" from "not food,"but will also sort good food from bad, fresh from rotten, nutritious from toxic. As in the case of the oil, sugar, and salt, the body knows not only what we need, but the right quantity and concentration. This protects us from too much of a good thing. One could build up from this simple example (we won't) to a discussion of all types of foods and the means by which the body attempts to get what it needs from Coke and Doritos. But to begin our present discussion, we need only to acknowledge that hunger exists and that what, and how much, we eat is regulated. The purpose of this regulation is to optimize function and ensure reproduction, but it seems also to be related to the overall size of the body. For the same reason we don't find ants the size of dogs, we don't see humans 2 cm or 4 meters tall. Regarding weight, we don't see adults who weigh 20 pounds, nor 2000 pounds. We can debate which parameters drive the regulation of human body size and appetite, but we cannot debate that the regulation exists to help our bodies remain within a certain range of optimum. It's impossible to think of any animal existing without mechanisms to produce a certain size and shape.

Normal Weight Gain

In medical school one day, I heard something in a lecture that caught my interest. A professor mentioned that the natural progression of weight gain in adults happened to be almost exactly one pound (0.45 kg) per year. It was considered normal, he said, for adults to put on about forty pounds from age 20 to 60. This had been a stable finding over the previous hundred years or so, the time frame for which we had weights for large enough groups of people to make such an observation. At that time, I happened to be 26 years old and weighed 126 pounds. The thought occurred to me that I'd be a good subject to test this hypothesis: whatever age I was throughout my adult life, plus 100 more pounds, should tell me my weight. Being naturally "skinny," I didn't focus on my body weight much, stepping on a scale only when required at a doctor's visit. I went through various career stages, including insane hours in the hospital, eating cafeteria food and losing sleep.

Vacations, graduations, fathering children, moving houses six or seven times. I noticed at age 32, I weighed 132 pounds. I was a bit squishy at that time, so I got back into the gym, began to lift weights and drink protein shakes. At age 35, I weighed 135.  Later, having a bit more free time available, I continued the weight lifting and resumed soccer and running. I had surgery on my knee and couldn't do aerobic exercise for many months. Some years I commuted by car, others by bike. I spent 12 years as a vegetarian, five years as a purposeful protein eater, a few more indulging in sour patch kids and beer.

At age 45, as I write this, my weight sits squarely at 145 pounds.

I happened to develop an interest in obesity in the middle of my career and I never noticed a reference for that professor's contention of what is normal weight gain and never saw it repeated. However, in 2013, a group of researchers from Duke published "Young Adult Weight Trajectories Through Midlife by Body Mass Category." In this study, they used data from 10,000 individuals (aged 25-33) involved in a different longitudinal study, who happened to be weighed yearly, to get a sense of what is "normal" for adult weight gain. Remarkably, their data almost perfectly corroborated my professor's contention (and my own experience).

On average, these 10,000 adults gained 1.17 pounds (0.53 kg) per year. They broke down the participants by body mass category and found that whether one was skinny or heavy at the start of the study, more than 98% of the men and 92% of the women were on a trajectory that sloped upward. Let's put this another way: you could tell a 25 year old man with decent confidence that there is a 98% chance he is going to be gaining weight over the next 20 years of his adulthood.

The Duke group referenced previous work by Mozaffarian et al. in 2011, which showed an average of 0.84 pounds (0.38 kg) per year. If we take these two papers together, which used different populations and different methods, we come to the, rather convenient, rule that adult humans are designed to gain a pound per year.

This pound per year rule complicates the discussion of "set points" and "homeostasis." We spend much time trying to figure out what’s wrong with the “homeostasis” of the human animal trying to remain at a stable weight, but forget that weight is almost never stable. It rises from the moment we are born until our very last decades. At the University of Colorado, researchers led by Dan Bessesen looked at these normal weight trajectories and tested what happens when you experimentally knock healthy humans off their steady weight gain path.

They did the same with lab rats, after observing that gradual weight gain is the norm for rats who are allowed to eat as much as desired, just like humans. What they found is that when either humans or rats are deliberately overfed or underfed, the tendency, after the intervention ceases, is to return, not to the previous weight, but to the expected, slightly higher, weight predicted on the pound-per-year trajectory (and the grams per week equivalent for rats). This explains the near constant observation by patients that they dieted for a period of time, lost weight, then gained it all back "and a little more."

In this case, the lead author suggests the use of the term "homeorrhesis" in lieu of homeostasis. The "rhesis" suffix denoting not stability, but a constant trend with a predictable direction. For obesity, the questionis not, "why am I gaining weight?" but rather, "why am I gaining so much weight?"

We are told that 1/3 of adults in the U.S. can be considered "overweight" and another 1/3 "obese." Those exact numbers and their true significance can be debated, but we cannot argue with the fact that there is a significant portion of the population that would be considered, by nearly anyone, as having trouble regulating their body size.

If it is becoming more common than not that adults are mis-regulated, specifically too heavy, the natural inclination would be to look for the cause, or causes, of this mis-regulation. The possibility that it is caused by a massive, collective, sudden loss of will power in all industrialized nations, simultaneously, seems unlikely to me.

We will look for the causes in our body and in the food environment.

References:

Bessesen D., Physiology and Behavior 2011; 104: 599-607.

Malhotra R. et. al. Obesity 2013 Sep; 21 (9): 1923-34.

Mozaffarian et al. NEJM 2011;364:2392-2404.