This is the second part of my response to Brad’s comment

Your post scratches at some scientific-sounding ideas of the involvement of genetics, but fails to cite any definitive research into inheritable weight-control issues. The mathematics behind maintaining a healthy weight are startlingly simple, and the mythology of “fat genes” and thyroid problems and impossible to shed pounds is merely there to sell books and gym memberships.

The body is a closed system and cannot create additional weight without an over consumption of calories. Joel S is correct — it’s literally impossible to gain weight by eating your “maintenance” number of calories without violating the first law of thermodynamics.

Previously, I talked a little bit about the scientific evidence for a genetic basis for obesity and gave a few links to some big papers on genes and obesity.

Now I’ll address the first law. So, first a minor nit to pick. The commonly repeated statement

it’s literally impossible to gain weight by eating your “maintenance” number of calories without violating the first law of thermodynamics

conflates energy balance and mass balance. This is somewhat important if you actually want to predict and measure weight loss. It is true that calories-in minus calories-out must equal the calorie content of the organism. However, not all tissue and body fluid have the same calorie/mass ratio. Thus, it is technically possible to gain mass while having a negative calorie balance.

As a super nerdy aside, mass balance must hold as well. That is mass-in minus mass-out must equal mass of the organism. So when the organism looses mass where does that mass go? Answer at the end.

So, yes calorie balance must hold. Calories-in minus calories-out must equal the calorie content of the organism.

Doesn’t this imply that we can control weight by controlling calories-in?


For several reasons. First, is that calories-in and calories-out are not independent. If you feed a load of sugar to a five year-old you will witness one of the natural calorie balancing processes the body engages in.

Tolerance to a glycemic spike is actually one of the earliest breakdowns in the calorie regulation mechanism and I suspect that it is in part responsible for the mild weight gain that most adults experience and have experienced for generations.

However, even in adults one will find that consistent calorie restriction results in exhaustion, drowsiness, and a reduction in body temperature. Thus restricting calories-in reduces calories-out.

Secondly, you have no direct control over the composition of the weight loss. It could be fat, muscle, bone, hair, blood, etc. In general it is a combination of all of the above. Even in bariatric surgery, which shows great success in fat reduction, muscle loss is common and hair loss is not infrequent.

Muscle loss in particular is counter-productive because muscle is much more metabolically active than fat. Thus a loss in muscle reduces the regular amount of calories expended and thus reduces calories-out.

The combination of these factors is likely responsible for the plateau that most dieters experience where continued calorie restriction does not result in additional weight loss. Surprisingly the plateau appears even in Very Low Calorie diets of 500 kcals a day or less. 

There is an intense debate over whether this could possibly be due to a reduction in the maintenance level to only 500 kcals. I am not sure where I side on this debate but it does seem that the plateau appears.

We should also note that hormone changes can induce changes in weight. The most obvious is human growth hormone. Since the first law must hold in all cases how is it possible that growth hormone can do what only a change in calorie balance can accomplish?

The short answer is that changes in body composition induce changes in calorie intake and expenditure. Teenage boys are famously hungry and they are all adding mass. The same is true for pregnant women.

Could it be that the same is true for the obese? I don’t know how we could rule it out.

Perhaps, a hormonal change induces the body to accumulate fat. The accumulation of fat then increases hunger to satisfy calorie balance. The individual then attempts to eat more. In addition, like pregnant women and teenage boys we should not be surprised that such an individual develops cravings for calorie dense food that would sicken a normal person.

Indeed, that’s one of the observations that makes us think this is hormonal. The behavior of many obese persons is not desirable to the non-obese. Many thin people do not want to eat ultra-high fat food.

Odd desires point to hormones. Again see the behavior of teenagers, both make and female, and pregnant women for notes on the strange desires that raging hormones can induce.

A more important observation, however, is that both thin and obese people seem to know how many calories they “need.” That is both the thin and the obese are typically in caloric equilibrium, neither gaining nor loosing weight. A person will often be 50lbs overweight for years.

How does their body “know” exactly the right number of calories to eat to maintain 50 lbs extra fat. It would be one thing if their weight was constantly fluctuating with the season, the opening of new restaurants, etc.

But that’s not what we observe. We observe either solid gaining trends or solid holding patterns. How does the body know that it needs exactly maintenance plus 500?

Moreover, how does the thin body know that it needs exactly maintenance? Most people have no idea what their maintenance level is and even if they did caloric estimates on food are only accurate to around 90% and not universally available. Are we really to believe that all thin people are counting up every calorie in and every calorie out to make sure they balance? 

No, almost certainly their hormones are telling them. They are telling them they desire food or that they have had too much and now food is repulsive. Their hormones are telling them they really need a hardy steak or they are just “in the mood” for a salad. Their hormones are monitoring body composition and issuing feedback to the brain to tell the brain what to eat.

Thus when someone becomes obese our first suspect should be a breakdown in this feedback loop. This is where I believe that research needs to focus. What is the nature of this feedback breakdown and how can it be fixed?

Answer: When we loose weight were does the fat mass go?

It turns into air and water. Fat is a primarily a chain of carbon and hydrogen atoms. The carbon combines with the oxygen we breathe to produce carbon dioxide, which we breathe out. The hydrogen combines with the oxygen we breathe to produce water, which we urinate out.

Most of the mass, however, is carbon and so we loose weight primarily because the air we breathe out is slightly heavier than the air we breathe in.