Why do people become obese?

Why do people become obese?

What are the causes of obesity? Why do people
get fat? Is it a sluggish metabolism, like many people believe? Or is it just a problem
of body build, like our friend Eric Cartman thinks? Someone claims it is it all written
in the genes. Someone else says it’s simply a problem of eating too much and moving too
little. Who is right and who is wrong? Well, all of them are at least in part right. We all see it every day. There’s the overweight
guy who lives a life of constant restrain, he doesn’t eat sweets, control the size
of his portions, he exercise religiously, and yet it never seems to be enough to shed
those extra pounds. At the same time, his skinny cousin who doesn’t go to the gym
and eats all the donuts he wants, never appears to gain a single pound.
In the eyes of many, this is proof that how much we eat or how much we move doesn’t
really matter to determine body weight. It all boils down to metabolism, body build or
genetics. But this idea is wrong. As we have already learned, weight variations
always involve an alteration of the energy balance: fat and weight accumulation is always
the result of eating more calories than we spend, and there’s no exception to this
basic rule of physics. We may have everything against us, our genes,
a sluggish metabolism, low thyroid hormones, big bones and whatever, but if we spend more
energy than we get with food, we will lose weight, if we eat more than we spend, we will
gain it. What is true however is that different people
have different susceptibilities to weight and fat accumulation due to both genetic and
environmental factors. The lucky cousin, who eats more and moves less, still manages to
keep a normal energy balance because somehow he is still burning more calories. His overweight
relative is eating less and exercising more, but this is still not enough to match energy
in and energy out. We have already examined many of the genetic
and metabolic factors that may influence energy intake and energy expenditure, and thus the
tendency to be more or less energy efficient. We already know, for example, that our basal
metabolism is affected by our gender which reflects body composition, by our age which
reflects hormonal changes, by our body build which reflects body surface area, and then
our levels of thyroid hormones, and of stress hormones. So for example people with hypothyroidism
are inevitably predisposed to weight accumulation. Taller individuals tend to have a body frame
with lower tendency to accumulate fat. Some individuals tend to dissipate more energy
with involuntary fidgeting movements. As we we already discussed, our eating behavior
is at least in part driven by biological hunger and satiety regulation systems, which differ
from one individual to the other. Some of us get satiated sooner, some other are more
hungry. In general, more than 200 genes have been
identified contributing to obesity, be it through impaired hunger and satiety signals,
increased fat uptake efficiency, ability to perform activities with lower energy expenditures,
different levels of hormones, and so on. On top of that, environmental and psychosocial
factor influence energy intake and energy expenditure.
We have seen for example that environmental temperature influences body metabolism. Many
medications also slow down metabolism or otherwise induce weight accumulation, such as hormone
replacement therapy, tranquilizers, some antidepressant drugs, cortisone or beta-blockers.
And then there are many psychosocial influences on our eating behavior, such as emotional
stresses and other psychological triggers, food availability, eating opportunities, cultural
factors, time and convenience, and many more. In summary, many genetic, metabolic, environmental
and psychosocial factors affect our energy intake and energy expenditure, and thus our
tendency to gain fat and weight, more or less easily.
However, physical activity and voluntary eating habits still have the final word. The case
of the Pima indians is illuminating, and it is a classic case study in nutrition science.
Pima Indians have a strong genetic predisposition to obesity, so much so that Pima living in
Arizona have one of the highest rates of obesity in the world, with an average BMI of 33. However,
Pima living in Mexico have much lower rates of obesity, and their average BMI is only
25. And yet, their genetic makeup is exactly the same. The only difference is their lifestyle.
The ‘americanized’ Pimas live a sedentary life and eat a US diet rich in empty calories,
while the Mexican Pimas follow a more traditional diet and do more physical labor. Like we already said many times, while it
is true that our genes strongly determine our predisposition to health and disease,
and therefore also our tendency to gain weight more or less easily, our genes almost never
determine by themselves the actual outcome, which is strongly controlled by the environment. If we look at the obesity statistics and compare
obesity rates today with less than 70 years ago, we can see a dramatic difference. Our
genes did not change in 70 years, because it takes thousands and thousands of years
for evolution to do its job. Our genes 70 years ago were the same as our genes today.
And yet, the incidence of obesity skyrocketed. Clearly, the responsibility is not our genes’.
What changed, instead, is our environment. The way we eat, and the way we live. We eat
more and we are much more sedentary than our great grandparents. In conclusion, we have seen that indeed there
are many genetic, metabolic, environmental and psychosocial factor that all contribute
to fat accumulation and obesity, but ultimately, the actual outcome is always the result of
the balance between energy intake and energy expenditure. Our genes can only determine
predisposition, but the final word is ours. With motivation and commitment, someone more
easily, someone with more effort, we all can make lifestyle choices and eating behavior
adjustments that in the end, will enable us to match energy intake with energy expenditure
and thus maintain a healthy weight. As we have already mentioned earlier in our
course, the body has a natural tendency to oppose weight variations. If we suddenly go
on a severely calorie restricted diet, our primordial genes will interpret it as an ongoing
famine, and will immediately activate emergency mechanisms to resist weight loss.
Thyroid hormones drop and basal metabolism slows down, we become more energy efficient
and perform many of the same activities with less energy, at the same time lipoprotein
lipase activity increases, so that fat uptake and accumulation into our cells is promoted,
and our brain will increase our hunger sensation, mainly as a consequence of dropping leptin,
so that it becomes very difficult to resist eating. As a result of this, even though we
decrease our energy intake, our energy expenditure also decreases and it soon adjusts to reach
a new equilibrium. From our body’s perspective, this makes
perfect sense. If I cut your salary in half from one day to the other, I’m sure you
wouldn’t keep spending the way you were doing before. Your body thinks, well, today
I only got 800 calories, I can’t keep spending 18 hundred as I was doing yesterday. And so
it does whatever it’s in his power to save energy, all the while giving you signals that
you should eat asap. This is the main reason why just cutting calories does not work by
itself as a weight loss strategy, because your body worries, goes in standby mode, adjusts
to the new level of energy intake, and weight loss soon plateaus.
On the other hand, deliberately gaining weight can be just as challenging, because in most
people the reverse will happen. Basal metabolism increases, we dissipate more energy and heat
through fidgeting and involuntary movements, and our hunger and appetite decrease to the
point that food eventually becomes repulsive. In summary, if we deliberately decide to increase
or decrease our weight, homeostatic mechanisms will start to operate to try and restore our
original weight. While this is nothing more than the umpteenth
(ennesima) application of the extraordinary homeostatic ability of our body, which goes
to extreme lengths to maintain the equilibrium of things, some researchers believe that as
far as weight is concerned, there is a little bit more than just homeostasis, and they have
proposed the so called set-point theory. According to this theory, each individual has a precise
genetically predetermined body weight, and it will do everything it can to achieve it
and maintain it. This would explain why a lot of overweight individuals tend to rapidly
gain their weight back after they go on a diet to lose it, and why many lean individual
don’t become obese even if they overeat. However, there are many evidences that this
theory is wrong, and the statistics I showed you about seventy years ago and today should
be enough to convince you. If a person overeats, he is much more likely to become overweight,
and if an obese person adopts appropriate lifestyle changes instead of just going on
a weight loss diet, he is much more likely to achieve and maintain a healthy body weight.
Like we said, indeed we have genetic predispositions to gain weight more or less easily, and indeed
we have homeostatic mechanisms that try to stabilize weight in the short term, but we
definitely do not have a genetically pre-set weight, and it is only our lifestyle, our
diet and our level of physical activity which can determine what our weight actually is.

5 thoughts on “Why do people become obese?

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