The Primary Causes of Obesity, According To Science


For some time now, there has been a little bit of a debate on genetics versus lifestyle in terms of losing weight and the causes of obesity and weight gain.

Some struggle to gain weight while many others struggle to lose weight.

With the thousands and thousands of weight loss products, diets, programs, and gimmicks, many people often blame their appearance and health on their genetics.

I can’t even begin to count how many times I have heard the comments of “I’m fat because of my genetics” or “You’re so lucky you can eat cookies. If I were to eat a cookie, I’d gain 10 pounds.”

Are these people right? Were they really cursed with “bad” genetics that has doomed their possibility of a “skinny” physique?

What are the causes of obesity and weight gain? Let’s see.

We explore that and more below.

Obesity: Genetics or Lifestyle?

Let’s go over what the studies show

Study 1

In a study conducted in 2009, 16 people (12 males and 4 females) were overfed 1000 calories per day for 8 weeks.

Since it is known that consuming an excess of 3500 calories equates to one pound of body fat, these individuals should have gained approximately 2 pounds a week and 16 pounds of fat over the course of the 8 week study.

As a basis of control for the study, the subjects were required to maintain very low exercise frequency over the course of the study.

This could be deemed a flaw of the study because there are always going to be variants as far as energy expenditure is concerned.

Throughout the study, there was an average of 432 cal/day stored and an average of 531 cal/day expended through energy expenditure per subject.

In simpler terms, an average of 432 calories a day were stored as fat while an average of 531 calories a day were burned off as energy.

When comparing the results of the study from a person-to-person basis, there was a lot of discrepancies.

Fat gain varied from .79 pounds to 9.3 pounds. Changes in BMR (basal metabolic rate) and TER (total energy requirement) could not explain the discrepancy in weight gain.

Study 2

In another study conducted on identical and fraternal twins, raised apart and raised together, show a highly genetic influence.

The study included 93 pairs of identical twins reared apart, 154 identical twins raised together, 218 pairs of fraternal twins raised together, and 208 pairs of fraternal twins reared apart.

The identical twins reared apart were probably the most important factor of the study considering just how similar identical twins’ genetic makeups are.

This study found that identical twins reared apart had a correlation in regards to BMI of .70, which is a very high correlation, and that the subjects’ environment had either minimal influence or no influence at all.

All groups in the study showed relatively high correlations further showing that genetics may play a big role in BMI.

Study 3

An interesting study conducted on the Pima Indians revealed a possible cause for the obesity epidemic in the United States and in many parts of the world.

The Pima Indians, residing in parts of Arizona and New Mexico, have an unusually high rate of obesity.

The study, conducted at BYU in Utah, studied the metabolic rates of 200 obese Pima Indians. The study showed that the obesity of this group of people may be linked to an increased metabolism.

In the past, this increased metabolism was a valuable trait for survival because it allowed them to metabolize food more effectively during periods of time when food was scarce, but has shown to cause many problems when food is in abundance.

Although the Pima Indians may be an extreme example, this could be a reason why obesity is so prevalent in the 21st century.

Obesity and gene mutation

In rare cases, causes of obesity and weight gain include a spontaneous mutation of a single gene (also known as monogenic).

These mutations have been found in genes that affect appetite control, food intake, and energy balance. In other rare cases, causes of obesity and weight gain include chromosomal abnormalities.

Examples of these include Prader-Willi and Bardet-Biedl syndromes. This form of obesity is often associated with mental retardation.

Who does obesity affect?

In America, and in most parts of the world, obesity affects people of all types. It affects people who are rich, poor, educated, and educated.

It affects those in developed nations as well those in undeveloped nations. As more and more studies are being conducted on the role of genetics and weight gain/loss, it is becoming more apparent that genetics are playing a big role in weight regulation.

Different people tend to gain weight in different areas and to bigger degrees than others. This obesity is often called polygenic obesity, meaning obesity caused by multiple genes.

Obesity and twin studies

Based on data from more than 25,000 twin pairs and 50,000 biological and adoptive family members, the weighted mean correlations are .74 for identical twins, .32 for fraternal twins, .25 for siblings, .19 for parent-offspring pairs, .06 for adoptive relatives, and .12 for spouses.

The twin studies are what has stood out in regards to the role of genetics on obesity. This correlation shows that genetics may play a big role in BMI.

The main problem with these studies is that it relies on the precedence that each of the twins have the same degree of shared environment, which is an assumption that may not hold true in the “real” world.

Obesity and gene chromosome 16

A study carried out in 2007 identified the first obesity-gene variants in the “fat mass and obesity-associated” gene on chromosome 16. Those that have this this gene variant have an approximately 25% chance of obesity than those without it.

A recent 2008 study showed another obesity-associated gene variant that lies on chromosome 18.

This chromosome lies close to the melanocortin-4 receptor gene, which lies close to the gene responsible for monogenic obesity.

This study showed that this variant influences fat mass, weight, and risk for obesity. It is also associated with higher energy intakes, greater long-term weight change, and a greater risk of diabetes in women.

Despite genetic factors playing many roles in the current obesity epidemic throughout the world, it is very unlikely to be the sole cause. The human gene pool stays relatively stable for many generations and it takes a long time for new mutations to spread across a population.

The recent obesity epidemic

In just the last 30 years alone, obesity has doubled worldwide.  Approximately 1.5 billion people worldwide are overweight while another half-billion is obese.

In the United States, obesity rates rose from 14.78% in 1978 to 31% in 2000.

This leads us to ask ourselves the question, “Why such a high rate of increased obesity in the last 40 years?”

The answer to that question has to do with a change in our lifestyles. It is really very simple.

We eat more and we exercise less.

The average American consumes approximately 3600 calories daily, which is way more than what is necessary.

With the rise of corporate jobs that consist of sitting at a desk for 40 hours a week, it is a lot harder to exercise and much easier to overeat.

Research on obesity-related genes is still in its early stages and a lot more research needs to be done before any kind of conclusion can be made, but the fact of the matter is that many people with the “obesity gene” are not obese.

Exercise and a good diet have the ability to counteract the genetic aspect of obesity.

Counteracting obesity with diet and exercise

In a recent 2008 study conducted on 17,058 Danes found that those who lived sedentary lifestyles and had the “obesity-causing” gene had a higher average BMI than those that lived sedentary lifestyles without the “obesity-causing” gene.

However, those who were active with the “obesity-causing” gene had BMI’s that were no higher or lower than those without the “obesity-causing” gene that were active.

In another study that combined and reanalyzed 54 different studies concluded that those with the “obesity” gene had a 23 percent higher risk of obesity than those who did not have it.

Once again, however, those who were active with the “obesity” gene were 30 percent less likely to be obese than inactive adults who carried the gene.

Bringing it all together

In summary, it is clear that genetics play a role in obesity. How big that role is, is still up for debate.

However, it is obvious that environment plays a huge role as well. The key causes of obesity and weight gain include, but are not limited to, the availability of foods at all times of the day, a high decrease in physical activity at “work,” and increased time spent watching television and engaging in sedentary activities.

Another important factor in the current obesity epidemic is the addition of highly processed, high-calorie foods, beverages, and fast food in most people’s diets.

The most important aspect of what we know about causes of obesity and weight gain, is that no matter how “bad” are genetics are, we can overcome them with physical activity and a good diet.

Put It into Practice: 3 Impactful Takeaways to Implement

  • Understand the role of genetics and lifestyle: Consider both genetics and lifestyle factors when addressing obesity and weight gain. While genetics may play a role, lifestyle factors such as diet and exercise can also have a significant impact.
  • Be mindful of food availability: Be aware of the availability of food at all times of the day and try to avoid overeating. Make conscious choices about what and when you eat to avoid consuming more calories than your body needs.
  • Increase physical activity: Incorporate more physical activity into your daily routine to counteract the effects of a sedentary lifestyle. Find ways to be active at work and during leisure time to help prevent obesity and weight gain.


Daniel Maman

Daniel is a certified personal trainer (ACE), has a Bachelors of Science degree in Sports, Exercise, and Wellness, and spends his free time keeping up to date with the latest research in health and fitness. In his free time, you can find him playing basketball, doing muay thai, camping, traveling, reading, and eating tomatoes like they're apples.

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