Your genes, which you inherited from your parents, have a large influence on whether or not you will become obese. So far researchers have identified at least 32 different genes that relate to obesity. Many of these genes change our feelings of hunger and fullness, so some people may feel hungrier than other people. There is evidence that our genes influence how much we prefer sweet, spicy or salty foods, or how much we like fruits, vegetables, meats and dairy products.
Your genes also influence your metabolism. About 90% of the difference in metabolism between people is due to differences in age, sex, body muscle, total body size and physical fitness. But about 10% of the difference in metabolism between people is due to their genes. However differences in resting metabolic rate are not enough to determine who becomes obese and who does not. When researchers compared the resting metabolic rate in women who used to be overweight but have dieted and become normal weight, with women who have always been normal weight, they found no differences in their resting metabolic rates. This shows that the women who used to be overweight became overweight for reasons other than a lower resting metabolic rate. Studies of twins also show that differences in resting metabolic rate do not explain obesity. Differences in activity level, such as time spent walking, fidgeting and standing, were most important in determining which twin would become overweight.
The increase in obesity in the past few decades cannot be explained by changes in human genes because it has occurred in only one or two generations. What has changed in the last thirty or forty years is our environment. Our genes determine if we are susceptible or resistant to diseases. But environmental factors determine if a susceptible person will develop the disease. For example, people with light colored hair and blue eyes are more genetically susceptible to sunburn if they are in the sun a long time. If we were recently seeing a large increase in sunburn, it would not be because more people have developed light hair and blue eyes. We would need to look for a change in our culture or environment to explain the rapid change. We may find a change in style of beach clothing that exposes more skin to the sun, or the fact that people are spending much more time in the sunlight. But the change in culture or environment causing an increase in sunburn would mainly affect those people with a genetic predisposition for it. In the same way, the rapid increase in obesity suggests that we should look for environmental changes rather than changes in genetic susceptibility.
This means that something in our environment has changed in the last 30 years to cause those of us who are genetically susceptible to obesity to become overweight. But genetic differences help to explain why some people are more obese and others are still normal weight, though they share the same environment. It seems contradictory to say that obesity is mostly due to genetics, and also mostly due to changes in the environment. Changes in the environment determine if most people in a population are tending to gain or lose weight, while genetic differences determine if any particular individual is more likely to become overweight in that changed environment.
We find that people sharing the same environment tend to have similar levels of obesity. For example, when people marry, their weight difference tends to decrease over time. Both spouses tend to gain or lose weight together. This suggests that it is their shared environment of eating and exercise that is most responsible for their weight changes. Also, when people move to the United States from countries with less obesity, they tend to gain weight. After being in the US for about 15 years, they are as likely to be overweight as someone who was born here.
Even if we are genetically prone to obesity in our current environment, we can change our behavior to prevent ourselves from becoming obese. People with a genetic tendency for heart disease can still reduce their risk by adopting a healthy diet and lifestyle. Individuals can change their diet and activity level to prevent themselves from becoming obese regardless of their genetic risk. But it will be more difficult for them than for someone without the genetic predisposition. To reduce obesity on a large scale, for the population as a whole, we will need to identify and change the environmental conditions that are making so many of us gain weight. But at the very least, knowing about the genetic differences should make those who don’t struggle to maintain a normal weight more charitable towards those who find weight loss to be very difficult.
Your genes also influence your metabolism. About 90% of the difference in metabolism between people is due to differences in age, sex, body muscle, total body size and physical fitness. But about 10% of the difference in metabolism between people is due to their genes. However differences in resting metabolic rate are not enough to determine who becomes obese and who does not. When researchers compared the resting metabolic rate in women who used to be overweight but have dieted and become normal weight, with women who have always been normal weight, they found no differences in their resting metabolic rates. This shows that the women who used to be overweight became overweight for reasons other than a lower resting metabolic rate. Studies of twins also show that differences in resting metabolic rate do not explain obesity. Differences in activity level, such as time spent walking, fidgeting and standing, were most important in determining which twin would become overweight.
The increase in obesity in the past few decades cannot be explained by changes in human genes because it has occurred in only one or two generations. What has changed in the last thirty or forty years is our environment. Our genes determine if we are susceptible or resistant to diseases. But environmental factors determine if a susceptible person will develop the disease. For example, people with light colored hair and blue eyes are more genetically susceptible to sunburn if they are in the sun a long time. If we were recently seeing a large increase in sunburn, it would not be because more people have developed light hair and blue eyes. We would need to look for a change in our culture or environment to explain the rapid change. We may find a change in style of beach clothing that exposes more skin to the sun, or the fact that people are spending much more time in the sunlight. But the change in culture or environment causing an increase in sunburn would mainly affect those people with a genetic predisposition for it. In the same way, the rapid increase in obesity suggests that we should look for environmental changes rather than changes in genetic susceptibility.
This means that something in our environment has changed in the last 30 years to cause those of us who are genetically susceptible to obesity to become overweight. But genetic differences help to explain why some people are more obese and others are still normal weight, though they share the same environment. It seems contradictory to say that obesity is mostly due to genetics, and also mostly due to changes in the environment. Changes in the environment determine if most people in a population are tending to gain or lose weight, while genetic differences determine if any particular individual is more likely to become overweight in that changed environment.
We find that people sharing the same environment tend to have similar levels of obesity. For example, when people marry, their weight difference tends to decrease over time. Both spouses tend to gain or lose weight together. This suggests that it is their shared environment of eating and exercise that is most responsible for their weight changes. Also, when people move to the United States from countries with less obesity, they tend to gain weight. After being in the US for about 15 years, they are as likely to be overweight as someone who was born here.
Even if we are genetically prone to obesity in our current environment, we can change our behavior to prevent ourselves from becoming obese. People with a genetic tendency for heart disease can still reduce their risk by adopting a healthy diet and lifestyle. Individuals can change their diet and activity level to prevent themselves from becoming obese regardless of their genetic risk. But it will be more difficult for them than for someone without the genetic predisposition. To reduce obesity on a large scale, for the population as a whole, we will need to identify and change the environmental conditions that are making so many of us gain weight. But at the very least, knowing about the genetic differences should make those who don’t struggle to maintain a normal weight more charitable towards those who find weight loss to be very difficult.