Differences in the bacteria that live in your colon may actually make you more or less likely to gain weight. There are many different kinds of bacteria in your colon that compete with each other. Some people have more of one type of bacteria while other people have more of another type of bacteria.
Our bodies are unable to separate the glucose molecules in dietary fiber. As the dietary fiber passes into your colon, bacteria separate the sugar molecules and change them into short-chain fatty acids. These fatty acids are then absorbed by your colon and used by your body for energy.
Some bacteria are very good at turning dietary fiber into fatty acids that our bodies can use for energy. Other bacteria are not as good at it, so more dietary fiber remains in the stool rather than being absorbed for energy.
A bacterium called Bacteroides is not very good at digesting the dietary fiber and converting it into fatty acids to be absorbed for energy. A different bacterium called Fermicutes is very good at turning dietary fiber into usable energy. So if your gut contains more of the Fermicutes and less Bacteriodes species, then more of the fiber you eat will be digested and absorbed rather than pass undigested through your bowels. This means that if two people eat the same high fiber meal, one person may digest more energy from the meal than another person because of the different type of bacteria that lives in her colon.
Studies in mice found that obese mice passed less undigested dietary fiber in their stool than lean mice. This suggests that the bacteria in the gut of the obese mice may be assisting the mouse in absorbing extra calories from their food. When researchers transferred gut bacteria from obese mice to lean mice, the lean mice began to gain more weight. A trial of a small number of humans found that obese people had more Fermicutes and less Bacteroides in their gut than thin people.
It is not clear how relevant these findings are to the current epidemic of obesity. It may be that a change in our diet, such as eating more corn-based products or using more antibiotics, has promoted the growth of one type of bacteria more than another. It may be possible that as we learn more about what regulates the type of bacteria in our gut, we can change our diets to promote the growth of gut bacteria that are less efficient at removing calories from the food we eat.
But changes in gut bacteria only change how efficiently your digestive tract removes calories from food you eat. So if you have bacteria that don’t effectively remove all the calories from the food you eat, you can eat more food and “get away with it” by not absorbing all the calories. If you do not eat excess amounts of food, it won’t matter that your gut efficiently absorbs the calories from the food you eat. You still won’t gain weight.
There are some virus infections in animals that cause obesity. One virus related to measles called the canine distemper virus causes obesity in mice by damaging the hypothalamus. When the virus damages the hypothalamus, the mouse no longer responds to satiety signals from the gut and the mouse gains weight. A different virus called the borna disease virus causes obesity in rats by damaging several brain areas including the hippocampus, amygdala and hypothalamus. Luckily, neither of these viruses infects humans.
But there is a different virus, called adenovirus, which causes obesity in animals and also infects humans. There are over 50 different types of adenovirus that infect humans. Some adenovirus’ cause infections similar to the common cold. Others cause bladder infections or irritation around the eyes. One type of adenovirus, called adenovirus-36 may contribute to obesity in humans. Almost one out of every three obese people have been exposed to adenovirus-36, while only about one in ten thin people have been exposed to the virus. When one twin has been infected with adenovirus-36 and the other has not, the twin exposed to the virus is more likely to be overweight
Adenovirus-36 causes fat cells to divide, which creates more fat cells, and causes the cells to store more triglycerides. This virus was first discovered in 1980 but it is likely that it was present for much longer, so it is possible that this virus may contribute to the recent increase in obesity.
But having extra fat cells is like having extra food storage containers. If you don’t have anything to store in the containers, you still will not be obese. A starving person would not keep from starving by being infected with this virus. Your body cannot create triglycerides out of nothing, even if it has a lot of space available for storing them. But if you overeat, this virus will help your body to efficiently store the extra calories and become obese.