Understanding How and Why We Get Fat
Why we get fat? We must first answer that question before we can solve how to get un-fat. Here are what a couple of experts think.
We don’t get fat because we overeat; we overeat because we get fat. Gary Taubes
Obesity is not an imbalance in calories; it is an imbalance in hormones. Robert Lustig, MD
Gary Taubes is the author the books Good Calories, Bad Calories and Why We Get Fat. Dr. Lustig is the author of the book Fat Chance and is a pediatric endocrinologist and director of the obesity clinic at University of California-San Francisco.
Do you believe one or both of these two statements that seem contrary to conventional wisdom? Actually both statements make sense once you understand the role of insulin and other hormones on weight gain. In fact, both men are making similar points, but in different ways. Here’s why.
Insulin: the Fattening Hormone
To understand why we get fat you need an understanding of the role of insulin. Insulin is a hormone produced in the pancreas that is released in response to carbohydrates in a meal. Insulin drives blood glucose or sugar out of the blood stream and into the cells where it can be used for energy.
But, perhaps more importantly insulin is the main fattening hormone of the body. Dr. Lustig believes that 75-80% of obesity is related to high insulin levels. The higher your insulin levels are the more likely you are to be overweight.
Why does insulin lead to fat gain? Insulin drives sugar into to our fat stores where it is stored as calories which can be burned for energy potentially later. But, elevated insulin levels lock those stored calories preventing those fat calories from being burned for energy. So as long as your insulin levels are elevated you cannot use calories stored in fat cells even during exercise.
Yet, your body needs an ongoing stream of energy to function. So what do you do if you cannot use those calories stored in fat for energy? You eat. You eat to provide an immediate energy source to your cells. The problem is if you eat too many bad calories from bad carbohydrates your insulin levels spike causing you to get fatter.
Because insulin prevents you from using those fat calories for energy you find yourself constantly eating to get enough energy to your cells. So you overeat because you are getting fatter. Now you can see why Taubes’ statement makes sense – we overeat because we get fat. Just remember – in the presence of elevated insulin levels you cannot burn calories stored as fat.
Insulin is not the only hormone that affects weight gain. Insulin inhibits another hormone called leptin. Leptin is produced in fat cells and sends a signal to the brain telling it that we have enough calories stored in fat and that we can stop eating. But, insulin blocks this leptin signal. When that happens we don’t feel full and we overeat.
Sex Hormones and Fat Gain
To understand why we get fat you need an understanding of the role of hormones that regulate appetite and the storing and utilization of energy. An enzyme in fat cells called lipoprotein lipase (LPL) pulls fats into fat cells making them fatter. The sex hormones estrogen and testosterone inhibit LPL
As we get older women make less estrogen and men make less testosterone. This enables LPL to work unfettered pulling in more and more fat. We get fatter as our sex hormones decline. Men become more apple-shaped and women pear-shaped. The reason for that is men carry more LPL in their abdominal fat and women carry more LPL in the fat around the hips and pelvis.
Also, testosterone helps both men and women maintain muscle mass. Both sexes lose testosterone with age. Muscle contains an abundance of insulin receptors so that muscle can use glucose for energy. We lose muscle mass as we age due to declining testosterone levels usually coupled with less physical activity. Thus, our ability to clear sugar from the bloodstream is hampered contributing to fat gain. Many thin elderly individuals are skinny fat. They make look thin, but they have too much body fat because of a loss of muscle mass.
Add in the problems with insulin and leptin and you can understand why Dr. Lustig says obesity is an imbalance in hormones. Getting fat is far more than simply consuming more calories than you expend which remains the conventional wisdom.
Article updated: 1/17/23, originally published 4/13/15