Are humans born with all the fat cells they will ever have?
No. New fat cells are made throughout our lives. When the white adipocytes store too much triglyceride, they get really big and they get “sick” and die faster. It’s the rate at which the white cells take up the fat to store it and then get rid of it that can impact whether someone gains a lot of weight and whether they can successfully lose it after reasonable effort.
The average lifespan of a white fat cell is 15 years. We have no idea yet of the lifespan of a brown fat cell.
Is there a single “fat gene”? What role do fat genes play in the likelihood of developing metabolic diseases and type 2 diabetes?
Genes are very important for influencing the development of obesity and probably influence 50%-70% of obesity, based on studies in populations of predominantly European origin. But that high percentage reflects the impact of hundreds of genes. For most people, there is no one gene that exerts all of the effects. There are extremely rare diseases where one gene is responsible. Currently, only 20% of the entire phenotypic variation in obesity can be explained by the thousands of loci identified so far.
Why is it “correct but too simplistic” to attribute the increasing rates of obesity to excessive triglyceride storage in white adipose tissue?
Saying obesity is caused by too much triglyceride storage ignores the reasons how and why the triglycerides got there. There are likely to be multiple contributing factors to drive obesity, and those have billions of dollars of policy implications. Is obesity resulting from portion sizes? Then we should work on educating the public on how to estimate their caloric intake. Is it the types of foods, such as ultra-processed foods? Then we can discourage eating certain food groups while promoting others. Is it about physical activity? Then we should prioritize exercise programs.
Why is obesity “not simply a failure of will power”?
Genetic factors in adipose tissue impact how easy it is to store triglycerides, how easy it is to get fat out of the tissue and burn it up, and what kinds of hormones are released by the tissue to regulate appetite, insulin resistance, and inflammation. Ten different people can all overeat the same amount of the same foods, yet there will be differences in the amount of weight gain and metabolic complications experienced. And at the brain level, some people will feel “full” sooner than others.
How can excess adipose tissue lead to disease? Do some people have “metabolically healthy obesity”?
Excess adipose tissue leads to chronic inflammation that can then cause insulin resistance, hypertension, fatty liver disease, and other complications. It appears that there are metabolically healthy obese people, but it is not clear if that is only a temporary state.
Could long-term brown adipose tissue activation help treat obesity or related metabolic disease?
Our research group at the NIH and others have shown that long-term brown adipose tissue activation produces metabolic benefit such as improved insulin resistance, lower plasma glucose, and higher HDL [good] cholesterol. However, there is no evidence yet that it will lead to actual weight loss.