It is no surprise that society is facing a public health crisis in the form of obesity. While excessive accumulation of body fat is a hallmark of obesity, body fat itself, in the right proportions and right anatomical locations, is essential for human health. Body fat plays numerous roles in human health and is primarily responsible for the storage and mobilization of energy. Yet, in this country, where the prevalence of obesity in American adults was approximately 42.4% for 2017-2018 (https://www.cdc.gov/obesity/data/adult.html), the main issue is unfortunately not healthy body fat, but rather the accumulation of excess body fat in unhealthy locations leading over time to diseased body fat. Over time this excessive dysfunctional body fat precipitates other conditions such as metabolic syndrome and cardiovascular disease. Therefore, many people are stunned to learn that healthy body fat is essential for living a metabolically healthy life.
The main physiological role of body fat is to store and mobilize energy depending upon your nutrient and hormonal status. When nutrients are plentiful, as after eating a meal, food is digested, and sugar and fat enter your blood stream. Sugar in particular signals to your pancreas to produce the hormone insulin which is essential for clearing blood sugar up into your muscles, a process termed insulin stimulated glucose uptake. Insulin also signals to your body fat resulting in the storage of excess sugar and fat here as opposed to other organs like the liver, which is exactly what body fat is meant to do. These processes occur in healthy, insulin sensitive individuals. Conversely, when nutrients are scarce, such as when we are fasted or have not eaten in a really long time, our body fat releases energy into circulation to be used by other tissues including the liver and the brain. This process occurs in response to two primary stress hormones named glucagon and cortisol and occurs in lean, healthy individuals. Therefore, body fat is essential for the proper storage and mobilization of energy in our bodies in a nutrient dependent manner. Overall, through these processes body fat makes a significant contribution to the overall maintenance of metabolic health.
So, what happens during obesity when individuals accumulate an excessive amount of body fat? For starters, this excess body fat accumulates in metabolically dangerous areas, such as around internal organs where it has significant negative health effects such as insulin resistance. Conversely, healthy body fat accumulates subcutaneously or just under your skin contributing to insulin sensitivity. Additionally, while healthy body fat properly stores and mobilizes energy in an insulin sensitive manner, unhealthy body fat is resistant to the energy storing effects of insulin resulting in the continual release of energy into circulation in the form of free fatty acids. Since this unhealthy fat is in such close proximity to internal organs, these free fatty acids end up going to the liver for storage and can ultimately result in a dangerous condition known as non-alcoholic fatty liver disease (NAFLD). Collectively, the accumulation of excess diseased body fat surrounding internal organs results in harmfully high levels of inflammation and can proceed the development of cardiovascular disease. Overall, body fat has garnered a negative reputation due to its pathophysiology during obesity. However, it is important to recognize that in a healthy context, the function of body fat is imperative for physiological and metabolic health.