You’ve likely heard it more than a few times: follow this diet plan or that exercise program and watch the fat “melt away”. Visually enticing, but impossible when it comes to actual fat cells found in the body. In fact, scientists have determined that from birth until the early 20s, the number of fat cells in the body increases, but after that point, the quantity of fat cells remains constant. As fat cells die, the body replaces them. This function is important as scientists seek to understand how to fight obesity as the U.S. rate amongst adults is at 36%- an all time high.
After early adulthood, not only does the quantity of fat cells not change, but contrary to what some believe- it also can’t be converted to muscle. Physiologically, fat cells are fat cells, and muscle cells are their own unique cells. When speaking about the two, it is comparing apples and oranges. And as muscles grow larger with exercise, fat cells shrink, but do not completely “melt away”.
Despite their differences, their relationship is important. Sources explain that during weight loss, fat is taken from fat cells and used to produce energy in the body along with other byproducts. Each day, you’ll consume a certain number of calories and your body will either use those calories to rebuild muscle, burn it as fuel, or store it as fat. Each individual requires a certain amount of calories to carry out basic functions- called our basal metabolic rate.
Along with other factors, not eating enough, in an effort to cut calories, can impact our metabolic rate. This leads to our body thinking it needs to help you survive and will slow metabolism down. Not eating enough calories can lead to muscle loss and your body holding onto fat, which can ultimately work against you, if you’re seeking to lose weight. This function- where the body shifts to hold onto fat cells- is linked to self-preservation etched into our biology- well before vanity was a driving reason for weight loss, and well before the dangers of obesity and carrying extra weight were understood.
Is fat really that bad?
Fat always seems to get a bad rap- but at the most basic level, our body requires it to function. Fat cells help our body sense respond to changes in energy balance. White fat cells release hormones such as leptin that impact food intake, insulin sensitivity, and insulin secretion. Brown fat produces chemical energy in the form of heat and defends against hypothermia, obesity, and diabetes.
All fat isn’t created equally, though. Brown fat, as we’ve shared before, is a type of fat within the body that contains mitochondria (an orangelle that generates energy within cells) by generating heat using the many smaller droplets of fat within the brown fat cells. During this process, called thermogenesis, the brown fat burns calories. This is unlike white fat cells, which are a single large, oily droplet that is otherwise relatively inert. Research has found that contrary to previous belief that only babies had brown fat, which helped them stay warm when they could not yet regulate their temperature, adults have brown fat too. And so began the research to figure out how to increase brown fat stores or to leverage existing brown fat stores to take advantage of its ability to burn calories and reduce stores of white fat in the process. It’s an intriguing approach when it comes to understanding how to utilize the body’s own functions to lose weight efficiently.
What happens to fat cells during weight loss?
Michael Jensen, an obesity researcher at the Mayo Clinic explains, “Fat cells’ main function is to hold on to lipids. These fatty molecules are the body’s main choice of energy reserve — each white fat cell encapsulates a drop of them. When we lose weight, these liquid fat reserves are drained to fuel the body. But the cell itself remains. Fat cells, or adipocytes, can grow or shrink dramatically, changing in size by up to a factor of 50”.
Studies also support what many of us have experienced first-hand. Adipocytes are prone to regaining their size, especially in situations where fat is “lost” rapidly, as some fad diets encourage people to do. The hormone leptin, which regulates appetite, seems to be the primary culprit in why this occurs. During weight loss, leptin levels decrease- causing appetite to increase. As more food is ingested, adipocytes then increase in size
“As they get smaller, those fat cells can rev up their machinery for fat storage,” Jensen says. “So when people do start taking in more fat in their diet, the fat cells are there just ready to grab onto it and store it for you.”
How do we know the amount of fat cells doesn’t change in adulthood?
Research carried out by Kirsty Spalding of the Karolinska Institute in Stockholm, Sweden, and her team found that there is a dynamic process of cell death and replenishment. The study recorded the number and size of fat cells, age, sex, and BMI of 687 people. Using previous data form children, Spalding’s team was able to show that the average number of fat cells rises until the age of about 20, and then remains relatively constant, and is closely linked with BMI.
The real intrigue of the study, though, came from utilizing a sliver of our history. The study focused on fat that was removed during liposuction from people that lived through the Cold War atomic bomb testing period (1955-63). The testing resulted in the atmosphere being more radioactive than typical, elevating levels of an isotope called carbon-14 in foods that were grown and consumed at the time.
The results showed that fewer fat cells had a heavy dose of carbon-14 than might be expected if these cells were never replenished. This shows that the cells have been subject to turnover in the decades since the atomic testing.
Childhood obesity and the impact later in life
Spalding’s research highlighted that in humans, fat cells increase throughout childhood, but it becomes a fixed quantity after the mid 20s. When adults gain or lose weight, this is not an increase in the total amount of fat cells (also called adipocytes), but the change in the existing fat cells’ size. Spalding’s research underlines the importance of managing childhood obesity, as the evidence from that study suggests that obese children may struggle to lose weight later in life.
Other research supports these findings, as children that are obese become obese adults, and the numbers are staggering. Studies show that obese children and adolescents were around five times more likely to be obese in adulthood than those who were not obese. Around 55% of obese children go on to be obese in adolescence, around 80% of obese adolescents will still be obese in adulthood and around 70% will be obese over age 30.
In a country battling high levels of obesity in adults, a serious focus needs to be on building and maintaining healthy lifestyles, a routine of nutritious diet, as well as regular physical activity for children to be in a position to avoid obesity and the associated health risks, later in life.
Losing weight and shrinking fat cells is a delicate balance that requires patience and time to achieve. Too much, too quickly and they tend to rebound. Preventing childhood obesity is the first step.
Further research to better understanding how fat cells replenish themselves may be the key to finding ways to safely lose weight and keep it off
-https://massivesci.com/notes/fat-cells-nutritional-memory-in-mice/
-https://www.discovermagazine.com/health/your-fat-cells-never-disappear-making-future-weight-gain-more-likely
-https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4985254/
-https://www.lifehealthcare.co.za/news-and-info-hub/latest-news/where-does-fat-go-when-you-lose-weight/
-https://www.nature.com/articles/news.2008.800
-https://rdcu.be/cSosB
-https://pubmed.ncbi.nlm.nih.gov/26696565/
-https://www.cdc.gov/vitalsigns/childhoodobesity/index.html