We all know someone who seems able to eat whatever they want without gaining weight.
It can be tempting to look at that person and conclude that their diet simply is not affecting them. But visible body fat only tells us part of the story. The more important question may not be why someone remains thin, but where their body is storing the excess energy they consume.
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Not All Body Fat Is the Same
The fat stored just beneath the skin is called subcutaneous fat. Although people often view this fat as unhealthy, it serves an important protective role.
Subcutaneous fat provides a relatively safe place for the body to store excess energy. When this tissue is functioning properly, it can expand to hold more fat and help keep fatty acids from accumulating in the bloodstream or in organs.
The problem begins when someone’s subcutaneous fat tissue reaches its storage limit—or when that person naturally has very little storage capacity to begin with.
The excess fat still has to go somewhere. It may begin accumulating in areas that were never designed to store large amounts of it, including the:
- Liver
- Pancreas
- Muscles
- Area surrounding the abdominal organs
This means two people can look equally lean on the outside while having very different metabolic health on the inside.
Everyone Has a Personal Fat Threshold
Researchers use the term personal fat threshold to describe the amount of fat an individual can safely store in subcutaneous tissue before fat begins spilling into organs.
That threshold is different for everyone.
One person may remain metabolically healthy at a relatively high body mass index, while another may begin developing fatty liver, insulin resistance, or type 2 diabetes at a body mass index that falls within the “normal” range.
In one study, people with type 2 diabetes who did not have obesity lost just over six percent of their body weight. Most achieved diabetes remission as fat levels in the liver, pancreas, and abdomen declined.
These individuals were not necessarily carrying too much fat according to population standards. They were carrying more fat than their own bodies could safely store.
That is why body weight and body mass index can be useful measurements, but they cannot tell the entire metabolic story.
Thin on the Outside, Fat on the Inside
Imaging studies have shown that people with similar ages, body weights, body mass indexes, and total body-fat percentages can have dramatically different amounts of fat surrounding their internal organs.
This pattern has sometimes been described as thin on the outside, fat on the inside.
A person may have a flat abdomen and very little visible body fat while still carrying substantial fat in the liver or around the abdominal organs. That internal fat burden may increase the risk of insulin resistance and other metabolic problems, even when nothing appears unusual in the mirror.
The scale measures total weight. It cannot distinguish between fat stored safely beneath the skin and fat accumulating in places where it may interfere with normal metabolic function.
Fat Storage Capacity Matters
Metabolic health is influenced not only by how much fat someone carries, but also by how their fat tissue expands.
Fat tissue can accommodate additional energy in two primary ways. Existing fat cells can grow larger, or the body can create additional fat cells to increase its storage capacity.
Those two processes may produce the same change on the scale, but they can have very different metabolic effects.
When existing fat cells become excessively enlarged, they can become less responsive to insulin, release more fatty acids into circulation, produce inflammatory signals, and outgrow their blood supply.
Once those cells can no longer store energy effectively, fat is more likely to accumulate elsewhere.
Research has found that enlarged fat-cell size can predict the future development of type 2 diabetes more effectively than total body-fat percentage. In other words, the health and structure of the tissue storing the fat may matter more than the amount of fat alone.
Insulin Helps Determine Where Energy Goes
Insulin also plays an important role in directing where energy from a meal is stored or used.
After eating, dietary fat circulates through the bloodstream in specialized particles. An enzyme called lipoprotein lipase helps tissues pull that fat out of circulation.
Fat tissue uses the enzyme primarily to store fatty acids, while muscle can use those fatty acids for energy.
Insulin encourages fat storage by increasing this enzyme’s activity in fat tissue. At the same time, insulin can reduce its activity in muscle, shifting muscle toward burning glucose instead of fat.
The strength of this response differs between individuals. In some people, a carbohydrate-rich meal may strongly favor fat storage. In others, the same meal may produce a much smaller change.
Physical activity can also influence this balance. Research involving endurance athletes found that only two weeks without training shifted enzyme activity away from fat burning in muscle and toward fat storage in fat tissue.
Movement does not simply burn calories. It can also influence how the body partitions and uses fuel.
Is It Just a “Fast Metabolism”?
People do differ in how much energy they burn, but the explanation is not always a dramatically faster resting metabolism.
Controlled overfeeding studies have found large differences in how much weight people gain when given the same caloric surplus. One of the strongest differences was the amount of energy burned through ordinary daily movement—not formal exercise, but standing, walking, shifting posture, pacing, and fidgeting.
Some people unconsciously increase this movement when they overeat, while others do not.
Genetics also appear to influence how the body responds to excess energy and where fat is stored. Studies involving identical twins have found that twins tend to gain similar amounts of weight and visceral fat when overfed, suggesting that biology plays an important role.
There may also be a simpler explanation in some cases: we rarely see everything another person eats. Someone who consumes a large lunch may naturally eat very little later in the day.
The Mirror Is Not a Metabolic Test
Someone who eats poorly and remains thin does not disprove the importance of insulin or metabolic health.
They may have a large and well-functioning capacity to store fat safely beneath the skin. They may naturally burn more energy through everyday movement. Or they may be storing excess fat internally, where it cannot be seen.
That is why leanness should not automatically be treated as proof of good metabolic health.
Measurements such as fasting insulin, triglycerides, HDL cholesterol, liver enzymes, glucose control, and liver-fat imaging can reveal risks that body weight and appearance may miss.
Being lean may be consistent with good metabolic health—but the mirror alone cannot confirm it.