When we talk about obesity or metabolic health, body weight is usually the main focus. BMI, scales and total body fat are often used as the primary indicators of health risk.
However, modern metabolic research suggests that how fat is stored may matter far more than how much fat someone has.
Two people can have the same body weight, BMI or even body fat percentage, yet experience very different metabolic outcomes. The difference often comes down to fat cell size, fat distribution and how well fat tissue can safely store excess energy.
How fat cells grow
Fat tissue can expand in two main ways.
The first is hypertrophy. This occurs when existing fat cells grow larger as they store more energy.
The second is hyperplasia. In this case the body creates new fat cells, allowing fat storage to be distributed across a greater number of smaller cells.
This distinction is important because small fat cells tend to remain metabolically healthy. Larger fat cells are more likely to become dysfunctional.
When fat cells enlarge beyond their optimal size, they often become inflamed and insulin resistant. At this stage they begin releasing fatty acids and inflammatory signals into the bloodstream. This contributes to metabolic problems such as fatty liver disease, systemic insulin resistance and eventually type 2 diabetes.
The adipose expandability concept
A useful way to understand this is through the idea of adipose tissue expandability.
Each person has a limit to how much fat their subcutaneous fat tissue can safely store. Once this storage capacity is exceeded, fat begins to accumulate in places it should not be.
This is known as ectopic fat storage and commonly occurs in organs such as the liver, pancreas and skeletal muscle. Think wagyu beef! Fat accumulation in these organs disrupts normal metabolic function and significantly increases the risk of metabolic disease.
This limit is sometimes referred to as a personal fat threshold. Importantly, this threshold varies from person to person.
In simple terms, the issue is not just how much fat someone carries. The real problem arises when the body runs out of safe places to store energy.
Why ethnicity can influence metabolic risk
Fat storage capacity and fat cell behaviour can also vary between ethnic groups.
Some populations tend to have fewer fat cells that enlarge more easily. Because these cells grow larger rather than multiplying, they reach the dysfunctional stage sooner.
As a result, metabolic diseases such as type 2 diabetes can develop at lower body weights compared with populations that have a greater ability to create new fat cells and store fat safely.
This pattern is often seen in parts of Asia. For example, countries such as India, China and Singapore report high rates of type 2 diabetes despite having significantly lower average BMI levels than many Western nations. In India in particular, type 2 diabetes is common even in individuals who would not be classified as overweight.
A similar trend is observed in countries such as Japan and South Korea, where obesity rates are relatively low but metabolic diseases are still present at meaningful levels.
In contrast, populations in countries such as the United States, Australia and parts of Europe generally have a greater capacity for fat cell hyperplasia, meaning the body can create more fat cells to store energy. While obesity rates may be higher in these populations, metabolic disease sometimes develops later because fat can initially be stored more safely under the skin rather than spilling into organs.
These differences help explain why some countries with relatively low obesity rates still experience high rates of metabolic disease, and why BMI alone is not always a reliable indicator of metabolic risk across different populations.
Why BMI or body composition alone can be misleading
These insights highlight an important limitation.
Two people with the same BMI, and even the same body fat percentage, may have very different metabolic profiles depending on:
- fat cell size
- where fat is stored
- how easily their fat tissue can expand
- their level of insulin sensitivity
In some cases, a relatively lean person with large dysfunctional fat cells may be metabolically worse off than someone carrying more total body fat in smaller, healthier cells.
The real focus: fat cell function
The key message is that metabolic health is determined more by fat cell function than by body weight alone.
Improving metabolic health is therefore not just about reducing calories or chasing a lower number on the scale. It involves addressing the factors that influence fat cell health and insulin sensitivity, including:
- improving insulin sensitivity
- reducing chronic inflammation
- maintaining regular physical activity
- prioritising sleep and stress management
- supporting healthy metabolic function overall
When these factors improve, the body becomes better able to store and use energy appropriately.
In other words, the goal is not simply less fat, but healthier fat.
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