Visceral Fat and the South Asian Metabolic Phenotype

A 40 year old man of Indian ancestry living in London, Toronto, Dubai or Sydney, with a BMI of 24 and no obvious excess weight, may carry a risk of type 2 diabetes similar to that of a man of European ancestry with a BMI of 30. This striking difference is one of the most consistent findings in metabolic research, and it has direct consequences for how practitioners assess risk. People of South Asian ancestry, from India, Pakistan, Bangladesh, Sri Lanka and Nepal and their diaspora communities, make up roughly a quarter of the world’s population. Understanding their distinctive metabolic phenotype is therefore a global issue, not a regional one.

The thin fat phenotype

Research comparing people of South Asian and European ancestry at similar BMI has found consistent differences in body composition. South Asian adults tend to have:

• a higher percentage of body fat at the same BMI;
• more visceral fat and more fat in the liver and muscle;
• less lean muscle mass;
• greater insulin resistance at the same body weight;
• in some studies, lower insulin secretory capacity.

An often cited comparison of 2 men with the same BMI, one Indian and one European, found that the Indian man had roughly twice the body fat percentage. Researchers described this as the thin fat or Y Y paradox, referring to the investigators who described it. The phenotype has been observed from birth: babies born in India were found to be smaller than European babies but to have relatively preserved fat and less muscle.

Consequences for disease

This body composition helps explain why South Asian populations develop metabolic disease earlier and at lower weights.

• Type 2 diabetes occurs more frequently and roughly a decade earlier on average. A large United Kingdom study estimated that the risk of diabetes associated with a BMI of 30 in white populations was reached at a BMI of around 24 in South Asian populations.
• Coronary heart disease also occurs earlier and more frequently, with more extensive disease in some studies.
• Fatty liver disease is common at lower BMI.
• Gestational diabetes is more frequent.
• Dyslipidaemia is typically characterised by raised triglycerides, low HDL cholesterol and small dense LDL particles. Some studies have reported higher lipoprotein(a) levels in some South Asian groups.

Why the difference exists

No single explanation accounts for the phenotype. Several contributors are likely.

Developmental origins

The developmental origins of health and disease hypothesis proposes that nutrition before and after birth shapes lifelong metabolic risk. Low birth weight followed by rapid weight gain in childhood is associated with later insulin resistance. In a study of pregnant women in Pune, India, the combination of low maternal vitamin B12 and high folate was associated with greater insulin resistance in their children, an observation that raised questions about micronutrient balance in pregnancy.

Genetics

Genetic studies have identified variants associated with type 2 diabetes and adiposity in South Asian populations, but genetics alone does not appear to explain the difference.

Rapid transition

Many South Asian populations have experienced rapid changes in diet and activity within 1 or 2 generations, including increased refined carbohydrate and oil intake and reduced physical activity, which may be particularly harmful for people with a phenotype adapted to scarcity.

Muscle mass and activity

Lower muscle mass reduces the body’s capacity to take up glucose, and levels of physical activity are lower on average in some South Asian communities, particularly among women and older adults.

Diet

Traditional diets vary widely across South Asia. Modern urban diets are often high in refined grains and sugar and lower in protein relative to energy, which may favour fat accumulation over muscle.

Adjusting thresholds

Because standard BMI thresholds underestimate risk in these populations, several bodies recommend lower thresholds.

Waist circumference and waist to height ratio are particularly valuable because they reflect central fat more directly than BMI. A waist to height ratio above 0.5 is a simple, widely applicable warning sign.

What this means for practice

1. Ask about ancestry as part of risk assessment, respectfully, because it affects thresholds and screening.
2. Measure waist circumference, not only weight and BMI.
3. Screen earlier for diabetes and cardiovascular risk, and consider lipoprotein(a) testing where guidelines support it.
4. Use appropriate thresholds for BMI and waist.
5. Prioritise muscle. Resistance exercise and adequate protein are particularly important where muscle mass is low.
6. Improve carbohydrate quality. Replace refined grains with whole grains, millets and legumes, reduce sugar, and adjust portion sizes of rice and bread, while keeping meals culturally familiar.
7. Consider pregnancy and early life. Screening for gestational diabetes, attention to maternal B12 and folate status, and support for healthy infant growth.
8. Look for fatty liver, which is common even at modest BMI.

Avoiding stereotypes

These are population patterns, not rules about individuals. South Asia is enormously diverse, and risk varies with region, diet, activity, socioeconomic status and many other factors. The point is not to assume that every South Asian patient has metabolic disease, but to avoid falsely reassuring those who do because their weight looks normal.

An illustrative case

A 36 year old software engineer of Bangladeshi heritage, born and raised in the United States, has a BMI of 24 and considers himself healthy. His father developed type 2 diabetes at 45. His waist is 94 cm, his waist to height ratio 0.55, his triglycerides are raised and his HDL is low. His HbA1c is 40 mmol/mol, at the lower edge of the prediabetes range by some criteria. An ultrasound shows fatty liver. By standard BMI thresholds he would not have been flagged; by appropriate thresholds and a waist measurement, he clearly warranted attention.

His plan includes resistance training 3 times a week, a reduction in white rice and sugary drinks, more legumes and vegetables, a protein rich breakfast and a regular sleep schedule. A year later his waist has fallen, his muscle strength has improved and his triglycerides, liver tests and HbA1c have all moved in the right direction.

A global responsibility

With South Asian communities living across every continent, recognising their distinctive metabolic phenotype is part of practising good medicine anywhere. The same principle applies more broadly: standard thresholds derived from one population do not always fit another, and good assessment adapts to the person.