The First Obesity Gene
In 2007, three independent genome-wide association studies converged on the same locus: a cluster of variants in the first intron of a gene called FTO on chromosome 16. The lead SNP, rs9939609, showed a robust association with body mass index across tens of thousands of individuals. It was the first common genetic variant reproducibly linked to obesity risk, and it sparked a decade of research into the biology of body weight regulation.
rs9939609: The Numbers
The rs9939609 SNP has two alleles: T (ancestral, lower risk) and A (derived, higher risk). The effect sizes, replicated across dozens of studies and hundreds of thousands of participants, are:
- TT genotype: Reference group. Average population weight.
- AT genotype: Approximately 1.2–1.5 kg higher average body weight. About 30% increased obesity risk.
- AA genotype: Approximately 3 kg higher average body weight. About 67% increased obesity risk (odds ratio ~1.67).
To put this in perspective: the entire FTO locus explains roughly 1% of the population variance in BMI. This makes it the single largest-effect common variant for obesity, yet it is a small contributor in absolute terms. Hundreds of other loci, each with smaller effects, collectively contribute to polygenic obesity risk.
It Is Not Actually About FTO
For years after the GWAS discovery, researchers assumed the obesity-associated variants directly affected FTO gene function. FTO encodes an RNA demethylase (an m6A eraser), and early studies tried to connect RNA methylation to appetite and metabolism. But the true mechanism turned out to be far more interesting.
In 2015, a landmark study by Claussnitzer et al. in the New England Journal of Medicine demonstrated that the rs9939609 region contains an enhancer that regulates IRX3 and IRX5, two homeobox genes located over one megabase away. The risk allele disrupts an ARID5B repressor binding site, causing IRX3 and IRX5 to be overexpressed in adipocyte progenitor cells. This shifts fat cell development from thermogenic beige adipocytes (which burn calories as heat) toward lipid-storing white adipocytes.
Exercise Attenuates the Effect
The best news about FTO is that its effect is modifiable. A 2011 meta-analysis by Kilpelainen et al., combining data from over 218,000 adults, found that physical activity reduced the per-allele effect on BMI by 27%. A more recent analysis suggested the attenuation could be as high as 40% in consistently active individuals.
The mechanism likely involves exercise-induced browning of white adipose tissue — physical activity stimulates the conversion of white fat cells toward a more metabolically active beige phenotype, partially counteracting the IRX3/IRX5-mediated developmental bias. Exercise also increases overall energy expenditure, compensating for the reduced basal thermogenesis.
Population Variation
The A allele at rs9939609 shows substantial frequency variation across populations:
- European ancestry: ~42% A allele frequency (AA ~16%, AT ~46%, TT ~38%)
- African ancestry: ~44–52% (varies by subpopulation)
- East Asian ancestry: ~12–20% (much lower, yet obesity is rising rapidly in East Asia due to dietary changes)
- South Asian ancestry: ~30–35%
The mismatch between allele frequency and obesity prevalence across populations underscores that FTO is a susceptibility factor, not a deterministic one. Environmental context — food availability, dietary composition, physical activity levels — overwhelmingly shapes population-level obesity rates.
What This Means for You
- AA carriers: You may have a modest metabolic headwind, but regular physical activity can reduce the effect by a third or more. Focus on consistent exercise rather than extreme diets.
- AT carriers: The effect is roughly half that of AA. Awareness without anxiety is appropriate.
- TT carriers: You lack this particular risk factor, but remember that FTO explains only ~1% of BMI variation. Healthy habits matter regardless of genotype.
Medical disclaimer: This article is for educational purposes only and does not constitute medical advice. Weight management involves complex interactions between genetics, environment, behavior, and medical conditions. Consult a healthcare provider for personalized guidance.