Is there an "obesity gene"? FTO, explained
"Is obesity genetic?" usually leads people to one gene: FTO, the closest thing science has to an "obesity gene." It's real — but the headlines wildly oversell it. Here's what FTO actually does, how small the effect truly is, and how to find your variant.
Quick reference: for the genotype-by-genotype breakdown, see the FTO appetite variant in our gene library.
What FTO is
FTO is the most-studied common gene linked to body weight, and the variant people look up is rs9939609. The A allele is the "risk" version associated with a higher tendency toward weight gain; the T allele is the lower-risk version.
Crucially, FTO doesn't seem to act on metabolism directly — it works mostly through appetite and satiety. People with the risk variant tend to feel full a little less, find high-calorie food a bit more appealing, and eat slightly more without noticing.
How big is the effect, really?
This is where the "obesity gene" label falls apart. On average:
- Each copy of the A allele is associated with roughly 1–1.5 kg more body weight.
- So AA (two copies) ≈ a few kilos heavier on average than TT.
That's a real but small effect. It nudges a probability; it doesn't set your weight. Body weight is shaped by hundreds of genes plus diet, activity, sleep, stress, and environment — FTO is one modest contributor among many.
The encouraging part: it's not fixed
One of the most useful findings about FTO: physical activity blunts its effect. In active people, the weight difference between FTO genotypes shrinks substantially. In other words, carrying the risk variant is not a sentence — it's a small headwind that lifestyle largely offsets. If anything, knowing you carry it is a reason to pay a bit more attention to satiety and movement, not to give up.
How to check your FTO variant
- Download your raw data (or from AncestryDNA / MyHeritage).
- Search it for
rs9939609and read your genotype (AA / AT / TT). - Or use our free DNA explorer — it checks this marker in your browser, nothing uploaded.
The bottom line
FTO earns the "obesity gene" nickname only because it's the best-studied one — the actual effect is a few kilos of tendency, heavily modifiable by activity. It's interesting context, not a diagnosis or destiny. For more of what your file reveals, see our guide to analyzing 23andMe raw data, or browse the rest of the Quanome blog.
Check your FTO variant — and the rest — privately
Quanome reads your raw DNA on your device, so you can look up markers like FTO without uploading your genome. Learn more about Quanome →
Frequently asked questions
Is FTO really an 'obesity gene'?
FTO is the most studied common genetic variant linked to body weight, so it's often called the 'obesity gene' — but the effect is modest. The risk version (rs9939609 A allele) is associated with a slightly higher tendency to overeat and carry a few extra kilos, not a guarantee of obesity.
How big is the FTO effect?
On average, each copy of the risk (A) allele is associated with roughly 1–1.5 kg more body weight, so two copies ≈ a few kg. It's a real but small effect — diet, activity, sleep, and dozens of other genes matter far more.
How does the FTO variant work?
It's mainly linked to appetite and satiety — people with the risk variant tend to feel full a bit less and eat slightly more. Notably, physical activity blunts the effect, so it's far from fixed.
How do I check FTO in my raw data?
Search your 23andMe or AncestryDNA raw file for rs9939609 and read your genotype, or use a tool that looks it up. AA carries two risk alleles, AT one, TT none — but treat it as a small nudge, not a verdict.
Get Quanome at launch
Interested in making sense of your DNA and health data privately? Join the waitlist for early access.