No weight gain, but your genes already changed: 6 weeks on a Western diet alters belly fat at the molecular level

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Stepping on the scale after a meal of burgers and fries and finding the number unchanged might feel like a pass — but new research suggests the body is already changing in ways the scale cannot show.

When healthy adults were fed a Western-style diet for six weeks, gene activity in their abdominal fat tissue shifted measurably, even though researchers carefully matched caloric intake to each participant's daily energy needs to prevent weight gain.

Among the genes that became more active were a large number associated with detecting smell. The more strongly those genes were switched on, the more liver fat and insulin resistance the participants showed.

A research team led by Andreas Pfeiffer at the Charité medical school in Germany published the findings in volume 12, issue 34 of the international journal Science Advances.

Little change on the scale, but a different story inside

It has long been known that a Western diet can trigger metabolic disease, but determining whether the diet itself is the cause — or whether weight gain is the real culprit — has proved difficult.

The research team recruited 92 participants — 46 pairs of twins — with an average age of 31. All were healthy and lean.

Participants first followed a low-fat diet for six weeks to bring them to a comparable baseline. They then switched to a Western high-fat diet for another six weeks, eating bread and butter, french fries, processed meats, hamburgers, sausages and pizza.

For both phases, the team measured each participant's daily energy expenditure in advance, cross-checked it against five days of food records, and provided individual nutritional counseling to keep body weight stable.

Despite the dietary shift, average body weight rose by just 0.25 kilograms, or 0.39 percent, over the six weeks.

In the blood, total cholesterol, LDL (low-density lipoprotein) and HDL (high-density lipoprotein) all rose within the first week and climbed further by week six. Triglyceride levels did not change.

Fasting insulin also rose within a week, though fasting blood glucose remained unchanged.

Inflammatory markers IL-6, IL-18 and TNF-alpha increased in the first week but returned to baseline or fell below it by week six. Only CRP, a general marker of inflammation, was slightly elevated at the six-week mark.

In abdominal fat tissue, however, the changes were far more pronounced. Of roughly 22,000 genes the team analyzed, 8,771 showed altered activity.

Smell-sensing genes switched on in belly fat

When the researchers grouped the altered genes by function, they found olfactory receptors — proteins that capture odor molecules and convert them into signals. Their presence in abdominal fat tissue was unexpected.

Examining the 15 olfactory receptor genes that showed the greatest change, the team found that participants with larger shifts in those genes also had higher levels of liver fat, visceral fat, insulin resistance, the inflammatory marker IL-18 and LDL cholesterol.

How much weight each participant gained also tracked with the degree of change in olfactory receptor activity.

Of the 15, the only one with a known trigger is OR7D4, a receptor that responds to an odor compound found in pork. Prior research has linked variants of OR7D4 to body mass index, body fat percentage, waist circumference and susceptibility to hunger. What the remaining 14 receptors respond to is not yet known.

The research team suggested that an increase in olfactory receptor activity could make fatty, calorie-dense foods more appealing.

However, the team said what the experiment confirmed was a correlation — that the variables moved together — not evidence that olfactory receptors cause metabolic dysfunction or disease. The researchers also noted that participants were Western Europeans and mostly lean, making it difficult to generalize the findings to other ethnic groups or people with obesity.

The paper reanalyzed data from the earlier NUGAT study rather than drawing on a newly recruited clinical trial.

Reference

DOI: 10.1126/sciadv.aef5698

Jonathan Bertram et al., "Isocaloric switch from low-fat to Western high-fat diet triggers transcriptomic reprogramming and ectopic olfactory receptor responses." Sci. Adv. 12, eaef5698 (2026).

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