Obesity from MC4R mutations came with lower cholesterol, not higher

The textbook version is simple: weight gain raises cholesterol and triglycerides, and that is one of the routes by which obesity damages arteries. A study published in Nature Medicine on October 16 describes a group of people for whom that chain does not hold [s1].

They carry loss-of-function mutations in MC4R, the gene for the brain melanocortin 4 receptor. The receptor is the best-established single-gene cause of human obesity, and in mice the same receptors regulate lipid metabolism as well as body weight [s1]. Whether that second role carried over to humans had not been established [s1].

What was measured

The researchers drew on the Genetics of Obesity Study, a cohort of 7,719 people, and identified 316 probands and 144 adult family members carrying loss-of-function MC4R mutations [s1]. Their lipid levels were compared against 336,728 controls from the UK Biobank [s1].

The comparison was adjusted for adiposity — the point of the exercise being to ask what the receptor does beyond what it does to body weight [s1]. On that adjusted comparison, adults with MC4R deficiency had lower total cholesterol, lower LDL cholesterol and lower triglycerides than controls [s1].

A second, independent test used the UK Biobank alone. Carriers of loss-of-function MC4R variants within that cohort had lower lipid levels and a lower risk of cardiovascular disease than non-carriers, again after accounting for body weight [s1].

The third strand was physiological rather than epidemiological. After a high-fat meal, people with MC4R deficiency showed a smaller postprandial rise in triglyceride-rich lipoproteins than controls, along with reduced metabolomic markers of fatty acid oxidation — a pattern the authors read as favouring the storage of triglyceride in adipose tissue rather than its circulation [s1].

Their conclusion is that central MC4Rs regulate lipid metabolism and cardiovascular disease risk in humans, and that this points to possible therapeutic approaches for cardiovascular risk reduction [s1].

Why the adjustment is the whole argument

Everything in this result turns on the phrase "after adjusting for adiposity." People with MC4R deficiency carry more weight than the general population; that is the defining feature of the condition. Without the adjustment, their absolute lipid numbers would not necessarily look unusual.

The claim is narrower and more interesting than "genetic obesity is harmless." It is that if you compare a person with MC4R deficiency to a person of the same adiposity without it, the carrier's lipids run lower. That implies the receptor is doing something to lipid handling that is separable from what it does to body weight.

Statistical adjustment for adiposity is not the same as an experiment that holds adiposity fixed. Body-mass index and waist measures are imperfect proxies for fat mass and its distribution, and residual confounding by body composition is the standard objection to analyses of this shape. The UK Biobank carrier analysis partly answers it by repeating the comparison inside a single cohort, but it does not eliminate it.

What this is not

This is a study of a rare monogenic condition. Loss-of-function MC4R variants account for a small fraction of obesity, and nothing here establishes that the same lipid decoupling operates in common, polygenic obesity. The postprandial experiments describe a mechanism consistent with the epidemiology; they do not prove it is the mechanism.

Nor does the finding say anything about how the cardiovascular risk of any individual should be assessed or managed. The lower risk observed in carriers is a group-level association measured against non-carriers in one cohort.

The therapeutic suggestion the authors raise — that central melanocortin signalling might be a target for lowering cardiovascular risk — is a hypothesis drawn from human genetics, which is roughly where most successful lipid drug targets have started. It is not a drug, and no trial has tested the idea.

The other half of the MC4R story this autumn

The same journal's October issue carried a research briefing reporting that tirzepatide drives weight loss in people with obesity due to MC4R deficiency [s2]. That is a separate question — whether an incretin drug works in people whose obesity has this specific genetic cause — and it matters clinically because melanocortin-pathway obesity has historically been treated as a distinct therapeutic problem.

Read together, the two reports sketch a group whose obesity responds to the same class of drug as everyone else's [s2], but whose lipid profile behaves differently from what their weight would predict [s1].

What to watch

Whether the adiposity-adjusted lipid difference replicates in other biobanks with MC4R carriers; whether common variation near MC4R shows the same directional effect on lipids that the rare loss-of-function variants do; and whether any programme attempts to separate the weight and lipid effects of melanocortin signalling pharmacologically.

This article describes genetic and observational research and is informational only. It is not medical advice and does not recommend any test, treatment, or genetic testing decision.

Sources

• [s1] Obesity due to MC4R deficiency is associated with reduced cholesterol, triglycerides and cardiovascular disease risk, Nature Medicine, 2025;31(12):4180-4188, published online 2025-10-16.
• [s2] Tirzepatide drives weight loss in people with obesity due to MC4R deficiency, Nature Medicine, 2025;31:3260-3261, published online 2025-09-05.