The Health Risks of Obesity: What Excess Weight Does to the Body

If you have been told that your weight is affecting your health, it is easy to hear it as a judgement rather than as medical information. It isn't meant that way. Obesity is a recognised chronic condition — one that quietly raises the risk of a long list of other problems — and understanding how it does that is the first step to doing something about it. This guide maps the main health risks honestly, explains why the location of fat matters more than the number on the scale, and ends with the part that often gets lost: how much of this risk is reversible.

What counts as obesity?

Obesity means carrying enough excess body fat to harm your health. The most common screening measure is body-mass index (BMI) — your weight in kilograms divided by your height in metres squared. In adults, a BMI of 25 or above is classed as overweight and 30 or above as obesity ¹.

BMI is a useful population tool but a blunt instrument for any one person: it can't tell muscle from fat, and it doesn't say where the fat is. A very muscular person can have a high BMI and be perfectly healthy, while someone with a "normal" BMI can carry risky fat around their organs. That's why clinicians increasingly pair BMI with waist circumference and metabolic blood markers — a theme we return to throughout this article.

Obesity is now extraordinarily common. The WHO estimates that in 2022, 1 in 8 people worldwide were living with obesity, and that in 2021 a high BMI contributed to an estimated 3.7 million deaths from non-communicable diseases ¹. This is not a fringe problem — it is one of the defining health issues of our time.

Why does excess fat cause disease?

The intuitive idea is that extra weight is simply a mechanical burden — more load on the joints, more strain on the heart. That's part of it. But the bigger story is that body fat is not inert padding. Fat tissue is an active organ that releases hormones and inflammatory signals into the bloodstream. When there's too much of it — especially the wrong kind in the wrong place — that signalling goes haywire and drives disease through three main pathways:

• Insulin resistance. Excess fat makes the body's cells respond poorly to insulin, the hormone that controls blood sugar. This is the engine behind type 2 diabetes, and it builds silently for years — often visible on a fasting insulin / HOMA-IR test long before standard blood sugar looks abnormal.
• Chronic low-grade inflammation. Overloaded fat tissue leaks a steady drip of inflammatory molecules, producing the kind of persistent, low-level inflammation that damages blood vessels and tissues over time.
• Mechanical and hormonal load. Extra weight raises blood pressure, strains weight-bearing joints, and alters levels of hormones including oestrogen and insulin-like growth factors — which is part of why obesity is linked to certain cancers.

Hold onto one idea: most of obesity's harm is metabolic, not cosmetic. That's also the good news, because metabolic damage responds to even modest change.

The major health risks, by system

Metabolism and blood sugar

This is the most direct link. Excess fat drives insulin resistance, and nearly 9 in 10 people with type 2 diabetes have overweight or obesity ². Obesity is also the central feature of metabolic syndrome — the cluster of a large waist, high blood pressure, high triglycerides, low HDL cholesterol and high fasting glucose that together multiply cardiovascular risk ². For the full picture of how blood sugar goes wrong, see our guide to diabetes.

Heart and blood vessels

Obesity raises blood pressure, worsens the cholesterol profile, and promotes the artery-narrowing process of atherosclerosis — together pushing up the risk of heart attack, heart failure, irregular heart rhythms and stroke ⁴. We cover this in depth in obesity and heart disease.

Liver

When fat spills over into the liver, the result is metabolic-associated fatty liver disease — now one of the most common liver conditions in the world. In some people it progresses to inflammation, scarring and even cirrhosis ². It is usually silent, picked up only on liver blood tests or a scan.

Cancer

This surprises many people: excess body weight is an established cause of at least 13 different cancers, including cancers of the bowel, breast (after menopause), womb, kidney, liver and pancreas ³. The link is real but often under-appreciated — we unpack it in obesity and cancer risk.

Joints and mobility

Carrying extra weight loads the knees and hips with every step, and the inflammatory signals from fat tissue add a second hit — which is why obesity is a leading risk factor for osteoarthritis ². More in obesity and joint pain.

Breathing, sleep and more

Excess weight is strongly linked to obstructive sleep apnoea (repeated pauses in breathing during sleep) and asthma, and is associated with gout, gallstones, kidney disease, fertility and pregnancy problems, and a higher risk of depression ². It rarely travels alone.

Does where the fat sits really matter?

Yes — arguably more than the total. Fat stored just under the skin (subcutaneous fat, the kind you can pinch) is relatively benign. Fat packed deep in the abdomen around the organs (visceral fat) is metabolically active, drains directly to the liver, and is the type most tightly linked to diabetes, heart disease and fatty liver. This is why a tape measure around the waist can predict risk better than the scale, and why two people at the same weight can have very different health. We give this its own article — visceral fat: why belly fat is the dangerous kind — because it's one of the most useful ideas in the whole topic.

What we see at the clinic

The people who come to us worried about their weight are rarely looking for a lecture — they've usually had plenty. What tends to help more is reframing the conversation around markers they can move: fasting insulin, blood pressure, waist circumference, liver enzymes. When someone sees those numbers improve after a few months of realistic change, it's far more motivating than chasing a goal weight. We also make a point of saying clearly that obesity is a medical condition with strong genetic and environmental drivers — not a character flaw — because the shame around it often gets in the way of getting help.

Common questions

Is BMI enough to tell if obesity is harming me? No. BMI is a screening starting point, not a verdict. Pair it with waist circumference and a few blood markers (fasting glucose or insulin, lipids, liver enzymes) for a much truer picture of metabolic health.

Can you be "fat but fit"? Up to a point. Fitness and good metabolic markers genuinely lower risk at any size, and some people with obesity are metabolically healthy. But that status often erodes over time, so it's best thought of as a reason to keep monitoring, not a reason to relax.

How much weight do I need to lose to benefit? Less than most people expect. Losing 5–10% of body weight produces measurable improvements in blood sugar, blood pressure and liver fat ². You don't need to reach an "ideal" weight to meaningfully cut your risk.

Are weight-loss injections a shortcut? The GLP-1 medicines (such as semaglutide) genuinely produce significant weight loss and improve several risk markers, but they are prescription medicines with side effects and trade-offs — best used within proper medical care, not bought online.

Key takeaway

Obesity matters because of what it does inside the body, not how it looks: it drives insulin resistance, inflammation and high blood pressure, and through them raises the risk of diabetes, heart disease, fatty liver, several cancers and joint disease. The encouraging flip side is that the same biology runs in reverse — even modest, sustained weight loss improves multiple risks at once. If you want to understand your own numbers rather than guess, that's exactly the kind of thing a consultation can map out.