Obesity as a Chronic Multisystem Condition

Few conditions are as common, as consequential or as misunderstood as obesity. It affects a large and rising share of adults and children worldwide. It increases the risk of more than 200 other conditions. And it is still widely treated as a simple failure of willpower, a view contradicted by decades of research into how the body regulates weight. Understanding obesity as a chronic, biologically regulated, multisystem condition changes how practitioners assess it, how they talk about it and how they help people manage it.

Defining obesity

Obesity has traditionally been defined by body mass index: a BMI of 30 or above in most populations, with lower thresholds recommended for some Asian populations. BMI is simple and useful at population level, but it has well known limitations for individuals. It does not distinguish fat from muscle, does not show where fat is stored and does not directly measure health.

In 2025, an international commission published in The Lancet Diabetes and Endocrinology proposed distinguishing between preclinical obesity, in which excess adiposity is present without current organ dysfunction, and clinical obesity, a chronic disease in which excess adiposity is causing organ or tissue dysfunction or limiting daily activities. It recommended confirming excess adiposity with waist measurements or direct body composition assessment rather than BMI alone. The proposal reflects a broader move towards defining obesity by its effects on health.

A multisystem condition

How the body regulates weight

Body weight is not simply the arithmetic result of calories eaten minus calories burned. It is regulated by a complex system centred in the brain, which integrates signals from fat tissue, such as leptin, from the gut, such as ghrelin and GLP 1, and from the environment. This system tends to defend a particular range of body weight.

When people lose weight, the system responds. Appetite increases, satiety decreases and energy expenditure falls further than would be expected from the reduced body size, a phenomenon called adaptive thermogenesis. In one study, changes in appetite hormones persisted a year after weight loss, favouring regain. This biology, not a lack of willpower, explains why most people regain weight after dieting, and why obesity behaves as a chronic, relapsing condition.

What contributes

Genetics

Twin and family studies suggest that a substantial proportion of the variation in body weight between people is heritable. Most genetic influence comes from many variants of small effect, many acting on appetite regulation in the brain. A small number of people have single gene causes, such as variants in the MC4R gene, which can cause severe early onset obesity.

Environment

The food environment, with abundant, inexpensive, energy dense and highly palatable ultra processed foods, large portions and aggressive marketing, interacts with genetic susceptibility. A controlled inpatient study found that people ate about 500 more calories a day on an ultra processed diet than on an unprocessed diet matched for presented nutrients, and gained weight. Environments that discourage walking and active travel also contribute.

Sleep and circadian disruption

Short sleep increases appetite and energy intake. Shift work is associated with weight gain.

Stress and mental health

Chronic stress, trauma and depression influence eating behaviour, and adverse childhood experiences are associated with obesity in adulthood.

Medicines

Many medicines promote weight gain, including some antipsychotics, antidepressants, antiepileptics, glucocorticoids, insulin and sulphonylureas, and some beta blockers. Reviewing medicines is part of every assessment.

Medical conditions

Hypothyroidism, Cushing syndrome, polycystic ovary syndrome and some genetic syndromes contribute in a minority.

The harm of stigma

Weight stigma, negative attitudes and discrimination based on body size, is common in healthcare as well as society. An international consensus statement concluded that weight stigma causes physical and psychological harm, reduces the quality of care and discourages people from seeking help. It recommended using person first language, such as “a person with obesity”, avoiding blame, and focusing on health rather than appearance. Practitioners can reduce stigma through their language, their clinical environment and their assumptions.

Treatment

Lifestyle intervention

Structured programmes combining dietary change, physical activity and behavioural support typically achieve average weight loss of around 5 to 10 percent, which substantially improves blood pressure, glucose, lipids, liver fat and mobility. Maintenance is the main challenge, and long term support improves results.

Medication

Newer medicines have transformed obesity treatment. In trials, semaglutide produced average weight loss of around 15 percent over about a year and a half, and tirzepatide around 20 percent. In the SELECT trial, semaglutide reduced major cardiovascular events by about a fifth in people with obesity and established cardiovascular disease without diabetes. Weight is typically regained after these medicines are stopped, consistent with obesity as a chronic condition requiring long term treatment. Side effects, cost, access and the loss of lean mass during rapid weight loss are important considerations. These medicines are prescribed and monitored by doctors.

Surgery

Bariatric surgery produces the largest and most durable weight loss and has been associated in long term studies with reduced mortality, diabetes remission and other benefits. It requires lifelong nutritional follow up.

The role of nutrition and exercise alongside medication

When people lose weight rapidly, particularly with medication, adequate protein intake and resistance exercise help preserve muscle. Nutritional quality matters when appetite is suppressed, to avoid deficiencies. Behavioural and lifestyle support remain relevant for long term health.

A functional approach

1. Ask permission to discuss weight, and ask what the person wants to achieve.
2. Build the timeline of weight change: when it began, what changed, what has been tried.
3. Identify mediators: sleep, stress, medicines, food environment, activity, mood, pain.
4. Assess health, not just weight: blood pressure, glucose, lipids, liver, sleep apnoea, joints, mood.
5. Set health focused goals, such as improving blood pressure, mobility or sleep, alongside or instead of weight targets.
6. Combine approaches, with referral for medication or surgery where appropriate.
7. Plan for the long term, with regular follow up.

An illustrative case

A 45 year old woman with a BMI of 38, type 2 diabetes, knee osteoarthritis and sleep apnoea has tried many diets, losing and regaining weight repeatedly. She feels blamed by previous clinicians. Her weight began rising after starting an antidepressant during a difficult period and after the birth of her second child. She sleeps poorly and works night shifts.

The conversation begins by acknowledging her experience and explaining the biology of weight regulation, which she finds relieving. Her doctor reviews her antidepressant and, after discussion, starts a GLP 1 receptor agonist that also treats her diabetes. Her sleep apnoea is treated. A dietitian helps her plan protein rich meals around her shifts, and a physiotherapist designs a strength and mobility programme for her knees. At a year she has lost 14 percent of her body weight, her HbA1c has improved substantially and her knee pain is less. Just as importantly, she has a care team she trusts and a plan for maintenance.

A chronic condition, treated as one

Obesity is not a lifestyle choice to be corrected by advice. It is a chronic, biologically regulated condition with many causes and many consequences. Treating it as such, with compassion, long term support and the full range of evidence based options, produces better outcomes than blame ever has.