18.8 — Obesity and Metabolic Syndrome

The energy balance equation is correct. Weight change equals energy in minus energy out. Physics is not negotiable.

And it is close to useless as advice, because it says nothing about what determines either side — which is hormonal, neural, genetic, environmental and behavioural all at once, and largely not under conscious control.

Body weight is regulated, and defended. The body behaves as though it has a set point, and it opposes attempts to move below it (Chapter 4.7). Which is why weight loss is achievable and weight loss maintenance is where nearly everyone fails — and why framing that failure as a lack of willpower is both wrong and harmful.

The measurements

BMI — weight divided by height squared. Under 18.5 underweight; 18.5 to 24.9 normal; 25 to 29.9 overweight; 30 and above obese.

And its limitations are substantial. It does not distinguish muscle from fat, so athletes are misclassified. It does not indicate where the fat is. And the thresholds differ by ancestry — lower cut-offs are used for South and East Asian populations, because metabolic risk appears at a lower BMI.

Waist circumference is more informative for metabolic risk, because it approximates visceral fat.

And the visceral–subcutaneous distinction is the important one.

Subcutaneous fat — under the skin, on hips and thighs — is metabolically relatively inert and may even be protective.

Visceral fat — around the abdominal organs — is metabolically active, inflammatory, and drains directly into the portal vein and therefore the liver (Chapter 7.5).

Which is why waist-to-height ratio — keeping your waist under half your height — is a better rough guide than BMI, and why someone with normal BMI and central adiposity can be metabolically unhealthy.

Why weight is regulated

Fat is an endocrine organ (Chapter 12.7).

Leptin signals energy stores to the hypothalamus.

And its asymmetry is the central problem. Low leptin is a powerful starvation signal — increasing hunger, reducing energy expenditure, and dominating behaviour. High leptin is a weak satiety signal.

Which makes evolutionary sense — for most of history the fatal error was starving — and it means the system defends against weight loss far more vigorously than against weight gain.

Ghrelin rises before meals and drives hunger. GLP-1, PYY and other gut hormones signal fullness.

And after weight loss, the hormones change in a direction that opposes it. Ghrelin rises, satiety hormones fall, and resting energy expenditure falls below what the new smaller body size predicts — adaptive thermogenesis.

These changes persist for years. A study following participants from a televised weight loss competition found metabolic rate remained suppressed six years later, and most had regained substantially.

So the biology actively opposes maintenance. This is not an excuse; it is an explanation, and it is the reason that treating obesity as a chronic relapsing condition rather than a one-off effort is the correct framing.

What drives the rise

Global obesity has roughly tripled since 1975. Human genetics has not changed.

Which means the causes are environmental, and the main candidates are:

Ultra-processed food — energy-dense, hyperpalatable, and easy to eat quickly.

And there is now trial evidence rather than only association. A controlled feeding study matching diets for calories, sugar, fat, fibre and macronutrients found that people ate around 500 calories more per day on the ultra-processed diet and gained weight — with food freely available in both arms. Something about the processing itself drives intake.

Portion sizes, which have increased substantially.

Sugar-sweetened drinks — liquid calories are poorly compensated for, meaning they do not reduce subsequent food intake the way solid calories do.

Reduced physical activity — though the evidence suggests diet has driven the rise more than activity decline, and activity matters more for maintenance and for health independent of weight.

Sleep deprivation — short sleep raises ghrelin, lowers leptin, and is consistently associated with weight gain (Chapter 24.7).

Medications — some antipsychotics, some antidepressants, steroids, insulin and sulfonylureas.

And genetics. Heritability of BMI is around 40 to 70 percent. Genes do not determine weight; they determine susceptibility to an environment. Rare single-gene causes exist — leptin deficiency, MC4R mutations — and the common form is polygenic.

The health consequences

Type 2 diabetes — by far the strongest association. Cardiovascular disease and hypertension.Non-alcoholic fatty liver disease (Chapter 9.4). Obstructive sleep apnoea (Chapter 8.5). Osteoarthritis — mechanical and inflammatory (Chapter 5.9). Several cancers — obesity is associated with at least 13 cancers, including endometrial, oesophageal, kidney, liver, breast after menopause and colorectal. Reduced fertility in both sexes. Gallstones, reflux, and increased surgical risk.

And metabolic syndrome — the cluster of central obesity, raised triglycerides, low HDL, raised blood pressure and raised fasting glucose. Three of five makes the diagnosis.

Its usefulness is contested — some argue it adds nothing to treating the components individually — and its value is in drawing attention to the clustering, which reflects a common underlying insulin resistance.

Treatment

And this section has changed more in five years than in the preceding fifty.

Lifestyle — dietary change, physical activity, and behavioural support.

Structured programmes achieve around 5 to 10 percent weight loss on average, with substantial individual variation, and maintenance is where they falter.

Which is not a reason to dismiss them. Even 5 percent loss produces measurable improvements in blood pressure, glucose and lipids.

On diets specifically: head-to-head trials of low-fat, low-carbohydrate, Mediterranean and other patterns consistently find similar average results at one to two years, with large individual variation. Adherence predicts outcome far better than composition does.

And physical activity has substantial health benefits at any weight — including in people who lose no weight at all. Fitness independently predicts mortality, which is one of the more useful findings in this field.

Medication — and this is the genuine change.

GLP-1 receptor agonists — semaglutide, and dual agonists such as tirzepatide.

They produce weight loss of 15 to 22 percent in trials — a magnitude previously achievable only with surgery.

And they reduce cardiovascular events, established in the SELECT trial in people with obesity and cardiovascular disease but without diabetes.

Their mechanism is largely appetite reduction, acting centrally, plus slowed gastric emptying.

The honest caveats: gastrointestinal side effects are common; the drugs must be continued, because weight is regained on stopping; the cost is substantial; and long-term data beyond a few years is still accumulating. Muscle loss alongside fat loss is a genuine concern requiring attention to protein and resistance training.

And the framing matters. These drugs work because obesity is a biological condition with a hormonal basis, not because people previously lacked discipline. That has been the most significant shift in how the condition is understood.

Bariatric surgery — produces 25 to 30 percent sustained weight loss, diabetes remission in a majority, and reduced long-term mortality.

And glucose improves within days, before meaningful weight loss — pointing to gut hormone changes rather than restriction alone (Chapter 12.5).

Its risks: surgical complications, nutritional deficiencies requiring lifelong supplementation and monitoring, and — importantly — an increased risk of alcohol use disorder and, in some studies, of self-harm, which requires screening and follow-up.

Weight stigma

And this belongs in a medical chapter rather than outside it, because it causes measurable harm.

Weight stigma is associated with worse mental health, avoidance of healthcare, and — counter-intuitively — weight gain rather than loss.

People with obesity report having symptoms attributed to their weight without examination, which delays diagnosis of unrelated conditions (Chapter 16.4). Equipment is frequently inadequate. And clinicians spend less time with them.

The evidence that stigma motivates weight loss is absent; the evidence that it causes harm is consistent.

Which has practical implications for care: asking permission before discussing weight, using person-first language, having appropriate equipment, and treating the presenting problem rather than assuming.

What actually helps someone

Realistic goals. 5 to 10 percent loss delivers most of the metabolic benefit, and framing success as reaching a "normal" BMI sets up failure.

Focusing on health markers rather than weight alone — blood pressure, glucose, fitness, sleep, energy.

Long-term support rather than short programmes, because it is a chronic condition.

Addressing sleep and stress, which are consistently under-addressed.

Treating contributory conditions — sleep apnoea, hypothyroidism, and reviewing weight-gaining medications.

And recognising that maintenance requires ongoing effort or ongoing treatment, which is true of every chronic condition and is only considered a moral failing in this one.

What the next page fixes

Chapter 18.9 covers cholesterol — what the numbers mean, which ones matter, and where the evidence is strong and where it has been overstated in both directions.