"Why am I gaining weight even though I haven't changed anything?" is one of the most frustrating questions in medicine — and the answer is almost always more complex than the classic "calories in, calories out" narrative suggests. While energy balance is ultimately the fundamental mechanism of weight change, the factors regulating both sides of that equation are far more complex than diet and exercise choices alone.
Medical Causes Worth Ruling Out
Hypothyroidism
An underactive thyroid reduces metabolic rate, causing weight gain, fatigue, cold intolerance, constipation, and dry skin. Even mild thyroid underfunction can produce a 5–10 kg gain over months. TSH testing identifies this easily correctable cause.
Polycystic Ovary Syndrome (PCOS)
The most common endocrine disorder in women of reproductive age, PCOS involves insulin resistance, androgen excess, and ovulatory dysfunction. Insulin resistance promotes fat storage — particularly abdominal — and makes weight loss significantly harder despite similar caloric intake. Up to 80% of women with PCOS are overweight or obese. Treatment with metformin improves insulin sensitivity and may facilitate weight loss.
Cushing's Syndrome
Excess cortisol — from a tumour or, more commonly, long-term corticosteroid medications — causes characteristic central obesity (truncal fat accumulation with thin extremities), moon face, stretch marks, and hypertension. Medication-induced Cushing's is the most common cause; review corticosteroid use when unexplained central weight gain occurs.
Medications
This is one of the most underappreciated causes of weight gain. Antidepressants (particularly paroxetine, mirtazapine, amitriptyline), antipsychotics (olanzapine, quetiapine), mood stabilisers (lithium, valproate), insulin, sulfonylureas, beta-blockers, and corticosteroids all promote weight gain through appetite stimulation, fluid retention, or metabolic effects. Always review medications when unexplained weight gain occurs.
The Biology of Weight Regulation
Body weight is regulated by an extraordinarily complex neuroendocrine system. The hypothalamus integrates signals from hormones including leptin (fat mass signal), ghrelin (hunger hormone), insulin, GLP-1, PYY, and cholecystokinin to regulate appetite, satiety, and metabolic rate. After weight loss, the body adapts by reducing resting metabolic rate (adaptive thermogenesis) and increasing appetite hormones — making sustained weight loss biologically much harder than losing it in the first place. This is not a willpower deficit; it is physiology.
Sleep Deprivation and Weight
Insufficient sleep (less than 7 hours) acutely increases ghrelin (hunger), decreases leptin (satiety), promotes food reward-seeking, and reduces impulse control — creating a hormonal environment that drives overconsumption. Chronic sleep restriction independently predicts weight gain and obesity.
Stress and Cortisol
Psychological stress elevates cortisol, which increases appetite (particularly for high-calorie, palatable foods), promotes abdominal fat deposition, and impairs sleep quality — a triple mechanism for weight gain.
What Actually Works Long-Term
- GLP-1 receptor agonists (semaglutide, tirzepatide): The most effective pharmacological weight-loss interventions ever developed, producing average losses of 15–22% of body weight in clinical trials while reducing cardiovascular events.
- Behavioural interventions: Consistent meal timing, reduced ultra-processed food, adequate protein (promotes satiety and preserves muscle during weight loss), and addressing emotional eating patterns.
- Sleep optimisation and stress management: Addressing these factors removes two major biological drivers of weight gain.
Sources
- Wilding JPH, et al. Semaglutide 2.4 mg in Adults with Obesity. NEJM. 2021.
- Taheri S, et al. Short Sleep Duration and Obesity. PLoS Med. 2004.
- Mayo Clinic. Unexplained weight gain — Causes. 2023.
- Endocrine Society. Obesity Guidelines. 2022.