Weight Gain: Causes, Health Risks & What You Can Do About It

How Common Is Weight Gain in the UK?

Excess weight and obesity affect a significant proportion of the UK adult population. According to NHS data, around 64% of adults in England are overweight or obese. This represents one of the most significant public health challenges in modern Britain — and it affects people across all age groups, backgrounds, and lifestyles.

The causes are rarely simple. Weight gain is almost always the result of multiple interacting factors: biological, behavioural, environmental, and social. Understanding what drives weight gain is the first step towards addressing it effectively.

Common Causes of Weight Gain

Weight gain occurs when calorie intake consistently exceeds calorie expenditure over time. But it is rarely as simple as “eating too much and moving too little.” Many biological, psychological, and environmental factors influence both how much we eat and how our bodies store and use energy.

Genetics and Weight

Genetic factors play a real role in weight regulation — influencing metabolic rate, fat distribution, hunger signalling, and the tendency to store energy. Studies of twins suggest that genetic factors may account for 40–70% of variability in BMI. This does not mean weight gain is inevitable or untreatable — but it does explain why some people find weight management significantly harder than others, even with identical lifestyles.

Health Risks of Excess Weight

Lifestyle Changes That Work

For most people, sustainable weight loss begins with evidence-based lifestyle changes. The most effective approaches are:

• A moderate calorie deficit — aim for 500–600 kcal below maintenance daily, achievable through diet alone or diet plus exercise
• Reduced ultra-processed food — replacing high-calorie, low-nutrient foods with whole foods, lean protein, vegetables, and fibre
• Regular physical activity — NICE recommends 150 minutes of moderate activity per week; even walking significantly helps
• Improving sleep quality — 7–9 hours per night supports healthy hunger hormone regulation
• Stress management — mindfulness, therapy, or structured relaxation can reduce cortisol-driven eating
• Keeping a food diary — consistently linked to better weight loss outcomes in clinical studies

These steps are effective for many people — but for those with significant excess weight or obesity-related health conditions, lifestyle changes alone may not be sufficient. This is where prescription weight loss medication can provide meaningful additional support.

When Might Medication Help?

NICE guidance recommends that prescription weight loss medication should be considered as an adjunct to lifestyle changes — not a replacement for them — in adults who:

• Have a BMI of 30 or above, or
• Have a BMI of 27 or above with a weight-related health condition (such as type 2 diabetes, hypertension, or sleep apnoea)
• Have been unable to achieve clinically meaningful weight loss through lifestyle changes alone

Medication works best as part of a broader programme including dietary change and increased physical activity. It is not a quick fix — but for eligible patients, it can produce weight loss significantly beyond what lifestyle alone achieves.

Weight Loss Treatment Options

The most commonly prescribed weight loss medication in the UK is liraglutide (Saxenda) — a GLP-1 receptor agonist given by daily injection that reduces appetite and calorie intake. Clinical trials show average weight loss of 5–10% of body weight over 56 weeks.

Newer GLP-1 receptor agonists — including semaglutide (Ozempic/Wegovy) — have also shown significant weight loss effects and are increasingly available through specialist pathways. Your prescriber will advise on the most appropriate option based on your medical history and current medications.