If you’ve struggled to lose weight, or found the scale creeping up despite your best efforts, you’re far from alone. The latest CDC data from the National Health and Nutrition Examination Survey shows that about 40% of US adults have obesity, nearly 1 in 10 have severe obesity, and around 1 in 5 children and teens are affected. Globally, the World Health Organization reports that obesity has more than doubled among adults since 1990.
Weight gain is rarely about willpower alone. It’s shaped by a mix of biology, environment, habits and health conditions, many of which are outside our direct control. Understanding these causes is the first step toward realistic, lasting change. Here are 10 of the most common causes of weight gain and obesity, what the research says, and what you can do about each.
How obesity is defined
Doctors usually start with body mass index (BMI), your weight in kilograms divided by your height in meters squared. A BMI of 25 to 29.9 is classed as overweight and 30 or above as obesity. BMI is a useful screening tool but doesn’t distinguish muscle from fat or show where fat is stored. Waist circumference adds valuable information, because fat around the abdomen carries higher health risks. In 2025, a global expert commission in The Lancet Diabetes and Endocrinology recommended using body fat measures and signs of organ dysfunction alongside BMI, to better identify who needs treatment.
1. Genetics and family history
Genes strongly influence appetite, how full you feel after eating, how your body stores fat and how much energy you burn. Twin and adoption studies suggest genetics explain a large share of the differences in body weight between people. Hundreds of gene variants each contribute a little. Genes don’t make weight gain inevitable, but they explain why two people eating the same diet can end up at very different weights, and why some people need more support than others.
2. Ultra-processed foods
Ultra-processed foods, such as packaged snacks, sugary cereals, processed meats, ready meals and fast food, are engineered to be cheap, convenient and very hard to stop eating. A landmark 2019 NIH trial found that when people ate an ultra-processed diet, they consumed about 500 more calories a day and gained weight within two weeks, compared with when they ate a minimally processed diet matched for nutrients. The 2025 to 2030 US Dietary Guidelines now explicitly advise cutting back on highly processed foods.
3. Sugary drinks and added sugar
Sodas, energy drinks, sweet teas, specialty coffees and fruit juices add calories without making you feel full. Large amounts of added sugar, especially fructose, may also promote fat storage in the liver and abdomen. Swapping sugary drinks for water or unsweetened alternatives is one of the simplest, most effective changes you can make.
4. Food marketing and the food environment
We’re surrounded by inexpensive, high-calorie food: drive-throughs, vending machines, delivery apps and supersized portions. Marketing, much of it aimed at children, makes these foods feel normal and desirable. Meanwhile, fresh produce can be harder to find or afford in some neighborhoods. This environment makes overeating the default and healthy choices harder work.
5. Hormones: insulin and leptin resistance
Insulin helps move sugar from your blood into cells and signals the body to store energy. When cells become less responsive (insulin resistance), the body produces more insulin, which can encourage fat storage and make weight loss harder. Insulin resistance is closely linked to abdominal fat, inactivity and type 2 diabetes.
Leptin is a hormone released by fat cells that tells your brain you have enough energy stored. People with obesity often have high leptin levels, but the brain becomes less sensitive to its signal. The result is that the brain thinks you’re hungrier than you are and defends a higher body weight.
6. Certain medications
Some medicines can cause weight gain as a side effect, including some:
- Antidepressants and antipsychotics
- Corticosteroids such as prednisone (especially long-term use)
- Diabetes medications such as insulin and sulfonylureas
- Anti-seizure drugs and some migraine treatments
- Certain blood pressure medicines (some beta-blockers)
Never stop a medication on your own. If you’ve gained weight after starting a new medicine, talk to your doctor; there may be an alternative with less effect on weight.
7. Poor sleep
Short or poor-quality sleep disrupts hormones that control appetite, raising ghrelin (the hunger hormone) and lowering leptin. Sleep-deprived people tend to crave high-calorie, sugary foods and have less energy to be active. Adults generally need 7 to 9 hours a night. Our article on omega-3s and insomnia includes the habits that improve sleep most.
8. Chronic stress
Long-term stress raises cortisol, which can increase appetite and favor fat storage around the abdomen. Stress also drives emotional eating, cravings for comfort foods and skipped workouts. Read more about the negative health impact of stress and practical ways to manage it.
9. Physical inactivity and too much sitting
Many jobs and commutes now involve sitting for most of the day, and screen time has increased leisure sitting too. According to the WHO, almost one in three adults worldwide don’t get enough activity. Inactivity lowers daily energy use and contributes to muscle loss and insulin resistance. Regular movement doesn’t just burn calories; it improves how your body handles food. See the benefits of regular physical activity.
10. Medical conditions
Several health conditions can contribute to weight gain or make weight loss harder, including:
- Hypothyroidism (an underactive thyroid)
- Polycystic ovary syndrome (PCOS)
- Cushing’s syndrome (excess cortisol)
- Menopause, which shifts fat toward the abdomen
- Depression and some other mental health conditions
- Sleep apnea, which is both a cause and a result of weight gain
If you have unexplained weight gain, fatigue, irregular periods or other new symptoms, see your doctor for a check-up.
Bonus: misinformation and diet culture
Conflicting advice, fad diets and “miracle” supplements leave many people confused and stuck in cycles of restrictive dieting and regain. Very restrictive diets can trigger rebound overeating and slow metabolic adaptation. Sustainable, evidence-based habits work better than extreme short-term fixes.
Why weight is so hard to keep off
Many people lose weight successfully, only to regain it over the following years. That isn’t a lack of discipline. When you lose weight, your body responds in ways that push you back toward your previous weight: hunger hormones rise, fullness hormones fall, and your body burns slightly fewer calories than expected for your new size. This “metabolic adaptation” can last for years.
Knowing this helps you plan. People who keep weight off long term tend to stay physically active (often around an hour a day of moderate activity), keep eating patterns consistent, weigh themselves or track habits regularly, and treat maintenance as an ongoing effort rather than the end of a diet. Small, sustainable changes are far more likely to last than dramatic ones.
It also helps to measure success in more than kilograms. Better blood pressure, blood sugar, sleep, energy, fitness and mood are all real wins, even if the scale moves slowly. A loss of just 5% to 10% of body weight can meaningfully improve many weight-related health problems.
What helps: evidence-based ways to manage weight
- Build meals around whole foods: vegetables, fruit, legumes, whole grains, nuts and quality protein.
- Prioritize protein and fiber to stay full longer.
- Swap sugary drinks for water.
- Move more every day, aiming for at least 150 minutes of moderate activity a week plus strength training twice a week.
- Protect your sleep and manage stress.
- Plan ahead with healthy snacks; see our list of healthy snacks for weight loss.
- Look after your gut; our guide to improving gut health explains why fiber matters.
- Focus on maintenance, not just loss. Our tips on how to sustain weight loss help you keep results.
Medical treatment is an option too
Obesity is now widely recognized as a chronic disease. For some people, lifestyle changes alone aren’t enough, and that’s not a personal failure. Newer medicines known as GLP-1 receptor agonists, along with related drugs, can lead to substantial weight loss, and in December 2025 the WHO issued its first global guideline on their use for adults with obesity as part of comprehensive, long-term care. Bariatric surgery is another effective option for severe obesity. Talk to your doctor about which approach might suit you.
Common questions
What is the most common cause of obesity?
There’s rarely a single cause. Most obesity results from a combination of genetics, an environment full of ultra-processed, high-calorie food, low physical activity, poor sleep and stress.
Can genetics cause obesity?
Genes strongly influence appetite, fullness and fat storage, which is why some people gain weight more easily. Genes don’t make obesity inevitable, but they shape how much effort and support you may need.
Can stress make you gain weight?
Yes. Chronic stress raises cortisol, increases appetite and cravings, and can promote fat storage around the abdomen.
Which medications cause weight gain?
Some antidepressants, antipsychotics, steroids, diabetes medications, anti-seizure drugs and beta-blockers can cause weight gain. Talk to your doctor before making any changes.
How much of the US population has obesity?
According to CDC data from 2021 to 2023, about 40% of US adults have obesity and nearly 10% have severe obesity.
This article is for general information and isn’t a substitute for medical advice. Speak with a healthcare professional about your weight and health.
Explore more weight management advice in our obesity and overweight section.
