What causes weight gain? 

Discovering your favorite pair of jeans are a little too snug, or the zipper on that dress you planned to wear tonight doesn’t want to budge can be disheartening, especially when you are in the dark as to why this is happening. So what causes weight gain? You might be surprised to learn it comes down to more than just diet and exercise.

As we get older, the number on the scale can fluctuate with significant life events, health status, and lifestyle changes. So, pinpointing the cause of unexplained weight gain can be tricky. This article dives into the various causes of weight gain and steps you can take to reverse weight gain if that’s your goal.

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Body weight is affected by much more than the calories we consume. Medications, sleep, hormone changes, and certain medical conditions can affect how the body stores fat. 

Keep reading to understand the most common causes of weight gain and when you should seek medical advice.

1. Medical conditions 

Obesity and unexpected weight gain may be triggered by underlying medical conditions that alter metabolism and cause a chemical imbalance, leading to fluctuations in weight.

Medical conditions associated with weight gain include (NIH, 2022; NIH, 2018; AHA, 2017; NAMS, n.d.):

• Hyperthyroidism is caused by an underactive thyroid leading to a slower metabolism. 

• Cushing’s syndrome is caused by the overproduction of the stress hormone cortisol leading to excess weight in the face, neck, trunk, and abdomen.

• Polycystic ovary syndrome (PCOS) is an imbalance of reproductive hormones that creates problems in the ovaries, often resulting in unexpected weight gain.

• Heart disease and heart failure can cause excess fluid stored in your body called edema resulting in swollen ankles, feet, legs, and abdomen.

• Kidney disease can cause fluid retention, leading to high blood pressure, excess water weight, and bloating.

• Menopause isn’t directly responsible for weight gain. Still, it is associated with decreased estrogen levels and changes in fat distribution, specifically in the abdomen, which can lead to weight gain.

• Type 2 diabetes and some medications used to treat it are associated with weight gain.

A routine blood test can help healthcare professionals rule out these conditions as possible causes of unexpected weight gain.

2. Eating habits 

The foods we consume keep our bodies working and provide us with energy. Eating more calories than you need, though, can contribute to weight gain. And too much of certain types of food can make this more likely. 

One study found the following food categories led to long-term excess weight gain in adolescents (Dong, 2015):

• Fat spreads (butter or margarine)

• Coated poultry (breaded or battered)

• Potatoes cooked in oil (french fries, potato chips)

• Coated fish

• Processed meats

• Desserts and sweets

• Sugar-sweetened beverages (soda, fruit drinks)

Another study found that ultra-processed foods make up 51% of total caloric intake, increasing levels of sodium, cholesterol, and fats resulting in long-term weight gain and higher body fat (Bielemann, 2015). 

That doesn’t mean you can never eat these foods—no food is inherently bad. But primarily eating a diet rich in whole grains, vegetables, and fiber while minimizing snacking can reduce fluid retention and constipation and help promote a healthy weight.

3. Lack of physical activity

Daily life offers many opportunities for inactivity. Working at a desk, watching TV, and commuting long distances contribute to an increasingly sedentary lifestyle, which studies show causes chronic health conditions and weight gain (Ladabaum, 2014).

Routine exercise is not only vital to weight loss, but it also has a strong effect on many aspects of your well-being and health, including (Warburton, 2006):

• Lowering cholesterol

• Controlling blood sugar and lowering the risk of type 2 diabetes

• Reducing blood pressure

• Minimizing inflammation

• Preventing cardiovascular disease

• Reducing stress, anxiety, and depression

• Promoting weight loss

• Helping to maintain a healthy body mass index (BMI)

4. Medications 

If you have sudden or unexpected weight gain, it might be from a medication you take. Many drugs can cause weight gain when taken over a long period.

Studies have shown that the following classes of medications can contribute to weight gain (Domecq, 2015):

• Antidepressants

• Atypical antipsychotics

• Anticonvulsants

• Drugs used to treat type 2 diabetes

• Corticosteroids

Speak to your healthcare provider if you are concerned that your medication is causing sudden weight gain. There may be an alternative treatment plan available.

5. Sleep disturbances

Quality sleep is vital to mental and physical health. Studies show that one in three people don’t get enough sleep, leading to a rise in various health conditions, including weight gain and obesity (CDC, 2017). 

One study found that people who had less than seven hours of sleep were more likely to have a higher BMI than those who got more sleep on average. People who got less sleep also had specific biomarkers associated with obesity, including salt retention, inflammatory markers, and decreased levels of leptin, a hormone that helps regulate appetite (Cooper, 2018).

Moderate to severe sleep apnea is also associated with sleep deprivation and weight gain. Sleep apnea can result in daytime sleepiness, leading to a lack of energy, less physical exercise, and poor diet choices (Nedeltcheva, 2010; Jehan, 2017).

Sometimes, we gain weight despite eating a healthy diet and exercising regularly. While it’s normal for weight to fluctuate over time, there are circumstances when you should seek help from a healthcare provider. Talk to a professional if you are experiencing any of the following in combination with sudden weight gain (Wilson, 2021; AHA, 2017):

• Chronic constipation

• Sudden excessive weight gain without a known cause

• Vision changes

• Uncontrolled hunger with sweating and heart palpitations

• Feel colder than usual

• Swollen feet

• Shortness of breath

• Hair loss

Losing weight is not easy, and different strategies work for other people. Eating right and increasing physical activity are proven strategies for weight loss.

A 2018 study found that dietary changes in people who successfully lost weight had three things in common: eating more vegetables, consuming less sugar, and eating more whole foods instead of processed foods (Gardner, 2018).

In addition to improving your eating habits and moving your body, other strategies to help you lose weight include (CDC, 2022):

• Reducing stress

• Getting plenty of sleep

• Drinking lots of water

• Eating high-fiber foods

• Tracking your eating

• Setting realistic goals

• Identifying resources for support and information 

• Making changes to your environment to limit overeating

Unexplained weight gain can be frustrating and stressful. It can be caused by several factors beyond diet and exercise, including certain medical conditions, medications, sleep quality, and stress. 

See a healthcare provider If you experience sudden weight gain in conjunction with other health changes such as swollen feet, hair loss, vision changes, and uncontrolled hunger.

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How we reviewed this article

Every article on Health Guide goes through rigorous fact-checking by our team of medical reviewers. Our reviewers are trained medical professionals who ensure each article contains the most up-to-date information, and that medical details have been correctly interpreted by the writer.

Current version

June 27, 2022

Written by

Amy Isler

Fact checked by

Chimene Richa, MD

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About the medical reviewer

Chimene Richa, MD

Dr. Richa is a board-certified Ophthalmologist and medical writer for Ro.