You may gain weight unintentionally as a result of certain things you consume, such as sugary foods and beverages. But sometimes weight gain can occur due to an underlying health condition.

Weight gain can be frustrating, especially when you don’t know what’s causing it.

While diet typically plays the largest role in weight gain, other factors like stress, lack of sleep, and taking certain medications may also contribute.

Here are 9 causes of unintentional weight gain.

Highly processed foods are typically packed with calories yet devoid of essential nutrients, such as protein and fiber, which keep you feeling full.

Foods like sugary cereals, fast food, and microwave dinners also contain a slew of harmful ingredients, such as:

Numerous studies link highly processed food to weight gain and rising obesity rates in the United States and around the world.

For example, a 2019 study in 19,363 Canadian adults found that those who ate the most ultra-processed foods were 32% more likely to have obesity than those who ate the least.

Another small study found that participants ate about 500 more calories per day on an ultra-processed diet than on an unprocessed diet. Eating more calories than you burn in a day is associated with weight gain.

To avoid unintentional weight gain, try cutting out processed meals and snacks and instead focusing on whole foods.

Regularly consuming sugary foods and beverages may lead to weight gain and increase your risk of chronic health conditions, such as diabetes.

Some foods high in added sugars include:

  • candy
  • cakes
  • soda
  • sports drinks
  • ice cream
  • iced tea
  • sweetened coffee drinks

In particular, sugary beverages are the largest source of added sugar in the United States and are strongly associated with weight gain.

A 2018 review of 30 studies in 242,352 children and adults tied sweetened beverage intake to weight gain and obesity.

A 2017 study in 11,218 females revealed that drinking one sugary soda per day led to 2.2 pounds (lbs) (1 kilogram (kg)) of weight gain over 2 years. As such, cutting out sweets might have the opposite effect.

Inactivity is a common contributor to weight gain and chronic diseases.

Working a desk job, watching TV, driving, and using a computer or phone are all sedentary activities.

A 2018 study in 464 people with obesity and excess weight found their average daily sitting time was 6.2 hours on working days and 6 hours on non-working days. Work-related tasks were the largest contributor, followed by watching TV.

Making a few lifestyle changes, such as exercising and sitting less, can make a big difference.

For example, a 3-month study in 317 workers found that replacing just 1 hour of sitting with 1 hour of standing during the workday reduced total fat mass and waist circumference while increasing lean muscle mass.

Research has also shown that engaging in excessive screen time contributes significantly to unintentional weight gain.

Some ways to help increase your level of physical activity during the day include:

  • taking a walk after dinner instead of watching TV
  • working out or walking during your lunch break
  • investing in a standing or treadmill desk
  • riding your bike to work
  • aiming to walk 10,000 steps each day

Yo-yo dieting refers to cycles of intentional weight loss followed by unintentional weight regain. This pattern is linked to an increased risk of weight gain over time.

In a 2019 study in 2,785 people, those who had dieted within the previous year had greater body weights and waist circumferences than those of non-dieters.

Other studies reveal that restrictive eating and dieting are likely to lead to future weight gain. This may be due to several physiological responses to such behaviors, such as changes in hunger and fullness hormones.

A 2015 study also found that most people who lose weight through restrictive dieting gain up to two-thirds of the weight back within 1 year and nearly all of it within 5 years.

To manage your weight long-term, try focusing on sustainable lifestyle changes, such as:

  • exercising
  • cutting out processed and sugary foods
  • eating nutrient-dense, whole foods rich in fiber and protein

Many lifestyle factors contribute to unintentional weight gain, but certain medical conditions may also play a role. These may include:

Binge eating disorder (BED) may also be responsible for unintentional weight gain. BED is categorized by recurrent episodes of uncontrollable overeating.

What’s more, certain medications, including antidepressants and antipsychotic drugs, may lead to weight gain.

Speak with a healthcare professional if you think your weight gain is associated with medications you’re taking or an underlying health condition. They could help modify or develop a treatment plan for you.

Sleep is essential for overall health and well-being, but poor sleep quality may trigger weight gain.

A 2022 review found that poor sleep is associated with eating more frequently and eating foods higher in carbohydrates and unhealthy fats – all of which may lead to weight gain.

Similarly, another 2022 review found the following sleep factors affected weight loss:

  • short sleep duration
  • frequent awakenings during the night
  • poor sleep quality
  • longer sleep onset

The authors also found that staying in bed and waking up later reduced the likelihood of getting enough exercise each week, which is important for avoiding weight gain.

It’s important to get at least 7 hours of sleep per night. Fewer than this is considered “short sleep,” which is associated with weight gain and obesity.

If you have poor sleep quality, try limiting screen time before bed, reducing your caffeine intake during the day, and going to sleep at a consistent time.

Eating a diet high in whole foods is an easy and effective way to promote weight loss and improve many other aspects of your health.

In fact, the most important factor in weight loss is choosing whole, minimally processed foods.

A 2018 study divided 609 adults with excess weight into groups that followed either a low fat or low carb diet for 12 months.

Both groups were instructed to:

  • maximize vegetable intake
  • restrict intake of added sugars, trans fats, and refined carbohydrates
  • eat mostly whole, minimally processed, nutrient-dense foods
  • prepare most meals at home

The study found that people in both diet groups lost similar amounts of weight — 12 lbs (5.4 kg) for the low fat group and 13 lbs (5.9 kg) for the low carb group.

This demonstrated that diet quality, not macronutrient content, was the most important factor in weight loss.

Incorporating whole foods into your diet doesn’t have to be difficult. Start by slowly adding more nutrient-dense whole foods, such as vegetables, fruits, beans, eggs, nuts, and seeds, into your meals and snacks.

Chronic stress is a common problem that can affect your weight.

High levels of the stress hormone cortisol have been shown to increase hunger and your desire for highly palatable, calorie-dense foods, which can cause weight gain.

Interestingly, stress management may promote weight loss.

In an 8-week study in 45 adults with obesity, those who engaged in relaxation techniques like deep breathing lost significantly more weight than those who only received standard dietary advice.

Spending time in nature, practicing yoga, and speaking with a therapist are some ways to relieve stress.

Learn more about 16 ways to reduce stress.

Overeating remains a prominent cause of weight gain.

If you take in more calories than you burn per day, you’ll likely gain weight.

Mindless eating, frequent snacking, and making calorie-rich, nutrient-poor dietary choices all promote excessive calorie intake.

Determining your calorie needs on your own can be difficult, so consult a registered dietitian if you need help managing overeating.

Some ways to help avoid overeating include:

  • paying attention to hunger and fullness cues by eating mindfully
  • following a high fiber, high protein diet rich in plant foods
  • drinking water instead of calorie-rich beverages
  • increasing your activity level

Learn more about 23 ways to stop overeating.

Why am I suddenly gaining weight?

Some causes of unintentional weight gain include underlying health conditions, being stressed, eating more highly processed foods, and not exercising.

How much unexplained weight gain is concerning?

The American Heart Association recommends getting medical attention if you gain 2 to 3 lbs in a day or 5 lbs in a week. This may be a sign of heart failure.

Why am I gaining weight when I don’t eat much?

Even if you don’t eat much, a sedentary lifestyle may still cause weight gain if you consume more calories than you burn. Stress, taking certain medications, and having an underlying health condition may also cause weight gain.

Many factors can contribute to unintentional weight gain.

Poor sleep, sedentary activities, and eating too many processed or sugary foods are just some of the habits that may increase your risk of weight gain.

A few simple steps, such as mindful eating, exercising, and focusing on whole foods, may help you manage your weight and improve your overall health.