13 Reasons You’re Having Unintentional Weight Gain

by Marixie Ann Obsioma, MT, undergrad MD on October 21, 2020
Last updated on May 23, 2021

It’s totally normal to gain weight as you age. After all, thanks to something called responsibilities, you probably aren’t moving around the same way you were younger. Less activity plus the natural loss of muscle mass can both contribute to a slower metabolism as you get older.

13 Reasons You’re Having Unintentional Weight Gain

But if you’re experiencing sudden weight gain that just doesn’t seem to make sense to you and it doesn’t seem to have to do with normal life changes, then it may be a sign that something odd is going on.

Ahead is a list of possible reasons that could be causing your unintentional weight gain.

1. Intake of Highly Processed Foods

Many healthy foods, such as oats, frozen fruit, and yogurt, are minimally processed.

However, highly processed foods, including sugary cereals, fast food, and microwave dinners, pack a slew of harmful ingredients, as well as added sugars, preservatives, and unhealthy fats.

What’s more, numerous studies link highly processed food to weight gain, in addition to rising obesity rates in the United States and around the world (1).

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 be obese than those who ate the least (2).

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

In fact, in a 2-week study in 20 people, participants ate about 500 more calories per day on an ultra-processed diet than on an unprocessed diet (3).

Thus, you should consider cutting out processed meals and snacks, focusing instead on whole foods.

2. Too Much Sugar

Regularly downing sugary foods and beverages, such as candy, cakes, soda, sports drinks, ice cream, iced tea, and sweetened coffee drinks, can easily enlarge your waistline.

Many studies link sugar intake not only to weight gain but also an increased risk of chronic health conditions, including type 2 diabetes and heart disease (4).

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

For instance, a review of 30 studies in 242,352 children and adults tied sweetened beverage intake to weight gain and obesity (5).

One study in 11,218 women revealed that drinking 1 sugary soda per day led to 2.2 pounds or 1 kg of weight gain over 2 years, meaning that cutting out sweets might have the opposite effect (6).

You can try gradually reducing your sugar intake to ease the process.

3. Lack of Whole Foods

If you regularly eat processed foods, switching to a diet that’s higher 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.

One study divided 609 adults with excess weight into groups that followed either a low-fat or low-carb diet for 12 months (7).

Both groups were instructed to maximize their vegetable intake, restrict their intake of added sugars, trans fats, and refined carbohydrates, eat mostly whole, minimally processed, nutrient-dense foods, and prepare most meals at home.

The study found that people in both diet groups lost similar amounts of weight — 12 pounds (5.4 kg) for the low-fat group and 13 pounds (5.9 kg) for the low-carb group. This demonstrated that diet quality, not macronutrient content, was the most important factor in their weight loss (7).

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.

4. Too Many Calories

Overeating remains a prominent cause of weight gain.

If you take in more calories than you burn per day, you’ll likely gain weight (8).

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

It can be difficult to determine your calorie needs on your own, so consult a registered dietitian if you struggle with overeating.

Some simple ways to 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, and increasing your activity level.

5. Insomnia

There’s nothing like a busted night of sleep to make a girl crave sugar and fat or anything to survive at work the next day, right? That’s because a missed shuteye does a number on your hunger hormones and metabolism. Sleeping too little raises ghrelin, the hormone that signals it’s time to eat, while lowering your levels of leptin, the hormone that conveys the “I’m full” feeling. The result?  A totally unsatisfying chow-fest the next day.

Putting off sleep to watch just one more episode? That hour could be contributing to sudden weight gain. Evidence showed that people who slept just one hour more per week lost more fat than those who slept an hour less (9). The people who slept less lost less even though everyone in the study ate the same number of calories, proportionate to their weight at the start of the study

6. Depression or Anxiety

When you’re stressed, you’re thrown into fight-or-flight mode and get a surge of adrenaline, along with a heavy dose of the hormone cortisol, which is supposed to help you restore energy reserves and store fat. 

The problem? Lots of us get chronically stressed sitting at a desk all day or just living a crazed life. When your cortisol levels stay elevated for a prolonged period, then your body continues storing fat, which can lead to weight gain.

If you’ve persistently felt down in the dumps or anxious, have trouble sleeping, feel fatigued, or you’ve lost interest in the stuff that used to make you tick, talk with an MD or mental-health pro who can make suggestions for getting back on track if stress seems to be the culprit behind your sudden weight gain.

7. Insufficient Physical Activity

Living a sedentary lifestyle is strongly associated with weight gain. And consuming lots of electronic and social media is the sedentary behavior that seems to most increase our risk of diabetes and obesity (10). 

Regular exercise not only burns calories directly, but it also can jazz up your metabolism so we burn more calories in between exercise sessions. Read about how much exercise you really need.

8. Smoking Cessation

Some people gain weight initially when they stop smoking tobacco products. Experts believe that this occurs both because nicotine suppresses appetite and because withdrawal symptoms may include stress, which can lead to overeating.

Research has shown that 1 kg is the average weight gain in the first month after quitting cigarette smoking (11). The majority of a person’s weight gain seems to occur during the first 3 months after quitting smoking, with the rate of weight gain appearing to slow down by the 6-month mark.

However, changes in weight as a result of quitting smoking can vary depending on the person. The same research found that 16 percent of people lost weight in the first year of not smoking while 13 percent gained over 10 kg.

9. Yo-Yo Dieting

Yo-yo dieting refers to cycles of intentional weight loss followed by unintentional weight regain.

Notably, this pattern is linked to an increased risk of weight gain over time (12).

In a 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 (13).

Other studies reveal that restrictive eating and dieting may lead to future weight gain due to your body’s physiological responses to such behaviors, such as changes in hunger and fullness hormones (14, 15, 16).

Plus, most people who lose weight through restrictive dieting gain back most or all of it within 5 years (11).

To keep weight off long term, you should focus on sustainable lifestyle changes. These include exercise, cutting out processed and sugary foods, and eating nutrient-dense, whole foods rich in fiber and protein.

10. Sedentary Lifestyle

Inactivity is a common contributor to weight gain and chronic diseases (17, 18).

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

A study in 464 people with obesity and excess weight found that 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 (19).

Making a few simple 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 (20).

Research has also shown that engaging in excessive screen time contributes significantly to unintentional weight gain (21, 22).

Even small adjustments, such as taking a walk after dinner instead of watching TV, working out or walking during your lunch break, investing in a standing or treadmill desk, or riding your bike to work, can counter weight gain.

11. Dehydration

There’s a reason behind the bloat, and it may have just as much to do with the water you forgot to drink as the food that you ate.

Experts say most of us aren’t drinking nearly enough water. That’s because many of us mistake the feeling of thirst for the feeling of hunger. Confusion, tiredness, and lightheadedness are all signs of even mild dehydration. Sounds a lot like how we feel when we’re hankering for a snack.

Mixed signals aren’t the only possible culprits behind your unexplained weight gain. Adequate hydration increases your metabolism. Without enough water, your cells can’t convert your food into energy quickly and efficiently.

12. Medications

Several prescription medications have been associated with weight gain, including:

  • Several drugs used to treat psychiatric disorders such as schizophrenia, depression, and bipolar disorder
  • Steroids
  • Birth control pills
  • Some diabetes drugs
  • Some drugs used for seizure disorders

If you are being treated for any of these problems with prescription medication, talk to your doctor about whether your medicine might be contributing to your weight gain.

13. Medical Conditions

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


When a young woman walks into a doc’s office with unexplained weight gain, the thyroid is the first place most physicians will investigate, and for good reason. A whopping one in eight women will develop a thyroid disorder in her life, according to the American Thyroid Association (23).

That butterfly-shaped gland in the neck is responsible for secreting a hormone that regulates the metabolism, and if you’ve got an under-active thyroid or hypothyroidism, the metabolism may slow down, triggering weight gain.

Women with hypothyroidism may also suffer from low energy levels or fatigue, dry skin, hair loss, hoarseness, or constipation. Notice any of them and you should book a chat with your doc who can check on your thyroid with a simple blood test if necessary.

Polycystic Ovary Syndrome

People with polycystic ovary syndrome (PCOS) may find that they gain weight easily around their middle. PCOS causes the ovaries to produce abnormally high levels of male sex hormones.

Other symptoms of PCOS include:

  • Irregular periods
  • Excess hair on the back, chest, or abdomen
  • Thinning hair or hair loss
  • Acne
  • Dark patches of skin around the armpits, breasts, or neck

Although there is no cure for PCOS, a doctor can recommend lifestyle changes, such as exercising more and eating a healthful diet that may reduce symptoms. Hormonal medications can also help control symptoms.

Small intestinal bacterial overgrowth (SICO)

The gut relies on good bacteria to function well, but there’s also bad bacteria chilling in your digestive tract. When that balance of good to bad gets thrown off, small intestinal bacterial overgrowth or SIBO can take place, triggering extra gas in your GI tract along with bloating, abdominal pain, diarrhea, and sudden weight gain.

Experts aren’t entirely sure how SIBO may trigger those extra pounds, but treatment for SIBO typically includes antibiotics to treat the bacterial overgrowth, according to the U.S. National Library of Medicine (24).

Heart Failure

Rapid weight gain or swelling in particular areas of the body can be due to fluid retention and may be a sign of heart failure.

According to the American Heart Association, a weight gain of more than 2–3 pounds over 24 hours or 5 pounds in a week could be a sign of heart failure.

However, it is vital to note that a person’s weight usually fluctuates by a few pounds over the course of a day. If their weight returns to normal and they do not have additional symptoms, the temporary increase could be due to normal bloating and fluid retention.

If blood is flowing slowly to and from the heart, it affects the function of other major organs in the body. As a result, fluid collects in the tissues, which causes weight gain and swelling.

People may experience weight gain with swelling in the:

  • Stomach area
  • Ankles
  • Legs
  • Feet

Other signs and symptoms of heart failure include:

  • Fluid collecting inside the lungs (pulmonary edema)
  • Frequent coughing
  • Feeling breathless when resting
  • Dizziness or confusion
  • Loss of appetite
  • Difficulty sleeping or lying flat

If people are experiencing any of these symptoms alongside rapid weight gain or unexplained swelling, they should seek medical attention.

Kidney Problems

Sudden weight gain or swelling in the body could be a symptom of kidney disease, such as kidney failure or nephrotic syndrome, which is damage to the kidneys.

If the kidneys are not functioning correctly, the body may retain fluid, resulting in weight gain. Damaged kidneys are not able to remove waste and fluids from the body properly, so these build up in the tissues.

Swelling due to kidney problems usually affects the legs, ankles, or feet.

Other symptoms that may indicate a problem with the kidneys include:

  • Fatigue
  • Not urinating very much
  • Urine that looks foamy
  • Itchiness
  • Loss of appetite
  • Muscle cramps
  • Joint pain
  • Headaches
  • Difficulty concentrating or confusion


If people experience rapid weight gain and their abdomen looks enlarged, this may indicate cirrhosis.

Cirrhosis is a condition in which scar tissue replaces healthy tissue in the liver, and it can cause fluid to build up in the abdominal cavity. This abnormal buildup of fluid is called ascites.

Other symptoms of cirrhosis include:

  • Swollen ankles
  • Breathing difficulties
  • Abdominal pain

If people are already having treatment for ascites, they should contact their doctor if they gain more than 2 pounds a day for 3 days in a row.


Acromegaly is a hormonal condition that happens when the pituitary gland produces too much growth hormone. It most commonly presents in middle adulthood.

The main symptom of acromegaly is an enlargement of the feet and hands. People may notice that their shoes or rings no longer fit properly. The lips, tongue, and nose may also become enlarged.

Other symptoms of acromegaly include:

  • Aching joints
  • A deeper voice
  • Skin tags
  • Excessive sweating
  • Obstructive sleep apnea and snoring

People with acromegaly may require surgery or radiation therapy to remove a benign tumor on the pituitary gland.

Key Takeaway

For people who have unintentional weight gain the most common cause, by far, is related to poor dietary habits and suboptimal physical activity.

But if you are gaining weight even if you think you are eating reasonably and getting a fair amount of physical activity, you should consult with your doctor about whether you might have a medical condition that’s potentially causing your weight gain.


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