Dr Nick Fuller
Leading Obesity Expert at the University of Sydney and founder of Interval Weight Loss.
Noticing the number on the scale creeping up without any major changes to your routine can be frustrating and confusing. You’re eating as you usually do, trying to stay active, yet the weight keeps piling on. If this sounds familiar, you’re not alone.
A study published in The New England Journal of Medicine found that adults typically gain an average of 0.5 to 1 kilogram per year as they age, often without realising it. But when the weight gain is more rapid, persistent, or feels entirely out of sync with your lifestyle, it’s worth looking deeper.
There are many potential causes of unexplained weight gain; some related to hormones, others to medications, digestion, sleep, or even stress. In many cases, it’s not just one thing but a combination of subtle factors working together over time.
In this article, we’ll walk you through seven possible causes of unexplained weight gain and help you understand when it might be time to consult a doctor.
If you’ve been gaining weight without making major changes to your diet or activity level, you’re not imagining things. Unexplained weight gain can be frustrating, and sometimes, it’s your body’s way of signalling something deeper. Let’s explore some of the most common causes worth knowing.
Hormones play a major role in regulating metabolism, appetite, and fat storage. When they’re out of balance, weight gain can happen even with a healthy lifestyle. Thyroid issues, especially hypothyroidism, slow down your metabolism and lead to fatigue, fluid retention, and weight gain. It’s more common than many realise; about 1 in 20 people will experience thyroid dysfunction in their lifetime.
Polycystic ovary syndrome (PCOS) is another common hormonal condition, affecting an estimated 6–13% of women of reproductive age. PCOS is linked to insulin resistance and increased fat storage, particularly around the abdomen, making weight management challenging even with diet and exercise. Hormonal imbalances also affect how your body uses energy, stores fat, and controls hunger.
Menopause and perimenopause come with hormonal shifts that can contribute to weight gain, particularly around the waist. The drop in oestrogen affects fat distribution, while declining muscle mass lowers overall calorie burn. In a study of midlife women, the average weight gain was approximately 2.25 kg, with those who experienced natural menopause gaining about 1.35 kg.
High cortisol levels, often triggered by chronic stress, can promote fat storage, especially in the belly area. Cortisol increases appetite and cravings for high-calorie foods, which may lead to overeating. Long-term stress also disrupts insulin function and metabolism. Cushing’s syndrome, though less common, involves excessive cortisol production and is known to cause rapid weight gain, especially in the face and upper body.
Some medical conditions, like heart failure or kidney and liver problems, can lead to fluid retention, which might appear as sudden or unexplained weight gain. This type of weight gain isn’t due to fat accumulation but to excess fluid, and it often comes with swelling or puffiness. If you’re seeing weight gain alongside fatigue, shortness of breath, or swelling in the legs, it’s important to seek medical advice.
Depression and other mental health issues can also influence weight. Some people may eat more as a way of coping with a low mood, while others might experience changes in metabolism or appetite as a result of the condition itself. Research shows that an increase in depression symptoms can lead to weight gain over time, particularly in people who are already overweight or obese.
Many medications list weight gain as a potential side effect, and for some people, the impact can be significant. Antidepressants, especially tricyclic antidepressants and selective serotonin reuptake inhibitors (SSRIs), have been linked to gradual weight gain in long-term users. A study of over 22,000 adults found that some antidepressants were associated with modest weight gain over 12 months, with variations depending on the specific medication.
Steroids, such as prednisone, are known to increase appetite and promote fat storage, particularly around the face, abdomen, and upper back. These effects can occur even in short-term use and are more pronounced with long-term or high-dose treatments. A recent study also shows that these medications may contribute to weight gain by reducing the activity of brown fat, the type of fat that helps burn energy in the body.
Some diabetes medications, especially insulin and drugs like pioglitazone, can also lead to weight gain by promoting fat storage and improving blood sugar control too efficiently. Managing weight while taking these medications can be a balancing act, particularly if dietary habits aren’t adjusted accordingly.
Beta-blockers, often prescribed for high blood pressure or heart issues, may lower metabolism and energy levels, making physical activity harder and leading to gradual weight gain. Though the gain is usually modest, it can be frustrating, especially when combined with other contributing factors.
Sleep plays a vital role in metabolism and hormone regulation. Inadequate sleep disrupts the balance of appetite hormones like ghrelin and leptin, making you feel hungrier and less full. Long-term research found that women who sleep 5–6 hours per night are more likely to gain weight and develop obesity compared to those who sleep 7 hours.
Stress affects the body in complex ways, often increasing cortisol levels and driving cravings for high-sugar or high-fat foods. Emotional eating, which uses food to soothe feelings of anxiety, sadness, or frustration, can easily lead to excess calorie intake without awareness. Studies confirmed that people under chronic stress are more likely to overeat and gain weight over time.
When sleep and stress aren’t managed well, the combination creates a perfect storm for unexplained weight gain, even if your diet hasn’t changed much.
Eating at irregular times or skipping meals can confuse your body’s hunger signals and make it harder to regulate appetite. When meals are inconsistent, your metabolism may slow down to conserve energy, making fat storage more likely. Some people also compensate by overeating later in the day, especially at night.
Mindless or unconscious eating, like snacking while working, eating in front of the TV, or finishing meals out of habit rather than hunger, can lead to calorie overload without you even realising it. Distracted eaters consumed significantly more calories and felt less full afterwards. Over time, these small habits add up, leading to gradual but steady weight gain.
In today’s screen-based, sedentary world, many people simply don’t move enough throughout the day. Even if you exercise for 30 minutes, sitting for the rest of the day can still lower your overall energy expenditure. The World Health Organization recommends at least 150 minutes of moderate activity per week, yet many adults fall short of this target.
Low activity reduces muscle mass and slows down your metabolism, especially as you age. It also makes it harder to burn off the extra calories from even occasional overeating. If your lifestyle includes long hours at a desk and limited movement, this could be one of the key reasons behind unexplained weight gain.
As we age, our metabolism naturally slows down. This means your body burns fewer calories at rest compared to when you were younger. Evidence shows that metabolic rate starts to decline noticeably after age 60, but even subtle changes begin earlier in mid-life.
At the same time, muscle mass decreases with age, a condition called sarcopenia. Since muscle burns more calories than fat, losing muscle means your body becomes less efficient at using energy. Unless you’re doing strength training or staying consistently active, this can lead to gradual weight gain.
Hormonal shifts in mid-life, including declines in oestrogen, testosterone, and growth hormone, also contribute to changes in fat distribution, making it easier to gain fat and harder to lose it.
Your gut microbiome, the trillions of bacteria living in your digestive system, plays a key role in how your body absorbs nutrients, stores fat, and manages hunger. Disruptions to this delicate balance, such as from antibiotics or a poor diet, can make weight regulation more difficult. Studies suggest that people with less diverse gut bacteria were more likely to be overweight or obese.
Poor digestion or frequent bloating can sometimes be mistaken for fat gain, especially when it causes the belly to appear distended. While not technically weight gain, it can feel and look similar. Food intolerances, such as to lactose or gluten, may also lead to inflammation and bloating.
When the gut isn’t functioning properly, it may send mixed signals about hunger and fullness, making it easier to overeat. Improving gut health through fibre-rich foods, probiotics, and a balanced diet can help restore balance and support a healthy weight.
Sometimes, unexplained weight gain is more than just a lifestyle issue. If you notice any of the following signs, it’s a good idea to check in with your doctor:
The weight gain continues for several weeks: A small fluctuation is normal, but if the gain is steady and doesn’t slow down, it may point to an underlying issue.
You notice swelling or fluid buildup: Puffiness in your legs, ankles, or abdomen could indicate fluid retention linked to heart, kidney, or liver problems.
You feel tired or weak all the time: Ongoing fatigue, especially when paired with weight gain, might signal a thyroid imbalance or other hormonal disorder.
Your appetite changes without a clear reason: Eating much more or much less than usual, without trying to, can be a sign that something in your body is off.
You have mood swings or depression: Mental health and weight are closely connected. If your mood is low and your weight is changing, both deserve attention.
Your menstrual cycles become irregular: For women, changes in hormones like oestrogen or progesterone can affect both cycle regularity and weight.
You’re gaining weight despite a healthy lifestyle: If you're eating well, staying active, and still gaining weight, it's time to dig deeper with professional help.
Unexplained weight gain can feel discouraging, especially when you’re doing your best to live a healthy lifestyle. But often, it’s your body’s way of telling you something important. Whether it’s due to hormones, medication, lifestyle shifts, or a hidden medical condition, identifying the root cause is the first step toward getting back on track. If you’re noticing persistent changes in your weight, don’t hesitate to speak with a healthcare professional. With the right support, answers and solutions are within reach.
Noticed extra kilos without changing your habits? Check out 7?Signs You Have a Slow Metabolism to uncover hidden factors, like thyroid imbalances, chronic stress, or hormonal shifts, that could be behind unexplained weight gain.
Dr Nick Fuller is the founder of Interval Weight Loss and is a leading obesity expert at the University of Sydney with a Ph.D. in Obesity Treatment. Dr Fuller is also the author of three best-selling books and his work been published in top ranked journals in the medical field, including JAMA, Lancet and American Journal of Clinical Nutrition.