Understanding weight gain: Causes, solutions, and healthy strategies

Weight gain is a complex issue that affects millions of people, often in ways that are frustrating, confusing, or difficult to manage. Whether it's happening gradually or suddenly, understanding what's going on in your body is the first step to feeling in control again.

This guide explores the many reasons why weight gain happens — from dietary and lifestyle factors to hormonal changes and hidden health conditions — and offers practical solutions to help you take a healthy, sustainable approach to managing your weight.

What causes weight gain?

At its most basic level, weight gain occurs when your calorie intake is more than your body needs to function. But it's not always that simple. Several other factors can influence how your body stores and uses energy.

For some, unexpected weight gain may feel unexplainable, especially if eating habits haven't changed. In these cases, it may be worth exploring why you're gaining weight even though you don't eat much or why weight gain seems to concentrate in specific areas, like the stomach.

Medical conditions and hormonal factors

Certain health conditions can cause weight gain by disrupting hormonal balance, metabolism, or appetite signals. For example, hypothyroidism, polycystic ovary syndrome (PCOS), and Cushing's syndrome are all known to contribute to weight changes.

Hormonal shifts during perimenopause or menopause can also make weight harder to manage, often leading to stubborn fat around the midsection. In some cases, these disruptions can contribute to visceral obesity, making it even more important to identify and manage any underlying issues [1].

Stress also plays a key role, as elevated cortisol levels — the body's primary stress hormone — can increase appetite and promote fat storage, especially around the belly [2]. If you're dealing with chronic stress or poor sleep, cortisol-related weight gain could be a factor.

In some cases, weight gain may stem from hormonal imbalances that have not yet been diagnosed. If you're noticing weight gain linked to ovulation, changes in your menstrual cycle, or fertility treatments, hormone-related fluctuations might be involved.

Medications and nutrient deficiencies

Certain medications — like antidepressants, steroids, and insulin — can cause weight gain as a side effect. For example, there's growing awareness around insulin-related weight gain, particularly in those with diabetes, as well as the potential side effects of hormone replacement therapy (HRT).

Nutritional deficiencies can also be a part of the puzzle. A B12 deficiency may lead to fatigue, which reduces physical activity, while iron deficiency can also slow your metabolism and contribute to weight changes. If you're feeling tired, cold, or unusually sluggish, it may be worth checking your nutrient levels.

You might also be surprised to learn that common digestive issues can play a role. For example, constipation can lead to temporary bloating and weight fluctuations, while undiagnosed conditions like fluid retention, gluten intolerance, or IBS may contribute to inflammation and weight gain over time.

Rare genetic conditions

While uncommon, some genetic disorders can cause intense hunger and persistent weight gain from a young age. These conditions are typically diagnosed early and managed with specialised medical care.

However, even without a rare diagnosis, body shape and fat distribution can be strongly influenced by genetics. If you've ever wondered why you only gain weight in your stomach, your genetic predisposition could be a part of the answer.

Lifestyle factors that influence weight

Beyond hormones and health conditions, daily habits and routines have a huge impact on how your body maintains or gains weight. Small changes in sleep, stress, diet or movement can add up over time.

Sleep, stress, and emotional eating

Lack of sleep disrupts hunger hormones like leptin and ghrelin, making you feel hungrier during the day [3]. When combined with elevated stress levels, this often leads to emotional eating or cravings for high-calorie comfort foods.

Emotional eating is often tied to mental health challenges like anxiety or chronic stress, which can affect both appetite and eating habits.

It's also important to recognise how seasonal changes affect weight. Some people experience winter weight gain due to lower activity levels, changes in sleep patterns, and increased appetite during colder months.

Movement and physical activity

Sedentary lifestyles are one of the leading contributors to weight gain, particularly when combined with high-calorie diets. Regular exercise helps regulate metabolism, burn calories, and build lean muscle, which supports a healthy weight over time.

That said, it's not always straightforward. Some women may feel frustrated when they're gaining weight despite working out, which could be due to factors like body recomposition, hormonal shifts, or water retention.

In many cases, what looks like weight gain on the scale might actually be an increase in muscle mass and a reduction in fat, especially in women focused on body recomposition. Incorporating strength training alongside cardio can support fat loss while preserving lean muscle, which can be helpful if you're working on managing your weight through exercise.

Dietary causes of weight gain

It's no secret that a poor diet can lead to weight gain, but not all foods affect your body the same way.

Sugary drinks like soft drinks, fruit juices, and energy drinks provide a large number of calories without keeping you full. Excessive intake of starchy carbohydrates, especially refined ones, can lead to fat storage if not balanced with physical activity. Still, carbs themselves don't make you fat — context and portion size matter.

Another hidden culprit? Alcohol. Many people underestimate how many calories are in a few glasses of their favourite beverage. Wine can contribute to weight gain when consumed regularly or in large quantities [4].

The quality of your calories

Not all calories are equal. A diet rich in ultra-processed foods can increase hunger and lead to overeating, while a balanced diet that focuses on whole foods, like vegetables, fruits, legumes, and lean proteins, can help with appetite regulation and weight control.

And when it comes to supplements, products like protein powder can be beneficial for muscle maintenance or growth, but taking them in excess can cause weight gain if they're not offset by your activity level.

Medical support and sustainable solutions

If you're experiencing sudden and unexpected weight gain, it's essential to consult a healthcare provider. Medical tests can help identify conditions like hypothyroidism, PCOS, or insulin resistance that may be affecting your weight. They may also assess factors like your body mass index (BMI) to get a clearer picture of what's going on.

For those considering medical treatment for weight loss, it's worth understanding what happens after treatment ends. For example, some people experience weight regain after stopping Mounjaro, especially if lifestyle habits aren't in place to support long-term change.

Creating a healthy plan

Working with a healthcare provider or dietitian can help you develop a tailored approach based on your individual needs. This might include a structured eating plan, exercise routine, or strategies for managing appetite and cravings.

If digestive issues are part of the picture, such as IBS or allergies and sensitivities, identifying and eliminating trigger foods can make a meaningful difference to both symptoms and weight.

Tracking your progress

Monitoring things like blood sugar, high blood pressure, energy levels, and sleep can give you better insight into what's working and what needs adjusting.

If this all sounds confusing or you're not sure where to begin, don't worry — support is available. Our team of clinicians and health experts is here to help guide you every step of the way.

When you join our Weight Reset Programme, you get access to far more than just weight loss medications like Mounjaro or Wegovy. You'll receive unlimited support from expert dietitians, a personalised strength and cardio plan to preserve muscle mass as you lose weight, tracking tools to keep you motivated, and access to a private community of women on the same journey.

The programme equips you with the knowledge, structure, and ongoing support you need to make informed decisions, so you can lose weight sustainably and keep it off well into the future.

Your path to long-term balance

Preventing weight gain isn't about perfection — it's about understanding your body, recognising your unique challenges, and making small, consistent choices that support your health over time.

For some, that means adjusting meal timings (like reducing late-night snacking before bed), while for others, it might be about uncovering how conditions like endometriosis or allergies impact weight regulation.

Life stages and hormonal changes also matter. If you're going through perimenopause or menopause, shifting hormones can affect appetite, metabolism, and fat storage, especially around the belly. These changes are completely normal, but they may require a tailored approach to exercise, nutrition, and support.

And sometimes, weight gain happens quickly and without any clear explanation. If you're experiencing sudden or unexpected weight gain, it's worth exploring all possible angles — from stress levels to sleep habits to less obvious medical factors like diabetes, IBS, or gluten intolerance.

Above all, remember that your weight does not define your worth. You deserve support, clarity, and care.

Putting it all together

Weight gain is rarely about just one thing — it's often the result of many interconnected factors that play out over time. Whether it's hormonal imbalances, lifestyle shifts, nutritional gaps, or medical conditions, understanding the why behind your weight changes is the first step toward making empowered, sustainable choices.

With the right support — whether that's clinical guidance, a structured programme, or a deeper understanding of your own body — it is possible to regain balance and feel more in control of your health.

You're not alone on this journey. And with the right tools, knowledge, and support system, long-term weight management is absolutely achievable.

Frequently asked questions

Sometimes it takes more than just a healthy diet to control your weight. Whether you need more calories, fewer calories, or you're simply not getting enough sleep, we address some common questions that people often ask when they seek support.

Why am I gaining weight if I don't eat much?

This could be due to a slower metabolism, hormone changes (like thyroid issues or PCOS), or hidden calories in food and drinks. Stress, sleep problems, and certain medications can also affect how your body stores fat, even if your food intake seems low.

Can you gain 2kg in a week?

Yes, but it's often not fat. Quick weight gain usually comes from water retention, inflammation, constipation, or even hormonal changes like your period. If it's temporary and fluctuates, it's likely not true fat gain.

What causes unexplained weight gain?

Unexplained weight gain can be triggered by hormone imbalances, medication side effects, gut issues, or stress. It's especially common during times of hormonal change, like perimenopause or after starting new medications.

Why am I always bloated and gaining weight?

Chronic bloating with weight gain might point to digestive issues like IBS, constipation, or food intolerances. Hormonal changes, stress, or poor gut health can also play a role.

How fast can you gain weight?

Weight can change quickly — even over a few days — but sustained fat gain typically takes longer. True fat gain requires a consistent calorie surplus over time, while short-term weight changes are often fluid or digestion-related.

Can hormones make you gain weight suddenly?

Definitely. Hormonal shifts from menopause, PCOS, thyroid dysfunction, or even stress can change how your body stores fat. Cortisol and insulin are two hormones commonly linked to weight gain around the belly.

Photo credit: Juniper Team