Common Causes of Weight Gain

Often, the causes of weight gain can be more complex than extra calories or a lack of regular exercise.

Read on for the current evidence on what can cause weight gain, why it often resists simple fixes and what professional support is available.

Medically reviewed by Danny Chu, HCPC-Registered Physician Associate (PA38670).

Explore Common Weight Gain Causes

For many adults, the causes run deeper and can be influenced by hormones, underlying health conditions, chronic stress, disrupted sleep, or their life stage.

It can help you:

What Can Cause Weight Gain?

Disclaimer: The information on this page is intended for general educational purposes only. This should not be considered a substitute for personalised medical advice.

Body weight can be influenced by a wide range of factors, not just calorie intake:

Metabolism determines how efficiently your body burns energy, and it naturally slows with age
Hormones regulate appetite, fat storage and energy use
Chronic stress can elevate cortisol, which can increase appetite and drive abdominal fat accumulation
Poor sleep interrupts hunger signals, making it harder to eat what your body really needs
Fluid retention can sometimes cause sudden or rapid changes in body weight
Certain medications, including antidepressants, steroids and some contraceptives, are associated with weight gain

Weight gain is rarely caused by a single issue. More often, it is an interplay of biology, behaviour, circumstance and other factors. Understanding what it is makes it far easier to respond effectively.

Hormonal Changes That Can Cause You to Gain Weight

Hormones act as chemical messengers that govern appetite, metabolism and fat distribution. When thyroid, stress or sex hormones fall out of balance, the effect on body weight can be significant. Hormonal changes are a well-documented contributor to unexplained weight gain, particularly during midlife.

Cortisol & Stress Hormones

Cortisol is your body’s primary stress hormone. While useful in short bursts, it can be damaging when chronically elevated.

Research published in Obesity found that higher baseline cortisol levels predicted greater weight gain over six months, along with increased food cravings driven by ghrelin (the so-called hunger hormone).

Elevated cortisol may contribute to increased fat storage, specifically in the abdominal area, which is associated with additional cardiovascular risk.

Insulin Resistance & Blood Sugar Changes

Insulin helps your body regulate blood sugar. When cells stop responding properly to insulin (a condition known as insulin resistance), blood sugar regulation is disrupted, appetite can increase, and the body becomes more resistant to the breakdown of stored fat.

A longitudinal study found that insulin resistance was a significant predictor of body weight gain over time.

What to look out for: Insulin resistance is closely linked to weight gain around the midsection (central obesity) and is a recognised precursor to type 2 diabetes.

Thyroid Function & Metabolism

Your thyroid gland controls your metabolic rate. An underactive thyroid gland (hypothyroidism) produces fewer hormones than the body needs, slowing metabolism.

Weight gain is a recognised effect of hypothyroidism, alongside other symptoms like fatigue and cognitive difficulties. Thyroid-related weight gain often develops gradually, and many people don’t realise an underactive thyroid is the cause until blood tests confirm it.

Menopause, Testosterone & Age-Related Hormonal Changes

Hormonal shifts in middle age affect sex hormones in women and men.

In women, research suggests that while overall weight gain during midlife is influenced by several factors, hormonal changes during menopause are associated with increased body fat and altered fat distribution.
In men, testosterone levels decline gradually from middle age onwards, a process associated with reduced lean muscle mass and increased fat accumulation. Men generally gain weight around the abdomen.

Lifestyle Factors That Lead to Weight Gain

Lifestyle factors are among the most common contributors to weight gain and can significantly influence how the body regulates energy balance, appetite and metabolism. Factors such as diet quality, physical activity and sleep patterns may all affect body weight over time, although their impact can vary from person to person.

Poor Sleep & Sleep Duration

Poor sleep disrupts the balance of the hunger hormones leptin and ghrelin. A meta-analysis found that short sleep duration was associated with significantly higher ghrelin levels (the hunger-stimulating hormone) and altered leptin levels. Both of these elements can drive increased food intake.

Inactive Lifestyle

Regular physical activity helps the body use energy more efficiently, supports muscle mass and improves overall health. In contrast, a sedentary lifestyle and prolonged sitting can reduce daily energy expenditure over time. Adding more movement into everyday routines, alongside structured exercise where possible, can help promote a healthy weight.

Food Intake, Ultra-Processed Foods & Calorie Intake

Calorie intake matters, but so does food quality. Recent evidence includes:

A randomised controlled trial found that participants on an ultra-processed diet consumed more calories and gained weight than those on an unprocessed, balanced diet. This was true even when both diets were matched for calories, sugar, fat and fibre.
A large study of over 110,000 adults further confirmed that higher ultra-processed food intake was associated with increased body mass index (BMI) and a higher risk of being overweight and developing obesity.
Another wide-ranging study across 107 countries and regions reported that daily soft drink consumption was associated with higher rates of overweight and obesity among adolescents. It suggests that reducing sugary drink intake may play an important role in supporting healthy weight management.

Underlying Medical Conditions That Cause Weight Gain

Various medical conditions can directly contribute to weight gain or create significant barriers to losing it. These include:

Thyroid conditions (such as hypothyroidism)
Polycystic ovary syndrome (PCOS)
Insulin resistance
Certain kidney, liver or heart diseases can contribute to fluid retention and rapid changes in body weight
Conditions that limit mobility or deplete energy levels
Rare genetic conditions (such as Prader-Willi or 16p11.2 deletion)

Some medications are associated with weight changes in certain individuals, such as:

Rapid or sudden weight gain without a clear cause warrants a conversation with your GP. Identifying an underlying condition is often the first step towards more targeted support.

Mental Health Conditions & Emotional Well-Being

Mental health and body weight are closely connected. Emotional well-being can affect appetite, eating habits, energy levels and motivation in ways that are often overlooked.

A 2023 clinical review found strong links between emotional eating, weight gain, depression, anxiety and chronic stress. Emotional eating was also identified as an important factor in recurrent weight gain.

Low mood and fatigue can make exercise, cooking and daily routines feel harder to manage. Anxiety and chronic stress may also lead some people to rely on comfort foods or high-energy fast foods as a coping response. In some cases, medications used to treat mental health conditions can also affect body weight.

Weight gain linked to emotional well-being is a physiological and psychological response, not a lack of willpower. At Envigore, we offer compassionate, informed support that addresses the emotional and physical sides of weight management together.

When Unexplained Weight Gain May Need Medical Support

Unexplained weight gain may sometimes indicate an underlying health issue. Signs that may require a discussion with your GP include:

Gaining weight quickly without changes to diet or activity

Noticeable changes in where your body stores fat

Weight gain alongside fatigue, hair loss or persistent cold sensitivity

Low mood or anxiety that feels connected to changes in eating habits

Medical Weight Loss Options & Professional Support

If lifestyle changes alone haven’t been effective, or if an underlying condition is making weight management harder, professional support can potentially make a difference. Several evidence-based weight-loss management approaches are available.

The services below are for informational purposes only. Suitability for any weight-management treatment is determined through individual clinical assessment. This content does not constitute a recommendation, promotion or advertisement for any prescription medicine.

Clinically Supervised Weight Management

For some people, lifestyle changes alone are not sufficient. Envigore offers clinically supervised weight management programmes with personalised dietary and lifestyle support with medical oversight.

Online Weight Loss Coaching

Online coaching provides personalised support to help you build sustainable habits around nutrition, movement and mindset. Rather than relying on rigid plans, coaching focuses on finding realistic approaches that fit into your daily life.

Long-Term Weight Management & Maintenance

For effective long-term weight management, you need to change your focus from restriction to sustainable habits. Understanding how to transition from active weight loss to maintenance is an important part of any long-term plan.

Support with Weight Loss Plateaus

A weight-loss plateau, where progress slows or stalls despite ongoing healthy habits, is a common part of many weight-management journeys. In many cases, it reflects normal physiological adaptations rather than a lack of effort. It does not indicate failure.

FAQs

Frequently Asked Questions

Need more answers about sudden weight gain?

Why am I gaining weight?

Weight gain can result from various factors, including changes in diet, reduced activity, poor sleep, stress, hormonal shifts or an underlying health condition. If the cause isn’t clear, speaking to a doctor to rule out medical factors is a useful first step.

What are the causes of weight gain in women?

Women can experience weight gain related to hormonal changes at various life stages, including the menstrual cycle, pregnancy, perimenopause and menopause. Conditions like PCOS and thyroid disorders are also more common in women and are known to affect body weight.

What are the causes of weight gain in men?

In men, declining testosterone levels can reduce muscle mass and increase fat storage. Stress, poor sleep, high alcohol intake and a sedentary lifestyle are also common contributors.

Can hypnotherapy help me lose weight?

Hypnotherapy may support weight loss by addressing emotional eating, stress-related habits and the psychological patterns that influence food choices. It is not a standalone solution but rather part of a broader approach that includes a healthy diet and more exercise, which some people find helpful.

Learn more about our weight-loss hypnotherapy service with a certified hypnotherapist.

What is a healthy weight?

BMI is a commonly used metric for estimating a healthy weight for an individual, but it doesn’t account for muscle mass or fat distribution. Waist circumference and metabolic health markers are also valuable indicators.

A healthcare professional can provide a personalised assessment to determine your healthy weight range.

Have you experienced rapid weight gain? Contact us to discuss the causes of weight gain or start your free online consultation.

Sources & Further Reading

At Envigore, we believe conversations around weight should be informed, supportive and free from judgment. Our content is shaped by clinical insight, patient experiences and trusted medical research to help readers better understand the factors that can contribute to weight gain and changes in body weight.

To support accuracy and transparency, we encourage readers to explore the medical resources and journals referenced throughout this page.

Tryon MS et al. ‘Chronic stress, cortisol, and food cravings in adults’. Obesity, 2013. PubMed PMID: 23512957.
van der Valk ES et al. ‘Glucocorticoids and HPA axis regulation in the stress-obesity connection: a comprehensive overview’. Clinical Obesity, 2024. PMC11907100
Chaker L et al. ‘Hypothyroidism’. JAMA, 2022 (updated 2025). PubMed PMID: 40900603.
Davis SR et al. ‘Understanding weight gain at menopause’. Climacteric, 2012. PubMed PMID: 22978257.
Kapoor E et al. ‘Adverse changes in body composition during the menopausal transition and relation to cardiovascular risk: a contemporary review’. Current Cardiology Reports, 2022. PMC9258798.
Peng Y et al. ‘Associations of short sleep duration with appetite-regulating hormones and adipokines: a systematic review and meta-analysis’. PLOS ONE, 2020. PubMed PMID: 32537891.
Hall KD et al. ‘Ultra-processed diets cause excess calorie intake and weight gain: an inpatient randomised controlled trial of ad libitum food intake’. Cell Metabolism, 2019. PMC7946062.
Beslay M et al. ‘Ultra-processed food intake in association with BMI change and risk of overweight and obesity: a prospective analysis of the French NutriNet-Santé cohort’. PLOS Medicine, 2020. PMC7451582.
Dakanalis A et al. ‘The association of emotional eating with overweight/obesity, depression, anxiety/stress, and dietary patterns: a review of the current clinical evidence’. Nutrients, 2023. PMC10005347.
Konttinen H. ‘Emotional eating and obesity in adults: the role of depression, sleep and genes’. Proceedings of the Nutrition Society, 2020. PubMed PMID: 32213213
Dubern B, Mosbah H, Pigeyre M, Clément K, Poitou C. Rare genetic causes of obesity: Diagnosis and management in clinical care. Ann Endocrinol (Paris). 2022 Feb;83(1):63-72. doi: 10.1016/j.ando.2021.12.003. Epub 2021 Dec 23. PMID: 34953778.
van Egmond LT, Meth EMS, Engström J, Ilemosoglou M, Keller JA, Vogel H, Benedict C. Effects of acute sleep loss on leptin, ghrelin, and adiponectin in adults with healthy weight and obesity: A laboratory study. Obesity (Silver Spring). 2023 Mar;31(3):635-641. doi: 10.1002/oby.23616. Epub 2022 Nov 20. PMID: 36404495.
Hu H, Song J, MacGregor GA, He FJ. Consumption of Soft Drinks and Overweight and Obesity Among Adolescents in 107 Countries and Regions. JAMA Netw Open. 2023;6(7):e2325158. doi:10.1001/jamanetworkopen.2023.25158

About Your Clinical Team

This content was medically reviewed and approved by Danny Gian Hou Chu, a HCPC-Registered Physician Associate (PA38670), who oversees all prescribing decisions at Envigore. All prescriptions are dispensed by Right Medicine Pharmacy, a GPhC-Registered Pharmacy (No. 9012382). Nutritional guidance and lifestyle support is provided by Veronica Giudice, a HCPC-Registered Dietitian (DT26570) and British Dietetic Association member.

Last medically reviewed: May 2026