Unexplained weight gain: 10 possible causes you shouldn't ignore

Have you gained weight without understanding why?
Before blaming yourself or questioning your habits, it’s important to remember one thing: your body never changes at random. Every physical change is a response to specific biological signals.

Behind unexplained weight gain, there are always underlying mechanisms to explore: lifestyle changes, chronic stress, hormonal imbalances, sleep disruption, medications, or metabolic conditions. The explanation is not always, and fortunately not always, found on your plate.

🔎 In this article, we help you understand the mechanisms involved and identify 10 possible causes of unexplained weight gain, so you can take informed, compassionate action.

1️⃣ Lifestyle habits and reduced daily movement

Daily habits have a significant impact on weight — often without us noticing. Sedentary behaviour, rushed meals, irregular schedules, and chronic fatigue all affect how the body manages energy.

A more sedentary lifestyle or an imbalance between energy intake and expenditure encourages fat storage. Long hours spent sitting, busy days, and limited physical activity can prompt the body to adapt by slowing down its metabolism.

Why does the body store fat more easily?

• Reduced movement → Lower energy expenditure
• Irregular or rushed meals → Disrupted metabolic signals and appetite regulation
• Stress and fatigue → Alter hunger and satiety signals and promote abdominal fat storage

Stress also activates the brain’s reward system via dopamine¹, increasing the tendency to seek comfort through food.

How to restore balance gently

• Move more without pressure (walking, yoga, Pilates, stretching)
• Eat more mindfully and slowly
• Learn to distinguish physical hunger from emotional or stress-related hunger

2️⃣ Chronic stress and persistently elevated cortisol

When faced with stress, the body produces cortisol, a hormone designed to help us cope. Short-term stress is not an issue. Chronic stress, however, leads to continuous cortisol secretion, which can cause:

• Metabolic disruption → Elevated cortisol promotes visceral fat accumulation around organs²
• Increased appetite → Particularly for sugary and fatty foods
• Digestive and gut imbalance → Stress alters the gut microbiota³ and influences eating behaviour and satiety⁴

Helpful stress-regulation strategies:

• Identify major stress triggers
• Take real breaks, even short ones
• Use breathing techniques (4-7-8, cardiac coherence)
• Choose gentle, regular movement over intense workouts

3️⃣ Dysregulation of hunger hormones (ghrelin and leptin)

Weight gain can also be driven by increased appetite — and this is biological, not a lack of willpower.

Hunger is regulated by:

• Ghrelin, which signals hunger

• Leptin, which signals satiety

When this balance is disrupted, hunger and fullness cues become blurred, leading to involuntary weight gain.

Why does this happen?

• Lack of sleep increases ghrelin and decreases leptin⁵

• Chronic stress stimulates ghrelin production

• Highly processed, sugar-rich diets worsen appetite dysregulation

How to support appetite regulation

Chrononutrition may help stabilise metabolism⁶:

• Protein-rich breakfast
• Balanced lunch with lean protein and fibre
• Light but satisfying dinner

Stress management and quality sleep are equally essential.

4️⃣ Poor sleep quality or sleep disorders

Sleep is a cornerstone of metabolic health. A large Brazilian longitudinal study (ELSA-Brasil) followed over 13,000 adults and showed that sleep disorders significantly increased the risk of weight gain and waist circumference expansion⁷.

Insufficient or poor-quality sleep:

• Alters hunger hormones
• Worsens insulin resistance
• Encourages fat storage

Adequate sleep is therefore a key lever in weight regulation 💤

5️⃣ Medications that affect metabolism

Some treatments can influence weight by altering appetite, metabolism, or fluid balance:

• Corticosteroids → Promote water retention and abdominal fat storage
• Antidepressants → May affect appetite and metabolic rate⁸
• Certain hormonal contraceptives → Especially those high in progestins, which may increase appetite and alter fat distribution⁹

💡 If weight gain coincides with starting a medication, discuss alternatives with your GP or gynaecologist.

6️⃣ Hypothyroidism: when metabolism slows down

The thyroid gland regulates metabolic rate. When thyroid function is reduced, the body burns fewer calories at rest.

Symptoms may include:

• Persistent fatigue
• Swelling or bloating
• Moderate but persistent weight gain

Hypothyroidism also causes water retention, creating a feeling of heaviness. With appropriate medical treatment and supportive lifestyle measures, balance can usually be restored¹⁰.

7️⃣ High blood pressure and fluid retention

High blood pressure (hypertension) is often silent but can contribute to unexplained weight gain.

When blood pressure is elevated:

• The kidneys eliminate excess water and salt less efficiently
• Fluid retention and swelling occur

Hypertension is also associated with chronic inflammation and insulin resistance, slowing metabolism and promoting weight gain. Women are particularly affected during menopause or in hormonal conditions such as PCOS.

8️⃣ Cushing’s syndrome and excess cortisol

Cushing’s syndrome is rare but serious. It involves prolonged exposure to high cortisol levels, either due to tumours or long-term corticosteroid use¹¹.

Excess cortisol leads to:

• Localised weight gain (face, abdomen, neck, upper back)
• Muscle loss
• Fragile skin and purple stretch marks
• Metabolic and hormonal disturbances

Medical evaluation is essential if symptoms suggest this condition.

9️⃣ Insulin resistance and type 2 diabetes

Insulin allows glucose to enter cells for energy. When cells become resistant:

1️⃣ Glucose uptake decreases
2️⃣ The pancreas produces more insulin
3️⃣ Excess insulin stimulates appetite and fat storage
4️⃣ Abdominal fat worsens insulin resistance

Over time, pancreatic exhaustion may lead to type 2 diabetes¹².

This is a hormonal feedback loop, not a failure of discipline.

🔟 Polycystic Ovary Syndrome (PCOS)

PCOS affects around 1 in 10 women and is often underdiagnosed.

Common symptoms include:

• Irregular or absent periods
• Excess hair growth
• Acne and oily skin
• Fatigue
• Mood changes
• Hair thinning¹³

Why does PCOS cause unexplained weight gain?

Around 70% of women with PCOS have insulin resistance¹⁴, leading to:

• Increased appetite
• Easier fat storage, particularly abdominal
• Difficulty losing weight despite balanced eating

Chronic stress associated with PCOS further amplifies these effects.

What to do if your weight gain feels unexplained

If you recognise yourself in several of these causes, don’t stay alone with your questions. Medical follow-up is essential to identify the underlying mechanisms and tailor appropriate care.

Understanding the cause is the first step towards regaining control — with clarity, compassion, and trust 💜