What Causes Weight Gain in Some Women but Not Others?

Edited by Abinaya Muralidharan, M. Pharm - October 09, 2026

Introduction

Some women appear to gain weight more easily than others. In such cases, not knowing why can be frustrating.

It is important to understand that weight change is complex and multifactorial, influenced by more than just food intake. For some women, a propensity for weight gain may be due to differences in metabolism and energy expenditure. Genetic and hormonal factors, including estrogen, can influence the distribution and type of adipose (fat) tissue.

Individual differences in lifestyle also affect body weight. Some medical conditions and medications can contribute to weight gain as well. Persistent or unexplained weight gain may sometimes warrant medical evaluation.

Can You Gain Weight Even If You Don't Eat Much?

Yes. Weight gain can occur even when someone feels they are not eating much. There is a misconception around “calories in vs. calories out”: while energy balance remains relevant, both intake and expenditure can be difficult to estimate, and this varies between individuals. Several factors, as elaborated below, may also influence how the body regulates weight, including:

• Energy expenditure and metabolic differences
• BMR, body composition, and daily movement
• Genetic differences in weight regulation
• Hormonal and life-stage changes
• Sleep, stress, and circadian rhythm
• Certain medications and underlying medical conditions
• Gut microbiome differences, although their role in weight gain remains uncertain

Why Perceived Intake and Actual Energy Intake Can Differ

Portion-size underestimation and calorie density

People may underestimate the amount they consume. Portion size and how often snacks are consumed impact the risk of being overweight or obese.

The food environment – the setting in which food is consumed – can also influence portion perception. People who eat at restaurants may have a skewed perception of normal portion sizes, potentially leading them to consume more food than needed. Furthermore, even small portions may be calorie-dense, so the type of food matters as well.

Liquid calories and snacks

The type of beverage consumed is important to consider. People may forget that these also add to total calories. For example, sugar-sweetened drinks can significantly add calories to a person’s daily intake.

Alcohol intake affects fat metabolism, and alcohol itself adds calories. That said, the relationship between alcohol consumption and weight gain is not so simple. Aside from what you eat and drink, other factors affect weight. 

Factors That Affect the Number on the Scale

Changes in activity levels

An increase in activities such as aerobic exercise can lower body weight. However, a person needs to exercise 150 minutes per week at a moderate or higher intensity to decrease body weight and waist circumference.

Water retention and constipation

These are non-fat contributors to body-weight changes. Constipation can temporarily increase body weight.

Examining calorie intake, energy, and metabolism can provide insights into changes in body weight.

Energy Expenditure & Metabolic Differences

The “calories in, calories out” (CICO) principle reflects the thermodynamic basis of energy balance, but “calories out” varies substantially between similar-sized individuals. Herman Pontzer's energy-expenditure research found variations based on age and life stage, reflective of behavioral and metabolic changes. The study identified four distinct metabolic phases across the human life course: infancy, childhood/adolescence, adulthood, and older adulthood. Daily energy expenditure was relatively stable from about age 20 to 60, but declined after age 60, even after accounting for body size and composition.

Total energy expenditure (TEE) is defined as the total amount of energy, measured in calories, that your body uses over a given time period, usually 24 hours. TEE is influenced by basal metabolic rate (BMR), thermic effect of food, exercise, and non-exercise activity thermogenesis (NEAT).

• Basal/resting metabolic rate (BMR/RMR) is the amount of energy required to maintain bodily functions at rest. This includes breathing, circulation, and maintaining body temperature.
• Thermic effect of food (TEF): This is the energy we use to digest, absorb, and process food.

Metabolic Adaptation After Weight Loss

• Adaptive thermogenesis (also known as metabolic adaptation) describes changes in energy expenditure that occur because of changes in calorie intake, temperature, or body weight. It describes a reduction in energy expenditure during or after weight loss or restrictive/yo-yo dieting that is more than expected by changes in the person’s body composition, but the change may disappear after weight stabilizes.
• Increased hunger after calorie restriction may also occur and is due to a rise in the hormone ghrelin.
• Lower energy expenditure can contribute to weight gain if the body burns fewer calories while energy intake stays the same. This may occur after weight loss, prolonged calorie restriction, or loss of lean mass.

BMR, Body Composition, and Daily Movement

How Age, Height, and Lean Muscle Mass Influence Resting Energy Expenditure

Energy metabolism is associated with age, fat-free mass, and body height. Taller people generally require more energy at rest. Fat-free mass includes the body’s non-fat tissues, such as muscles, organs, and bones, and is an important determinant of resting energy expenditure. These tissues require energy to maintain their normal functions even when the body is at rest. For example, muscle tissue uses energy even when a person is not exercising.

Resting energy expenditure also tends to decrease with age. Changes in fat-free mass and its composition are among the factors that contribute to this decline.

When a person loses weight, some muscle and other lean tissue may also be lost, which can decrease resting energy expenditure. With less fat-free mass to maintain, the body generally requires fewer calories at rest.

Muscle-Mass-to-Fat Ratio

This shows what your body weight consists of, and it is computed by dividing total skeletal mass by total fat mass. There is a misconception that “muscle weighs more than fat”. However, they weigh the same. The difference is that muscle is denser, so the same weight of muscle takes up less space than the same weight of fat. Therefore, body weight alone does not indicate the amount of weight that comes from muscle versus fat.

NEAT and Daily Energy Expenditure

NEAT (non-exercise activity thermogenesis) is the energy expended through everyday activities outside structured exercise, including standing, walking, changing posture, and even spontaneous movement such as fidgeting. Because these activities use energy, differences in everyday activity can contribute to differences in total energy expenditure.

Genetic Differences in Weight Regulation

Genes can influence body weight. Body mass index (BMI) heritability among twins is high, about 0.84–0.85 during adolescence. Among parents and children, the heritability of BMI is about 39%.

Why Some Women Seem Naturally Resistant to Weight Gain

Genes involved in appetite regulation and body fat storage may play a role. Variants in the genes FTO and MC4R have been associated with overweight and obesity risk. These genetic variants are also related to differences in appetite-related traits, but they do not specifically determine a person’s body weight. Higher natural NEAT, more lean muscle mass, favorable genetics, efficient satiety signaling, and smaller appetite may affect weight gain.

Some genetic pathways may also influence susceptibility to weight gain. Experimental research has linked TAp63 activity in hypothalamic POMC neurons with resistance to diet-induced obesity in female mice, but an effect in humans has not been established as yet. Some genetic variants may provide relative protection against weight gain and obesity. Rare variants in genes such as GPR75 have been associated with a lower BMI and risk of developing obesity. Body weight is not always associated with health.

Lower body weight does not always mean better metabolic health

The “thin outside, fat inside” (TOFI) concept describes people who appear lean or have a normal BMI, but they have high amounts of visceral or ectopic fat deposits.

BMI does not show where fat is stored, so you may have a normal body weight but have fat accumulation around internal organs or in tissues such as the liver and muscle. This can be associated with insulin resistance.

Hormonal & Life-Stage Factors in Women

Hormones and life-stage can affect body weight and BMI.

Why Do Women Gain Weight During Menopause?

Estrogen declines during perimenopause/menopause, and there is a shift toward abdominal fat storage, which can result in weight gain.

Postmenopausal changes, including declining estrogen, are associated with lower muscle strength and mass and increased fat accumulation, which may heighten the risk of developing sarcopenic obesity.

Other Life Stages

Menstrual cycle changes

Women can gain weight during their period. Due to fluid retention, a woman may gain about 0.5kg during menstruation.

Pregnancy and postpartum changes

Women who gain more weight during pregnancy than expected also may retain more weight after giving birth. Changes in physical activity, diet, and sleep after childbirth may affect postpartum weight retention.

PMOS and Insulin Resistance

PMOS (formerly known as PCOS) is associated with insulin resistance. This can affect both metabolic and hormonal regulation. Insulin resistance may contribute to higher insulin levels, altered androgen production, and difficulty controlling weight. Excess body fat worsens insulin resistance and PMOS, creating a cycle in which metabolic and hormonal abnormalities reinforce one another. PMOS and insulin resistance can also occur at any body weight. Insulin resistance can eventually lead to type 2 diabetes.

Hormonal Changes and Weight Gain

Thyroid dysfunction

Hypothyroidism is a potential contributor to weight gain and increases your risk of obesity. This condition is more prevalent in women than men. In women with PMOS, lower LH levels are associated with higher body weight, although this relationship does not establish that low LH causes weight gain. 

Can low testosterone cause weight gain in women?

In men, low testosterone may influence body composition by reducing lean muscle mass and increasing fat mass, but there is no good evidence that low testosterone directly causes weight gain in women – some data show the opposite pattern [45.46].

Gut Microbiome and Weight

Individual variation in gut-bacteria composition may lead to possible differences in energy extraction from food, but there is no evidence that this causes changes in weight gain.

In a recent twin study, the heavier and leaner twins did not differ significantly in microbial diversity.

Gut-driven inflammation and increased intestinal permeability “leaky gut” may affect insulin sensitivity by letting lipopolysaccharide (LPS) from bacteria enter the bloodstream. Although gut permeability can trigger inflammation, more research is needed to fully establish a link with metabolic diseases or weight changes.

Sleep, Stress & Circadian Rhythm

Obstructive sleep apnea may make it hard to manage weight due to disrupted sleep, intermittent hypoxia, changes in appetite regulation, and reduced daytime activity. But being overweight is also a major risk factor for developing or worsening sleep apnea.

Shift work and circadian misalignment can affect glucose metabolism and other metabolic factors, leading to metabolic syndrome. Poor sleep reduces insulin sensitivity regardless of changes in fat composition.

A lack of sleep can disrupt hormones such as cortisol, leading to increased appetite. Higher ghrelin and lower leptin levels may occur with less sleep of 4 hours a night. These factors can promote weight gain in females, including increased visceral fat accumulation.

In addition to increasing calorie intake, poor sleep can reduce physical activity levels. Studies have shown that it can also lead to a tendency to prioritize taste over nutrition, potentially contributing to weight gain.

Sleep-deprivation-driven cravings can occur because of the impact on ghrelin. People may crave carbohydrates and energy-dense (high-calorie) foods. So, women may eat less but still consume more calories and gain weight because of the type of food they eat. Furthermore, stress, in addition to lack of sleep, can impact the body, exacerbating weight gain.

Can Stress Alone Cause Significant Weight Gain?

Chronic stress can activate the hypothalamic–pituitary–adrenal (HPA) axis and modify cortisol regulation. Cortisol affects glucose and fat metabolism. This may then promote the accumulation of visceral fat.

Long-term stress may increase cravings for energy-dense foods and lead to “stress eating,” which can increase calorie intake. Stress eating is more common among obese individuals. Stress eating and emotional eating both involve cravings rather than true hunger.

Medications That May Cause Weight Gain

• Antidepressants, corticosteroids, some antiseizure medications, beta-blockers, insulin, and some antipsychotics may cause weight gain.
• Hormonal contraceptives and hormone replacement therapy (HRT) may or may not cause weight gain. Many have no effect, but others like Depot Medroxyprogesterone Acetate (DMPA) increase the risk of fat deposition.
• Other medications may affect appetite, metabolism, or fluid balance.

It is recommended not to stop or change medications without medical advice.

Other Underlying Medical Conditions

Depression and other mental health problems may cause reduced or increased appetite and activity.

Less common endocrine or metabolic disorders, including Cushing's syndrome, can lead to weight gain. In Cushing’s syndrome, excess cortisol leads to fat accumulation.

Clinically significant fluid retention in the tissues can occur with heart disease or kidney problems. This rapid weight gain due to a medical condition is not the same as ordinary short-term water-weight fluctuation.

Depending on the reasons behind weight gain, there are some strategies that can be helpful for women.

What Can Women Do If They Gain Weight Despite Their Efforts?

• Keep a realistic food and activity record.
• Prioritize protein and fiber-rich foods, which also helps you maintain muscle.
• Focus on sustainable dietary habits rather than extreme restriction.
• Combine resistance training and aerobic exercise while maintaining regular daily physical activity.
• Improve sleep quality.
• Manage chronic stress.
• Consult a doctor about medications and any health conditions that could be influencing weight gain.

When Unexplained Weight Gain Needs Medical Attention

• Rapid or significant weight gain without a clear diet or lifestyle change.
• Thyroid clues: fatigue, cold intolerance, hair thinning.
• PMOS clues: irregular periods, acne, excess hair growth.
• Cushing's signs, such as a rounded face and hump on the back.
• Swelling or persistent fluid retention.
• Significant changes in appetite or thirst.

What tests may be considered?

Typical bloodwork may include a thyroid panel, fasting glucose or HbA1c, with oral glucose tolerance testing (OGTT) considered in PMOS, and a hormone panel, depending on clinical context.

Conclusion

Differences in satiety signaling and naturally smaller appetite can influence eating behavior, but so can sleep, stress, medications, and age. Genetic predisposition may influence your susceptibility to weight gain, but both environment and lifestyle continue to affect body weight. It is important to maintain a healthy weight regardless of age. Signs of rapid weight gain should be evaluated by a primary care physician, who may check blood pressure and perform other tests to determine whether an underlying medical condition is present.

Frequently Asked Questions

Can thyroid problems cause weight gain with no other symptoms?

Yes, hypothyroidism can lead to weight gain, and it can sometimes be asymptomatic or present with few symptoms.

Can a “slow metabolism” contribute to unexplained weight gain?

A slow metabolism by itself doesn’t necessarily cause weight gain, but some medical conditions can affect metabolism and contribute to weight gain.

Does a woman gain weight during ovulation?

Studies have not consistently shown significant weight gain during ovulation, but there can be fluid retention causing weight gain during menstruation.

Does creatine cause weight gain in women?

Creatine may increase body mass while increasing fat-free mass and slightly reducing body-fat percentage. However, available evidence in women does not show that it consistently causes significant weight gain.

Do women gain weight after a hysterectomy​?

It depends. In premenopausal women, they can gain weight after a hysterectomy. Weight gain is common during menopause even without a hysterectomy, but weight gain is greater if both ovaries are removed.

Can eating fewer calories cause weight gain?

Eating too few calories does not make the body gain fat even with a sustained energy deficit. But prolonged food restriction can lower energy expenditure and make you hungrier. This can make weight loss harder to maintain and, ultimately, lead to weight gain.

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