People often discuss ways to lose weight, including counting calories, hitting the gym, increasing protein intake, trying intermittent fasting, or following the latest diet trends. Social media is flooded with weight-loss tips, transformation stories and fitness advice. But there is another side of the conversation that rarely gets the same attention: people who struggle to gain weight despite eating more, working out and trying every possible way to bulk up.

That conversation has resurfaced following the latest episode of Samay Raina’s India’s Got Latent Season 2, featuring Karan Aujla, Tanmay Bhat, Rahul Dua and Gurleen Pannu. Contestant Gaurav Madaan left the judges and audience in splits with a parody of the iconic song ‘Khamoshiyan’, replacing its romantic lyrics with jokes about gym life, muscles and the everyday struggles of being extremely skinny.

Madaan’s deadpan humour, witty lyrics and comic timing earned him Rs 1 lakh, but the performance also struck a chord with many viewers who related to his struggles. Beyond the laughter, it has reignited a discussion that often gets overlooked: Why is gaining weight so difficult for some people? Is it simply a matter of eating more, or do genetics, metabolism and underlying health conditions also play a role? Let’s dive in.

Why is weight gain a challenge for many

While people are often told to “just eat more” if they want to gain weight, scientific evidence suggests that gaining weight is not equally easy for everyone. Research indicates that genetics, metabolism, hormones and differences in how the body processes nutrients may all influence a person’s ability to gain weight and build muscle.

A 2020 study on people with constitutional thinness (CT) – individuals who remain naturally underweight despite being otherwise healthy – found that simply increasing calorie and protein intake did not necessarily translate into weight or muscle gain. Researchers overfed participants for two weeks by providing an additional 600 kcal per day, along with extra protein, carbohydrates and fat, and compared their response with that of normal-weight individuals.

Although the constitutionally thin participants consumed more calories and maintained a positive energy balance, they did not gain lean muscle mass to the same extent as the control group. The researchers also observed differences in appetite-regulating hormones and protein metabolism, suggesting that their bodies responded differently to excess nutrition.

The study concluded, “The blunted muscle energy mechanism… suggests a resistance to lean mass gain in the constitutional thinness phenotype.”

These findings indicate that, for some people, increasing calorie and protein intake alone may not be enough to produce substantial weight or muscle gain.

This variation in weight gain is also reflected in broader research. Another 2020 systematic review, which examined more than 300 overfeeding studies conducted over periods ranging from one day to six months, found that overfeeding generally led to weight gain across adolescents, adults and older individuals. However, the response was not identical for everyone.

The review reported that genetics, insulin sensitivity, obesity susceptibility, birth weight and diet composition all influenced how individuals responded to excess calorie intake. It also noted that overfeeding altered hormones such as insulin, triiodothyronine and growth hormone, while changes in fat storage and muscle metabolism differed between individuals.

The authors concluded that while increased energy intake usually results in weight gain, there is evidence that genetic and metabolic factors contribute to why some people gain weight more readily than others.

Further evidence came from a 2021 systematic review and meta-analysis examining constitutional thinness. After reviewing 39 clinical studies, researchers found that constitutionally thin individuals generally consumed normal amounts of food, yet remained underweight.

The review also found a trend towards a higher resting metabolic rate relative to fat-free mass, suggesting that these individuals may have a more metabolically active lean tissue. Unlike people with anorexia nervosa, they also showed normal levels of several hormones involved in growth and reproduction, reinforcing that constitutional thinness is a distinct biological condition rather than an eating disorder.

The researchers concluded that, “Constitutionally thin people present an atypical phenotype highly different from anorexia nervosa.”

More recently, a 2025 review revisited the concept of constitutional thinness and highlighted how little scientific attention it has received compared with obesity.

The authors noted that although the condition was first described more than a century ago, fewer than 50 clinical studies have investigated constitutional thinness over the past 90 years, compared with hundreds of thousands of studies on obesity.

The review posed a question that continues to challenge researchers, “How could a young, healthy person with normal food intake present such an underweight state that resists body-weight gain?”

According to the authors, constitutional thinness represents “a unique model questioning basic dogmatic knowledge about energy balance and metabolism,” suggesting that body weight is influenced by far more than calorie intake alone.

Expert insights: Why eating more isn’t always enough 

According to Ms Edwina Raj, Head of Services – Clinical Nutrition & Dietetics, Aster CMI Hospital, Bengaluru, gaining weight is not simply a matter of eating more. While a calorie surplus is necessary, factors such as genetics, metabolism, hormones and overall health play a crucial role in determining whether a person gains weight or builds muscle. 

“Some people naturally have a faster metabolism, meaning they burn calories more quickly even at rest. Genetics can influence body type, muscle-building potential and fat storage,” she said.

Raj explained that hormonal conditions such as hyperthyroidism can increase energy expenditure, while digestive disorders may reduce nutrient absorption despite adequate food intake. She added that stress, poor sleep and chronic illnesses can also affect appetite and metabolism.

“If someone is eating enough but still struggles to gain weight, it is important to assess lifestyle, medical history and underlying health conditions rather than assuming they simply need to eat more,” she said.

She noted that weight and muscle gain depend on much more than calories and protein alone. Adequate nutrition, proper digestion, hormonal balance, quality sleep and progressive strength training are all essential for building muscle effectively. According to Raj, some individuals naturally have a higher metabolic rate or greater levels of spontaneous physical activity, causing them to burn more calories than expected, while others may not absorb nutrients efficiently because of gastrointestinal conditions. Hormonal disorders, chronic infections and nutrient deficiencies can also interfere with muscle growth, she added.

“Inadequate recovery, overtraining or inconsistent exercise intensity may also limit muscle development. If someone follows a structured nutrition and exercise plan for several months without progress, a medical evaluation can help identify any underlying causes,” Raj said.

Explaining the difference between constitutional thinness and an underlying health condition, Raj said people with constitutional thinness are naturally lean but generally healthy. They usually have stable body weight, normal energy levels, a good appetite and no symptoms suggestive of illness.

However, she cautioned that warning signs such as unexplained weight loss, excessive hunger or thirst, frequent urination, chronic diarrhoea, abdominal pain, persistent fatigue, rapid heartbeat, heat intolerance, recurrent infections or muscle weakness may indicate an underlying medical condition.

“If weight remains very low despite adequate nutrition, or if these symptoms are present, a medical evaluation, including blood tests and other investigations, is recommended to identify or rule out an underlying disorder,” she said.

Raj advised that the healthiest approach to gaining weight is to gradually increase calorie intake through nutrient-rich foods rather than relying on junk food or excessive supplements.

“Meals should include adequate protein, healthy fats, whole grains, dairy, nuts and seeds, along with regular strength training to encourage muscle gain instead of excess body fat. Good sleep and stress management also support healthy weight gain,” she said.

She added that while some people are naturally lean, medical evaluation is advisable if weight loss is sudden, digestive symptoms or persistent fatigue develop, or there is no improvement despite following a balanced, calorie-rich diet and exercise programme for several months.

Also read: Why 90 minutes of weight training a week could be enough for boosting longevity