- AIIMS-trained gut doctor explains why some gain weight despite eating little
- He says people have different 'Basal Metabolic Rates' affecting calorie burn at rest
- Muscle mass also increases calorie burn three to five times more than fat tissue
We all know someone who swears they “barely eat” but still struggles with weight gain. And then there is that other person who snacks all day and never seems to gain a kilo.
It feels unfair, confusing and frustrating. But the truth is, weight gain is not just about how much you eat. It is about how your body burns what you eat.
At the centre of this conversation is metabolism.
More specifically, Basal Metabolic Rate (BMR). This is the energy your body uses just to stay alive. Breathing, thinking, digesting food and pumping blood – all of it counts. And here is the key point: everyone's metabolic “engine” is different.
On Wednesday, Dr Saurabh Sethi, MD, MPH, a gastroenterologist trained at AIIMS, Harvard and Stanford University, explained this in an Instagram post. He broke down why some people gain weight even when they eat very little, while others do not.
Dr Sethi explained that some people are born with a faster BMR. This can be due to genetics, thyroid function, hormones, or higher muscle mass. These people burn more calories even while resting. Others have a slower BMR, which means fewer calories burned at baseline, even if food intake is low.
One big factor here is muscle mass. Muscle burns 3 to 5 times more calories than fat, even when you are doing nothing. So people with more muscle burn more calories all day long. This is why those who are active or exercise regularly can often eat more without gaining weight.
On the other hand, metabolism can slow down after:
- Crash dieting
- Long-term calorie restriction
- Muscle loss with age
Dr Sethi also pointed to hormonal imbalances. Conditions like insulin resistance make it easier for the body to store fat and harder to burn it. This is common in fatty liver, PCOS, prediabetes and diabetes.
According to him, people who gain weight despite eating little often have:
- Slowed metabolism
- Low muscle mass
- Hormonal imbalance
- High cortisol or hidden stress
- Poor sleep
- Post-diet metabolic adaptation
And those who seem to “eat anything” and stay lean usually have:
- Higher metabolic rate
- Better insulin sensitivity
- More muscle
- More daily movement
Dr Sethi stressed one important thing. “It's not laziness. It's physiology.”
The real goal, he said, is not to eat less. It is to help your body burn better. Build muscle, manage stress, sleep well and move consistently. Even small daily movements add up more than people realise.