Picture this scene at a dinner table: On one side sits Nathan, a skinny guy who somehow manages to put away half a rotisserie chicken, a heaping portion of meatballs, a mixed stuffed vegetable platter, and a large serving of seasoned rice. Oh, and don’t forget the two whole roasted Cornish hens sitting next to his salad and bread. Nathan weighs about 156 pounds, despite this being his typical meal.
Now look across the table at Sam, who’s eating three tablespoons of rice, a boiled egg, and a small plate of sautéed vegetables. Sam weighs around 198 pounds, even though he always eats these modest portions.
The kicker? After dinner, Nathan might casually suggest they grab a couple of shawarma sandwiches to munch on while walking home. Despite the enormous amounts of food Nathan consumes, his weight consistently hovers around 155 pounds. Meanwhile, if Sam tried to indulge just once and eat, say, a couple of Cornish hens, his weight would noticeably increase.
Sound familiar? This scenario plays out in countless households across America. We all know someone nicknamed “the refrigerator’s best friend” – that person who devours everything in sight, in massive quantities, yet somehow remains slim as ever.
So what gives? Why do some people seem to have metabolic superpowers?
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It’s All About Energy Balance (But It’s Complicated)
First things first – let’s get one thing straight. If someone eats a lot without gaining weight, they’re not magical or “blessed with good genes” (well, not entirely). It comes down to a balance between energy consumed and energy expended.
Weight gain or loss results from an equation: the balance between food entering our body and being stored versus the energy our body burns. If the energy gained (food) is less than the energy lost, a person will lose weight. The opposite is also true.
But this oversimplifies what’s actually happening in our bodies. The body processes food and uses energy in complex ways. When there’s an imbalance in how energy from food is used, we see either weight loss or gain.
When Being Thin Isn’t Healthy
Being thin isn’t always something to celebrate, contrary to what many believe. Like obesity, extreme thinness carries health risks. There’s a fine line between being fit and being underweight.
Fitness gives the body an attractive, healthy appearance, while being underweight is concerning and can accompany various medical problems.
How Do I Know If I’m Underweight?
There’s an indicator we can use to determine if a person’s weight is healthy: Body Mass Index (BMI). It’s a simple calculation that helps assess whether our weight is appropriate for our height.
You can calculate your BMI using any iPhone or Android phone by searching for “BMI calculator” in the app store. Many free apps will do the job. Just input your weight and height (using metric values – centimeters and kilograms), and you’ll get your BMI number.
If your BMI falls between 18.5 and 24.9, your body weight is normal. If it’s higher, you’re overweight, and if it’s lower, you’re underweight. A BMI between 25 and 29.9 means you’re slightly overweight, while over 30 indicates obesity.
What Causes Someone to Be Underweight?
The causes can be grouped into two main categories: psychological and physical.
Psychological Causes
Among the most significant psychological factors affecting body weight is depression. Depression is typically associated with weight loss since those suffering from it often experience decreased appetite.
Severe stress and anxiety are also major causes of weight loss.
A lesser-known psychological condition is anorexia nervosa, or the fear of weight gain. It’s a psychological disorder characterized by an intense fear of gaining weight, leading to distorted body image. People with this condition drastically reduce their food intake, even when they’re already underweight, which can lead to serious health risks. It’s considered one of the most dangerous weight-related psychological disorders.
Physical Causes
One of the most common physical causes of thinness is an increased metabolic rate, often resulting from an overactive thyroid gland.
Metabolism is simply the process of breaking down chemical compounds in food to extract energy needed by the body. If metabolic rate is low, the rate of calorie burning decreases, and excess food gets stored as fat – a common obesity problem.
Conversely, if metabolic rate is high, the body burns calories rapidly, leaving little food to be stored. The body might even need to burn existing fat reserves. In cases of extreme thinness, the body can’t benefit from all the nutrients in food.
The Thyroid Connection
The thyroid gland secretes hormones that regulate metabolism. It’s a small butterfly-shaped gland located at the base of the neck, just below the Adam’s apple. Among the main hormones it produces are thyroxine (T4) and triiodothyronine (T3), which affect nearly every cell in the body and regulate how your body uses fats and carbohydrates. They also help control body temperature, heart rate, and protein production.
When the thyroid becomes overactive and produces excessive hormones, it disrupts burning processes, heart rate, and body temperature. Together, these cause continuous weight loss regardless of food intake – what’s known as “unintentional weight loss.”
Beyond Thyroid: Other Factors Affecting Metabolism
Is thyroid hyperactivity the only thing affecting metabolism? Not at all. Other factors influence metabolic rate and weight loss.
Gender, for example. Males generally have higher metabolic rates than females, which is why men need more daily calories than women. Metabolic rate is also affected by factors like age and body size.
Medical Conditions That Cause Weight Loss
Beyond what we’ve discussed, some illnesses can temporarily reduce body weight. The flu and high fever might shed a few pounds, but after recovery, weight typically returns to normal.
However, other diseases cause permanent thinness, including diabetes. Although many diabetics are overweight, some cases with insulin resistance lead to significant weight loss.
Tuberculosis is another wasting disease where weight loss is a primary symptom. TB is particularly prevalent in parts of southern Africa.
Enzyme deficiencies, especially digestive enzymes, can also cause weight loss as they impair proper digestion and absorption of food, leading to noticeable weight reduction.
The Bottom Line
While most of us struggle with keeping weight off, there are those among us fighting the opposite battle. Understanding the science behind metabolism helps us recognize that staying thin despite eating mountains of food isn’t magic – it’s biology at work.
If you’re significantly underweight despite eating well, consider consulting with a healthcare provider to rule out underlying conditions. And if you’re that person who can eat everything without gaining an ounce? Maybe ease up on mentioning those shawarma sandwiches around your friends who are counting every calorie!
