We all know that one person. The one who seems to defy the very laws of nature and metabolism. They munch on burgers, indulge in desserts, and never seem to put on a single kilo. While the rest of us are counting calories, dreaming of shedding those stubborn few kilos, this friend is on the opposite end of the spectrum, wishing they could gain some weight. And the most frustrating part? They can’t!

For many of us, the journey with food is a constant balancing act. We think, we choose, we limit, and we constantly monitor ourselves to ensure we don’t go overboard. So, watching someone effortlessly enjoy their food without a care in the world can be, well, a tad unbearable.

I’ve been scrolling through comments on various posts about metabolism and energy balance lately. The sentiment is almost unanimous: a collective sigh of “Why can’t I eat like that without gaining weight?” The pain of this perceived metabolic injustice is palpable, especially among those scratching their heads, wondering, “Why is it like this?”

Theories abound. Some say it’s a naturally fast metabolism, a gift that allows these lucky few to burn off calories at lightning speed. Others point to hormones, genetics, or even the idea that some people just don’t absorb food as efficiently. These theories often seem like the most logical explanations. After all, they’re the first things that pop into our minds, and they sound pretty convincing.

But is that the whole truth? Is it just a simple game of genetic roulette, or is there more to the story? As we dive deeper into the world of nutrition and metabolism, it’s essential to challenge our assumptions and seek the real answers.

The Myth of the “Fast Metabolism”: What’s Really Going On?

When we talk about healthy individuals, the differences in their metabolism are minimal and can’t explain the vast disparities we often observe. Simply put, the idea of a “fast metabolism” doesn’t cover the full scope. For a healthy individual (and it’s crucial to emphasize ‘healthy’), metabolism is largely determined by body composition and activity levels rather than any mysterious factors.

One might think that variations in the thyroid gland, which influences metabolic rate, could be the most apparent culprits. However, this would require us to step away from the notion of being “conditionally healthy.”

To provide a general perspective, the average man requires about 2,500 calories a day to maintain their weight, while the average woman requires around 2,000 calories. However, it’s essential to note that these are average figures, and individual basal metabolic rates (BMR) can vary based on factors like muscle mass, activity level, and overall health.

It’s also worth noting that as age increases, BMR tends to decrease. This decline is partly due to a decrease in muscle mass as one ages and partly because of hormonal and neurological changes. This is why it’s often harder to maintain or lose weight as one gets older.

In one study, the resting (basal) metabolism of individuals with hyperthyroidism was found to be elevated by 40%. After treatment (which reduced hormone activity), it decreased but was still 13% above average.

In another similar study, where individuals with hyperthyroidism were treated, the resting metabolism significantly decreased by an average of 490 kcal per day, dropping from 2090 kcal to 1600 kcal.

Indeed, these individuals had an elevated metabolism, which hindered their ability to gain weight. But here’s the catch: hyperthyroidism is a severe condition. If diagnosed, one would likely rush to get treatment rather than celebrate the lost kilos, which might include muscle mass.

Gastrointestinal Issues

Imagine a scenario where the gastrointestinal tract (GIT) doesn’t absorb the full amount of food one consumes. Instead of utilizing 2000 kcal, only 1500 kcal gets processed. Sounds like a potential reason for weight differences, right? However, significant changes in body composition are typically caused by severe conditions like Crohn’s disease or ulcerative colitis. These are conditions where a staggering 20-40% of consumed calories literally go down the toilet. While I haven’t personally encountered such cases, from descriptions, it’s clear that this is far from being a “beneficial” situation.

There are also less severe GIT disorders that might limit one’s food choices or the quantity they can consume. This often leads to individuals eating foods they don’t particularly enjoy, resulting in reduced overall intake. As for acute food poisoning, it usually lasts a few days. While there might be a temporary weight loss during this period, it’s not significant and typically rebounds shortly after recovery.

Testosterone: The Muscle Builder and Fat Reducer?

Testosterone. It’s often hailed as the powerhouse hormone, especially when it comes to muscle growth. There’s a plethora of research on the topic, so much so that it’s almost redundant to list them all. It’s a well-established fact: testosterone is excellent for muscle development.

However, the intriguing aspect of testosterone isn’t just its relationship with muscle mass but its connection to leanness. It turns out that individuals with higher testosterone levels tend to be leaner rather than more muscular. Several studies have found that men with elevated testosterone levels have less body fat compared to those with lower levels. Surprisingly, there wasn’t a significant difference in muscle mass or lean body mass between the two groups.

Now, here’s the twist: while testosterone is known to aid muscle growth, it doesn’t inherently possess fat-burning capabilities. So, what’s the real story behind this?

It’s all about reverse causality: higher testosterone levels don’t necessarily reduce fat levels. Instead, having lower body fat can boost testosterone levels. Here’s why: fat tissue produces an enzyme called aromatase, which converts androgens into estrogens. So, the less fat you have, the more testosterone and the less estrogen you’ll have in your system.


Type 1 diabetes can indeed lead to sudden weight loss. This happens when excess glucose (or calories) is excreted through urine. However, this weight loss is typically temporary and stabilizes once insulin therapy begins.

Eating Disorders

Eating disorders are another factor to consider. They don’t just lead to weight gain; they can also cause significant weight loss. Fears surrounding food, self-imposed dietary restrictions, and conditions like bulimia can all play a role.

The popular image of someone who eats endlessly, remains inexplicably slim, and is otherwise perfectly healthy doesn’t quite match up with reality. Delve a little deeper, and you’ll often find painful extremes. Any medically documented cases where individuals eat a lot, don’t gain weight, or even lose weight are almost always accompanied by a slew of health issues that no one would wish for.

These are classic examples of situations where a perceived “benefit” (like not gaining weight) is actually a symptom of a larger problem. It’s a bug, not a feature. And if you were to experience such a “benefit,” you’d likely be eager to address it through medical treatment.

But What About That Friend of Yours? Perfectly healthy, full of energy, never complains about any health issues. Yet, they eat whatever they want, and never seem to gain a pound. What’s their secret?

The Hidden Factor: The Thermic Effect of Food

When it comes to weight and metabolism, there’s a less-discussed player in the game: the thermic effect of food (TEF). While differences in the gastrointestinal tract and nutrient absorption do exist, it’s not quite accurate to label it as “incomplete absorption.” Everything gets absorbed, but the efficiency of this absorption varies between lean and overweight individuals.

Here’s an interesting fact: the thermic effect is generally lower in overweight individuals. For instance, the TEF of pure dietary fat in overweight people is almost negligible. This means they derive 100% of the calories from fat. In contrast, lean individuals have a TEF for fats that can reach up to 15%. This means that fats are oxidized before they’re fully absorbed.

This reduced thermic effect is linked to a slower fat oxidation rate in those carrying extra weight. Despite having ample energy stored as fat, their bodies are somewhat reluctant to use this reserve (to a certain extent, as there’s always some fat being oxidized).

The thermic effect of carbohydrates also tends to be lower in those with insulin resistance, making it harder for them to absorb glucose from the bloodstream.

Moreover, the TEF of mixed meals made from processed foods is lower than that of whole foods. For instance, a whole grain bread with cheddar cheese has a TEF of 19.9%, while white bread with processed cheese stands at just 10.7%. That’s a significant difference in energy expenditure for dishes with similar macronutrients and caloric content.

The body’s fat content determines how many calories will be derived. On average, the TEF of overweight individuals might hover around 10%, while lean individuals can oxidize up to 25% of the energy they consume (source). For a 2000 kcal intake, this translates to a difference of 200 kcal versus 500 kcal. That’s quite significant.

This may be the first major distinguishing feature of those who are naturally slender. It’s a subtle yet powerful factor that plays a crucial role in the complex world of metabolism and weight management.

Could It Really Be Metabolism?

There’s a common belief that metabolism plays a significant role in our body weight. One particular study aimed to identify differences between individuals with a low body mass index (BMI <18) and those with a normal range (>23).

Interestingly, while thyroid hormones (T3, FT3, FT4) were slightly elevated in the lean group, suggesting a marginally higher resting metabolism, the study found that these very lean individuals generally moved less throughout the day. The kicker? They “typically consumed less food than those with a normal weight.”

Surprising, right? Just when we thought we had it figured out.

However, there’s a caveat to this study. In their metabolism calculation formula, they oddly fixed the thermic effect of food (TEF) at 10% for everyone. But as we now know, this parameter can vary based on body fat percentage and food composition. So, even though the lean group “ate less than expected,” it’s possible that it wasn’t an accelerated metabolism at play. Instead, it might be the lean individuals’ ability to burn more energy before absorbing it.

The Less Obvious Factor: Metabolic Adaptation

Many are familiar with this concept, especially those who’ve embarked on weight loss journeys. It’s that frustrating phase where, as you lose weight, you need to consume fewer and fewer calories to continue shedding pounds. But here’s the thing: metabolic adaptation swings both ways. It can also work efficiently when increasing calorie intake.

For instance, as the body accumulates more fat, it might ramp up fat burning through spontaneous activity.

This is the second major distinguishing feature of those who are naturally slender. It’s not just about the calories consumed but how the body adapts and utilizes those calories, showcasing the intricate dance of metabolism and body composition.

The “Restless Phenotype”: The Secret Behind Some Slim Figures?

Have you ever noticed that some people just can’t sit still? They’re always moving, tapping their foot, or adjusting their posture. This phenomenon is known as the “restless phenotype.” It’s the body’s innate way of preventing weight gain by staying constantly in motion. On the flip side, there’s the “economical phenotype,” where every movement is deliberate, and nothing is done without a specific purpose.

But here’s the catch: when it comes to the fidgety folks, it’s not that their excess energy just magically disappears. These individuals genuinely move more, making countless micro-movements throughout the day to offset any surplus fat accumulation.

Imagine placing one of these restless individuals in a sealed metabolic chamber, roughly the size of a room, for 24 hours. This chamber can calculate a person’s energy expenditure, revealing how many calories they burn. Inside, there’s not much to do. One can sit, lie down, type, or take a restroom break.

While this might not be a natural setting for many (given the lack of movement, activities, or workouts), it’s an excellent way to determine how much energy someone can expend while essentially being stationary. It also mirrors the lifestyle of those who claim to “sit at a computer 24/7 munching on chips and burgers.” And guess what? Such a study exists!

The results? Hold onto your hats: the difference in energy expenditure between individuals, adjusted for body size, ranged from 100 to 800 kcal per day! The researchers attributed this variance to “fidgeting.”

So, our slender friends aren’t just lucky; they’re fidgety. And they might just be fidgeting a lot more than the rest of us. It’s a subtle reminder that sometimes, it’s the small, almost imperceptible actions that can make a significant difference.

What’s the Real Deal?

After diving deep into the world of metabolism, food consumption, and body movement, what’s the ultimate takeaway? It turns out that slim individuals tend to burn more energy during digestion and often exhibit what’s known as a “fidgety phenotype.” This means they’re naturally inclined to move around more, not just with purposeful actions but with spontaneous, restless movements. It seems that these individuals eat just the right amount for their bodies. And if they do consume more than expected? Well, they simply move more. There’s no magic or secret formula here.

Sure, some people might have hit the genetic jackpot, effortlessly maintaining their weight without conscious effort. But transforming into one of these naturally slim individuals doesn’t require genetic engineering or a PhD in rocket science.

The good news? You too can harness these qualities. The key factors that influence weight management, such as the thermic effect of food (TEF) and testosterone levels, can be optimized. As you shed body fat, both TEF and testosterone levels increase. Moreover, regular strength training can further boost testosterone. The efficiency of your movements, metabolism, and heightened energy expenditure can all be enhanced with consistent workouts.

So, instead of envying the effortlessly slim, it’s time to take action. Embrace the journey, harness the knowledge, and work towards your own version of a healthy, balanced body. After all, it’s not about luck; it’s about understanding and leveraging the body’s natural mechanisms.


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