It's Not Just a "Good Problem to Have"
Every summer, people stress about their bodies — but it's not always about losing weight. While many struggle with excess fat, others deal with the opposite problem: they simply can't gain weight no matter how much they eat. Being underweight comes with its own set of frustrations, yet people rarely take it seriously. Comments like "I wish I had your problem" or "just eat more" are easy to brush off, but for people who genuinely struggle to put on weight, those remarks sting.
Two Core Reasons Skinny People Stay Skinny
Why is gaining weight so difficult for some people? Broadly speaking, there are two main causes of being chronically underweight:
- Low digestive efficiency and nutrient absorption
- Poor eating habits
1. Digestive Efficiency and Nutrient Absorption
To maintain a stable body weight, the energy you take in must balance the energy you expend. According to the first law of thermodynamics, energy cannot be created or destroyed — it can only change form. When you consume more calories than your body burns, the surplus gets stored as glycogen and body fat. This was an efficient survival mechanism for our ancestors during times of scarcity.
That said, the human body is far too complex to reduce to a simple calories-in, calories-out equation. Even people with identical metabolic rates who eat the same number of calories can experience very different rates of weight gain. A study involving 12 pairs of identical twins demonstrated this clearly: when participants consumed 1,000 calories above their maintenance level, everyone gained weight — but the rate of gain varied dramatically. Some pairs gained at similar rates, while others showed nearly a threefold difference in weight gain.
The Role of Gut Microbiome
A major reason people absorb nutrients differently comes down to the gut microbiome. The human gut houses over 100 trillion microorganisms, and one of their key functions is helping digest food that the body wouldn't be able to break down on its own.
A classic example is lactose intolerance. People who lack the enzyme lactase cannot properly digest the lactose in milk, leading to digestive distress. But people with a healthy, well-balanced gut microbiome can break down lactose without any trouble. Interestingly, even lactose-intolerant individuals can usually digest yogurt because the fermentation process has already broken the lactose down into lactic acid and simpler sugars — in other words, yogurt is essentially pre-digested milk.
Research increasingly shows that the composition of gut bacteria directly influences how efficiently the body absorbs energy from food. Because everyone's microbiome is different, two people eating the exact same meal can end up with very different amounts of usable energy — which means very different weight outcomes. For people in this category, paying close attention to what they eat and how they eat it matters enormously.
2. Poor Eating Habits
Most underweight people genuinely believe they eat a lot. But when you actually log and calculate total daily calorie intake, the numbers usually tell a different story — one where calories consumed don't exceed the body's total daily energy expenditure.
Here's where many people make a critical mistake: they calculate their calorie needs using only their basal metabolic rate (BMR). But BMR only accounts for the energy your body uses at complete rest — just to keep your heart beating and lungs breathing. To find your true maintenance calories, you need to add your total daily activity level (TDEE) on top of BMR. And to actually gain weight, you need to eat above that maintenance number.
As a general benchmark, building 1 kg of muscle requires a surplus of roughly 7,000 to 8,000 calories over time, in addition to consistent strength training. For someone targeting 1 kg of muscle gain per month, that works out to an extra 200–300 calories per day above their TDEE. That total — TDEE plus the surplus — is what skinny people actually need to hit every single day to make progress.
Many underweight people train hard in the gym but struggle with food intake. Without hitting that caloric target consistently, even an intense training program essentially functions as a very efficient diet — not a bulk.
The Truth About Weight Gainer Supplements
The supplement industry markets weight gainers as a solution for people trying to gain mass. In reality, gainers were originally developed for a completely different purpose: to help athletes engaged in intense training replenish carbohydrates quickly, support performance, and speed up recovery. High-intensity exercise depletes muscle glycogen and often suppresses appetite, making it difficult to eat enough real food post-workout. Gainers were designed to fill that gap efficiently.
A common ingredient in weight gainers is maltodextrin — a fast-digesting carbohydrate. Research shows maltodextrin can improve endurance performance in cyclists and help maintain strength output during resistance training. These benefits are real — but they apply specifically when gainers are consumed the way they were intended: around workouts, when insulin sensitivity is elevated and the body can efficiently shuttle carbohydrates into muscle glycogen rather than storing them as fat.
The problem arises when people use gainers two or three times a day for the sole purpose of gaining weight. Fast-digesting carbohydrates spike blood glucose rapidly. The body stores excess carbohydrates as glycogen first, but glycogen storage capacity is limited. What's left gets converted to fat — and because rapidly absorbed calories don't get distributed evenly throughout the body, that fat tends to accumulate around the abdomen. This effect is compounded in men, whose hormonal profile already directs fatty acid storage predominantly toward visceral fat.
It's also worth noting that people who don't exercise regularly or who have low muscle mass tend to have reduced insulin sensitivity, making them more susceptible to metabolic issues. Additionally, East Asians tend to have smaller pancreases and lower insulin secretion capacity compared to people of European descent, which increases the risk of blood sugar dysregulation.
The Right Way to Bulk Up
Healthy weight gain should be driven primarily by muscle growth — not just an increase in overall body mass. That requires two things: a consistent caloric surplus and adequate protein intake.
Beginners often hear that protein supplements aren't necessary early on. This is misleading. Beginners may actually need as much — or more — protein than experienced lifters, because early-stage muscle growth is rapid. This is commonly known as "newbie gains": in the first few months of training, beginners can build muscle at a pace that would take a seasoned lifter a year to match. Providing the body with enough protein during this window is critical to maximizing that growth.
As training experience increases, the body becomes more efficient at protein metabolism, which means protein needs may slightly decrease over time. But getting enough protein consistently is difficult without conscious effort — it's not like carbohydrates, which are abundant in almost every food. From a practical and economic standpoint, there's little reason to use a weight gainer to hit a caloric surplus. A more sensible approach is using a quality whey protein supplement paired with a couple of bananas — cheaper, more nutritious, and far less likely to cause unwanted fat accumulation.
Key Takeaways
- Being underweight is a real concern, not something to dismiss or envy.
- The two primary causes are low nutrient absorption (often related to gut microbiome composition) and chronically insufficient calorie intake.
- To gain weight effectively, calculate your TDEE — not just your BMR — and add a daily surplus of 200–300 calories.
- Prioritize protein intake, especially as a beginner, to maximize muscle protein synthesis during the newbie gains phase.
- Weight gainers are not designed for weight gain as a primary goal. If you use one, limit it to once per day around your workout and consider reducing the serving size.
- For a caloric surplus, a protein shake plus a banana or two is more practical, economical, and metabolically sound than relying on a high-carb gainer.
