Why You Are Skinny (8 reasons and solutions by an ex-skinny guy)

Why you are skinny.

I used to be the typical ectomorphic scrawny kid on the block who found it impossible to gain weight. Below, I explain why skinny guys usually stay underweight, and solutions you can apply to solve the problem.

Poor nutrition is the primary reason why people stay skinny. Calories and protein are essential for gaining weight and putting on muscle. Therefore insufficient intake in either can cause a person to be underweight.

I was previously very skinny but gained weight by eating more food and lifting heavy weights.
I was an ex-skinny guy who gained weight by increasing calorie/protein intake and lifting heavy weights!

8 Reasons Why You Are Skinny And What To Do

1) Living A Physically Demanding Lifestyle

Doing long hours in a physically-intensive job and undereating can cause you to stay underweight.

For me, I was at my skinniest doing long hours in a restaurant working 9-hour shifts standing, walking, running, and carrying all day. Not only was this burning a lot of calories, but I also had very little time to eat nutritious meals. Sometimes I wouldn’t even eat all day until dinner.

I was putting myself in a calorie deficit without even realizing it. As a result, I was unable to maintain (let alone gain) weight.

Solution: It’s unrealistic to stop your job. But consider going on a bulking diet. Make an active effort to increase food intake. Calorie-dense foods like olive oil, fruits, and confectionary are easy to consume and are packed with energy.

2) Leading A Sedentary Lifestyle

The opposite of the above is also true- leading an inactive lifestyle can cause your muscles to atrophy (waste away).

Muscle mass grows as a result of exercise and good nutrition. So if you’re eating badly and sitting on the couch all day, you can make a good bet that this is a reason why you are so skinny.

Solution: combine a bulking diet with a regular training routine (see my other post for the best exercises for skinny guys to build muscle). Lifting heavy weights is one of the best things a skinny person can start doing to gain quality lean muscle weight.

Alternatively, you can check out my full guide on how skinny people put on muscle at home.

3) Insufficient Daily Protein Intake

This is a common reason why people fail to put on muscle weight even though they work out.

Protein serves as the building block for muscle, making it essential for building and maintaining lean mass.

So if you’re a skinny person who doesn’t eat enough high-protein foods, then you can be sure to stay underweight.

Speaking from experience, it’s so easy to overestimate how much protein you’re actually taking in daily.

Solution: start including more protein-rich foods in your diet. Good sources include red meat, poultry, whey powders, fish, soy, and legumes. Track your intake using an app or spreadsheet. Prep high-protein foods at home to take to work

If you’re interested in an example of what I eat to build muscle, you can check out my 140g protein meal prep here.

4) Not Eating Enough Calories

This is another common reason why people can train hard and still not gain noticeable muscle mass. It’s also the most common reason why people fail to gain weight.

Calories are used by your body to fuel the muscle repair process which is essential for growth (also called muscular hypertrophy).

It’s so easy to overestimate calorie intake, especially as a skinny guy with a fast metabolism and a low appetite.

Solution: go on a bulk. Meal prep high-calorie foods to take to the office or school. Include calorie-rich snacks like protein shakes, confectionery, and fruit in your diet. Add a tablespoon of olive oil to your shakes. This sounds weird, but trust me, you can’t taste it and it’s an easy way to take in an extra ~120 calories.

You can check out my other article to find out how long a typical bulk lasts.

5) Doing Too Much Cardio

Cardio is generally healthy when done as part of a well-structured training and nutrition plan. But doing too much cardio AND undereating can be a reason why people stay skinny.

Solution: increase your calorie intake to match the energy expended from cardio. Swapping cardio sessions for resistance training can also help you to put on lean muscle weight (if you’re eating properly).

You can check out my other article for a free bodyweight resistance training program you can do at home without weights.

6) Lack Of Fast-Twitch Muscle Fiber Activity

Fast-twitch fibers are responsible for quick explosive muscle contractions. They are the opposite of slow-twitch fibers which are responsible for low-intensity contractions over sustained periods.

A unique property of fast-twitch fibers is that they’re particularly responsive to growth.

This is why athletes that perform explosive movements (sprinters, powerlifters, weight lifters, etc) usually look jacked.

We all have different amounts of fast-twitch fibers in our muscles (this is genetic). But the good news is you can adopt specific workouts to target these fibers.

Solution: skinny people should adopt a heavy weight training program to recruit fast-twitch fibers. This promotes muscle growth and helps you to put on lean weight. Combine this with a caloric surplus and high-protein diet (i.e. a bulk).

You can check out my other post to find out how much weight a skinny person should lift to gain muscle.

7) Long Limbs Can Make You Look Skinny

People who have an ectomorphic type of body tend to have long arms and legs. This can give the visual impression of smaller muscle size for any given amount of mass.

Therefore two people could have the same amount of muscle, but the person with the longer limb will look skinnier.

Solution: this is based on genetics and partly out of your control. But you can improve your diet and combine this with intense weight training to increase muscle size.

8) Not Getting Enough Sleep

Sleep is essential for muscle repair and growth. Insufficient sleep can also make you lethargic and lower your appetite.

As a result, you’re less likely to train and eat well.

Consequently, you will find it hard to gain muscle and put on weight.

Solution: get your recommended 7-8 hours of sleep per night. If you’re like me and struggle to get a good night’s rest, you can try meditation apps and decrease screen time in the few hours before bed.

Most Common Reason Why People Are Skinny

Speaking from experience (and countless hours spent researching my own skinny to muscular transformation), the 2 most common reasons are:

  • Not eating enough. See point 4.
  • Insufficient protein intake. See point 3.

Other reasons like lack of sleep, leading a physically demanding lifestyle, etc, will exacerbate the problem. But the root issue is usually due to poor nutrition and burning more energy than you take in.

If you’re tired of being skinny like, you can go to guide to find out how I gained 10lbs of muscle fast as a skinny guy.


I’ve shared 8 common reasons why you may be skinny.

Being unable to overall body gain weight is usually related to eating less than your body needs to sustain your daily activities.

Similarly, being unable to gain muscle weight is usually due to insufficient calorie intake, low protein intake, and lack of resistance training.

You can apply the solutions described to stop being skinny!

You may also be interested in the downloadable Kalibre Blueprint PDF which details exactly how I gained 40lbs of lean muscle (it’s 100% free!). It details the exact exercises and nutrition (with printables) I used to go from skinny to ripped!


I'm Kal (B.S, M.S)- a health & fitness writer and owner of Kalibre Fitness. I love to nerd out on weight training and nutrition. My primary interests are in muscle hypertrophy mechanisms and strength development. You can connect with me in the "Contact Us" section below!

Recent Posts