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Being underweight is defined as having a body mass index (BMI) below 18.5 although know that BMI measurements on their own are not perfect. In the US, 1% of men and 2.4% of women 20 years and older are underweight.

It can actually be just as bad for your health as being obese. Studies show it can raise your risk of infections, lead to osteoporosis, lead to age-related muscle wasting (sarcopenia) and more.

According to one study, being underweight was associated with a 140% greater risk of early death in men, and 100% in women. These were actually higher than the risks for obesity.

There are also many people who are not clinically underweight but still want to gain some muscle and mass. Now for either scenario, the main principles are the same.

Eat More Calories Than Your Body Burns

The most important thing to gain weight is to eat more calories than your body needs. Without doing that, you simply won’t gain any.

The quickest way to determine your calorie needs is using an online calorie calculator which I'll link up in the video description.

Aim for 300-500 calories per day above your maintenance level, or 700-1000 calories if you want to gain weight fast. Keep in mind that calorie calculators only provide estimates.

Binging on soda and donuts may help you gain weight, but that's not the right kind. It's very important to eat mostly healthy foods so that you build muscle and still nurture metabolic health.

Eat Lots of Protein

The single most important nutrient for gaining healthy weight is protein. After all, muscle is made of protein.

For weight gain aim for 0.7-1 gram of protein per pound of body weight (1.5 – 2.2 grams of protein per kilogram). You can even go above that if your calorie intake is very high.

Healthy high-protein foods include meats, fish, eggs, many dairy products, legumes and nuts. Protein supplements like whey protein can also be useful if you struggle to eat large amounts.

Although protein is king, you'll also need to eat a lot of carbs and fat to exceed your calorie requirements each day. So following a low-fat or low-carb diet won't be helpful.

Lift Weights and Improve Your Strength

In order to make sure that the excess calories go to your muscles instead of just your fat cells, it's absolutely crucial to lift weights.

The gym is the perfect environment for this, and you should aim to train 2-4 times per week. If you are new to it all, consider hiring a qualified personal trainer at the beginning.

You may also want to consult with a doctor if you have skeletal problems or any sort of medical issue.

At the end of the day, changing your weight is a marathon, not a sprint. That goes for both losing and gaining. And while it does take a long time, if you are consistent then you will find a lot of success in the long run.

Further reading: https://authoritynutrition.com/how-to-gain-weight/
Calorie calculator: https://authoritynutrition.com/how-many-calories-per-day/

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Study mentioned in video:

Study 1: http://www.ncbi.nlm.nih.gov/pubmed/18519281