About two thirds of people in the US are either overweight or obese (1Trusted Source).
However, there are also many people with the opposite problem of being too skinny (2Trusted Source).
This is a concern, as being underweight can be just as bad for your health as being obese.
Additionally, many people who are not clinically underweight still want to gain some muscle.
Whether you’re clinically underweight or simply struggling to gain muscle weight, the main principles are the same.
This article outlines a simple strategy to quickly gain weight — the healthy way.
What Does Underweight Really Mean?
Being underweight is defined as having a body mass index (BMI) below 18.5. This is estimated to be less than the body mass needed to sustain optimal health.
Conversely, over 25 is considered overweight and over 30 is considered obese.
Use this calculatorTrusted Source to see where you fit on the BMI scale (opens in a new tab).
However, keep in mind that there are many problems with the BMI scale, which only looks at weight and height. It does not take muscle mass into account.
Some people are naturally very skinny but still healthy. Being underweight according to this scale does not necessarily mean that you have a health problem.
Being underweight is about 2–3 times more common among girls and women, compared to men. In the US, 1% of men and 2.4% of women 20 years and older are underweight (2Trusted Source).
What Are the Health Consequences of Being Underweight?
Obesity is currently one of the world’s biggest health problems.
However, being underweight may be just as bad for your health. According to one study, being underweight was associated with a 140% greater risk of early death in men, and 100% in women (3Trusted Source).
In comparison, obesity was associated with a 50% greater risk of early death, indicating that being underweight may be even worse for your health (3Trusted Source).
Another study found an increased risk of early death in underweight men, but not women, suggesting that being underweight may be worse for men (4Trusted Source).
Being underweight can also impair your immune function, raise your risk of infections, lead to osteoporosis and fractures and cause fertility problems (5Trusted Source, 6Trusted Source, 7Trusted Source).
What’s more, people who are underweight are much more likely to get sarcopenia (age-related muscle wasting) and may be at greater risk of dementia (8Trusted Source, 9Trusted Source).
Several Things Can Cause Someone to Become Underweight
There are several medical conditions that can cause unhealthy weight loss, including:
Eating disorders: This includes anorexia nervosa, a serious mental disorder.
Thyroid problems: Having an overactive thyroid (hyperthyroidism) can boost metabolism and cause unhealthy weight loss.
Celiac disease: The most severe form of gluten intolerance. Most people with celiac disease don’t know that they have it (10Trusted Source).
Diabetes: Having uncontrolled diabetes (mainly type 1) can lead to severe weight loss.
Cancer: Cancerous tumors often burn large amounts of calories and can cause someone to lose a lot of weight.
Infections: Certain infections can cause someone to become severely underweight. This includes parasites, tuberculosis and HIV/AIDS.
If you’re underweight, you may want to see a doctor to rule out any serious medical conditions.
This is particularly important if you have recently started losing large amounts of weight without even trying.
There are several medical conditions that can cause unhealthy weight loss. If you’re underweight, see a doctor to rule out a serious health problem.
How to Gain Weight the Healthy Way
If you want to gain weight, it’s very important to do it right.
Binging on soda and donuts may help you gain weight, but it can destroy your health at the same time.
If you’re underweight, you want to gain a balanced amount of muscle mass and subcutaneous fat rather than a lot of unhealthy belly fat.
There are plenty of normal-weight people who get type 2 diabetes, heart disease and other health problems often associated with obesity (11Trusted Source).
Therefore, it’s absolutely essential to eat healthy foods and live an overall healthy lifestyle.
The next chapter looks at several effective ways to gain weight fast, without ruining your health at the same time.
It’s very important to eat mostly healthy foods even when you’re trying to gain weight.
Eat More Calories Than Your Body Burns
The most important thing you can do to gain weight is to create a calorie surplus, meaning you eat more calories than your body needs.
You can determine your calorie needs using this calorie calculator.
If you want to gain weight slowly and steadily, aim for 300–500 calories more than you burn each day according to the calculator.
If you want to gain weight fast, aim for around 700–1,000 calories above your maintenance level.
Keep in mind that calorie calculators only provide estimates. Your needs may vary by several hundred calories per day, give or take.
You don’t need to count calories for the rest of your life, but it helps to do it for the first few days or weeks to get a feel for how many calories you’re eating. There are many great tools out there to help you.
You need to eat more calories than your body burns to gain weight. Aim for 300–500 calories per day above your maintenance level for slow weight gain or 700–1,000 calories if you want to gain weight fast.
Eat Plenty of Protein
The single most important nutrient for gaining healthy weight is protein.
Muscle is made of protein and without it most of those extra calories may end up as body fat.
Studies show that during periods of overfeeding, a high-protein diet causes many of the extra calories to be turned into muscle (12Trusted Source).
However, keep in mind that protein is a double-edged sword. It’s also highly filling, which may reduce your hunger and appetite significantly, making it harder to get in enough calories (13Trusted Source, 14Trusted Source).
If you’re trying to gain weight, aim for 0.7–1 grams 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.
High-protein foods include meats, fish, eggs, many dairy products, legumes, nuts and others. Protein supplements like whey protein can also be useful if you struggle to get enough protein in your diet.
Protein forms the building blocks of your muscles. Eating sufficient protein is required to gain muscle weight instead of just fat.
Fill up on Plenty of Carbs and Fat and Eat at Least 3 Times per Day
Many people try restricting either carbs or fat when trying to lose weight.
This is a bad idea if your goal is to gain weight, as it will make it harder to get in enough calories.
Eat plenty of high-carb and high-fat foods if weight gain is a priority for you. It is best to eat plenty of protein, fat and carbs at each meal.
It is also a bad idea to do intermittent fasting. This is useful for weight loss and health improvement but can make it much harder to eat enough calories to gain weight.
Make sure to eat at least three meals per day and try to add in energy-dense snacks whenever possible.
To gain weight, eat at least three meals per day and make sure to include plenty of fat, carbs and protein.