About two thirds of people in the US are either overweight or obese (1).

However, there are also many people with the opposite problem of being too skinny (2).

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.

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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 calculator 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 (2).

Summary Being underweight is defined as having a body mass index (BMI) below 18.5. It’s much more common in women and girls.

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 (3).

In comparison, obesity was associated with a 50% greater risk of early death, indicating that being underweight may be even worse for your health (3).

Another study found an increased risk of early death in underweight men, but not women, suggesting that being underweight may be worse for men (4).

Being underweight can also impair your immune function, raise your risk of infections, lead to osteoporosis and fractures and cause fertility problems (5, 6, 7).

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 (8, 9).

Summary Being underweight can be just as unhealthy as being obese — if not more. People who are underweight are at risk of osteoporosis, infections, fertility problems and early death.

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 (10).
  • 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.

Summary 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.

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 (11).

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.

Summary It’s very important to eat mostly healthy foods even when you’re trying to gain weight.

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.

Summary 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.

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 (12).

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 (13, 14).

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.

Summary Protein forms the building blocks of your muscles. Eating sufficient protein is required to gain muscle weight instead of just fat.

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.

Summary To gain weight, eat at least three meals per day and make sure to include plenty of fat, carbs and protein.

Again, it’s very important to eat mostly whole, single-ingredient foods.

The problem is that these foods tend to be more filling than processed junk foods, making it harder to get in enough calories.

Using plenty of spices, sauces and condiments can help with this. The tastier your food is, the easier it is to eat a lot of it.

Also, try to emphasize energy-dense foods as much as possible. These are foods that contain many calories relative to their weight.

Here are some energy-dense foods that are perfect for gaining weight:

  • Nuts: Almonds, walnuts, macadamia nuts, peanuts, etc.
  • Dried fruit: Raisins, dates, prunes and others.
  • High-fat dairy: Whole milk, full-fat yogurt, cheese, cream.
  • Fats and oils: Extra virgin olive oil and avocado oil.
  • Grains: Whole grains like oats and brown rice.
  • Meat: Chicken, beef, pork, lamb, etc. Choose fattier cuts.
  • Tubers: Potatoes, sweet potatoes and yams.
  • Dark chocolate, avocados, peanut butter, coconut milk, granola, trail mixes.

Many of these foods are very filling, and sometimes you may need to force yourself to keep eating even if you feel full.

It may be a good idea to avoid eating a ton of vegetables if gaining weight is a priority for you. It simply leaves less room for energy-dense foods.

Eating whole fruit is fine, but try to emphasize fruit that doesn't require too much chewing, such as bananas.

If you need more suggestions, consider reading this article on 18 healthy foods to gain weight fast.

Summary You can add sauces, spices and condiments to your foods to make it easier to eat more. Base your diet on energy-dense foods as much as possible.

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

Go to a gym and lift 2–4 times per week. Lift heavy and try to increase the weights and volume over time.

If you’re completely out of shape or new to training, consider hiring a qualified personal trainer to help you get started.

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

It’s probably best to take it easy on the cardio for now — focus mostly on the weights.

Doing some cardio is fine to improve fitness and well-being, but don't do so much that you end up burning all the additional calories you’re eating.

Summary It’s very important to lift heavy weights and improve your strength. This will help you gain muscle mass instead of just fat.

Combining a high calorie intake with heavy strength training are the two most important factors.

That being said, there are several other strategies to gain weight even faster.

Here are 10 more tips to gain weight:

  1. Don't drink water before meals. This can fill your stomach and make it harder to get in enough calories.
  2. Eat more often. Squeeze in an additional meal or snack whenever you can, such as before bed.
  3. Drink milk. Drinking whole milk to quench thirst is a simple way to get in more high-quality protein and calories.
  4. Try weight gainer shakes. If you’re really struggling then you can try weight gainer shakes. These are very high in protein, carbs and calories.
  5. Use bigger plates. Definitely use large plates if you’re trying to get in more calories, as smaller plates cause people to automatically eat less.
  6. Add cream to your coffee. This is a simple way to add in more calories.
  7. Take creatine. The muscle building supplement creatine monohydrate can help you gain a few pounds in muscle weight.
  8. Get quality sleep. Sleeping properly is very important for muscle growth.
  9. Eat your protein first and vegetables last. If you have a mix of foods on your plate, eat the calorie-dense and protein-rich foods first. Eat the vegetables last.
  10. Don't smoke. Smokers tend to weigh less than non-smokers, and quitting smoking often leads to weight gain.
Summary There are several other things you can do to gain weight even faster. These include drinking milk, using weight gainer shakes, adding cream to your coffee and eating more often.

It can be very difficult for some people to gain weight.

That's because your body has a certain setpoint of weight where it feels comfortable.

Whether you try to go under your setpoint (lose weight) or over it (gain weight), your body resists changes by regulating your hunger levels and metabolic rate.

When you eat more calories and gain weight, you can expect your body to respond by reducing your appetite and boosting your metabolism.

This is largely mediated by your brain, as well as weight-regulating hormones like leptin.

So you should expect a certain level of difficulty. In some cases, you may need to force yourself to eat despite feeling stuffed.

At the end of the day, changing your weight is a marathon, not a sprint. It can take a long time, and you need to be consistent if you want to succeed in the long run.