If you’re underweight and looking to gain weight, it’s very important to do it right.
Consuming soda, donuts, and other junk foods may help you gain weight initially, but it can increase your risk of heart disease, diabetes, and cancer (1Trusted Source, 2Trusted Source, 3Trusted Source).
A healthier approach to gaining weight involves gaining a balanced amount of muscle mass and subcutaneous fat rather than a lot of unhealthy belly fat (4Trusted Source).
A recent animal study suggests that belly fat specifically may be correlated with increased cancer risk (5Trusted Source).
A high percentage of body fat also increases your chances of developing type 2 diabetes and other health problems, even if you are not overweight (6Trusted Source, 7Trusted Source).
To gain weight healthily, focus on eating more nutrient-dense foods and living an overall healthy lifestyle that involves exercising, getting enough sleep, and reducing stress, if you can.
This article outlines simple strategies to quickly gain weight — without increasing your risk of disease.
It’s important to eat mostly healthy foods even when you’re trying to gain weight.
Why is gaining weight important?
While about 74 percent of adults in the U.S. are either overweight or obese, there are also many people with the opposite problem of being underweight (8Trusted Source, 9Trusted Source).
Some individuals may be naturally underweight due to genetics or a fast metabolism. Being underweight may also be indicative of poor nutrition or underlying health conditions (9Trusted Source).
Additionally, many people who are not clinically underweight may still want to gain some muscle and put on weight for other reasons.
Whether you’re clinically underweight or simply struggling to gain muscle weight, the main principles are the same.
What does it mean to be ‘underweight’?
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 (10Trusted Source).
You can use this calculatorTrusted Source to see where you fit on the BMI scale.
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.
According to the Centers for Disease Control and Prevention (CDC), being underweight is about twice as common among women, compared with men. In a U.S. survey that grouped participants into men and women, 1.3% of men and 2% of women 20 years and older are underweight (9Trusted Source).
Being underweight is defined as having a body mass index (BMI) below 18.5. It’s more common in women.
Health risks linked to being underweight
Being underweight can be harmful to your health.
One study found that clinically underweight individuals have nearly two times the risk of death compared to obese individuals, suggesting that being underweight may be worse for your health than being obese (11).
Additionally, another study found that self-reported health outcomes fell significantly in underweight individuals after the age of 38, indicating that a low BMI later in life may come with increased health risks (12Trusted Source).
Being underweight can impair your immune function, raise your risk of infection, lead to osteoporosis and fractures and cause fertility problems (13, 14, 15Trusted Source, 16Trusted 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 (17, 18Trusted Source).
For children, it can also lead to growth and development problems (19Trusted Source).
Being underweight can negatively affect your health. People who are underweight are at risk of osteoporosis, infections, fertility problems, and early death.
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Causes of being underweight
There are several medical conditions that can cause unhealthy weight loss, including:
- Eating disorders: This includes anorexia nervosa, a serious mental health disorder (20Trusted Source).
- Thyroid problems: Having an overactive thyroid (hyperthyroidism) can boost metabolism and cause unhealthy weight loss (21Trusted Source).
- Celiac disease: The most severe form of gluten intolerance. Overall, the incidence of celiac disease has risen throughout the 20th and 21st centuries (22Trusted Source).
- Diabetes: Having uncontrolled diabetes (mainly type 1) can lead to severe weight loss (23Trusted Source).
- Cancer: Cancer can cause the body to be in a hypercatabolic state, which leads to increased energy being used. It can cause someone to lose a lot of weight but the severity and weight loss can depend on the type of cancer and the age of the person. (24Trusted Source, 25Trusted Source).
- Infections: Certain infections can cause someone to become severely underweight. This can include parasites, tuberculosis, and HIV infection (26Trusted Source, 27Trusted Source, 28Trusted Source).
If you’re underweight, you may want to make an appointment with a doctor to rule out any serious medical conditions.
This is particularly important if you have recently started losing large amounts of weight without trying.