While being overweight or obese can lead to many medical complications, your health can be in danger if you are severely underweight.
Furthermore, many people who are underweight find themselves struggling to gain muscle. It can be frustrating at times to remain skinny despite efforts to achieve your ideal body weight.
This article will outline key strategies you can safely implement to help you gain weight.
What does it mean to be Underweight?
Many of us are familiar with the terms overweight and obesity which are commonly calculated using the body mass index (bmi). You can use calculators to determine your bmi  based on your height and weight.
Those that are overweight (bmi>25) and obese (bmi>30) should be counseled on diet and exercise due to the increased risk of cardiovascular disease and diabetes mellitus type 2 .
Typically, a normal bmi is in the range of 18.5-24.9. However, if you have a bmi less than 18.5 you are considered underweight . Girls are typically 2-3 times more likely to be underweight than men.
Keep in mind that the bmi only assess your height and weight, and is one number in the scheme of your health. Thus, you can still be healthy with a bmi that is labeled “underweight” or “overweight” .
Athletes are lean and strong but are often labeled as “overweight” or “obese” based on their bmi, but this is because they have so much muscle.
Similarly, if you are very tall and skinny, with lower muscle mass, you may have a lower bmi indicating you are underweight.
What are common causes of being underweight?
There are various reasons one may be underweight which may be due to genetics, fast metabolism, lack of food, or chronic illness such as hyperthyroidism, diabetes mellitus type 1, cancer, infections, anorexia, malabsorption and inflammatory bowel disease .
Anorexia nervosa is a psychological disorder related to intense fear of food that leads to restriction of food intake that leads to a low body weight, given the patient’s age, sex, developmental trajectory, and physical health .
Anorexia nervosa is more common in adolescent and young females with an estimated lifetime prevalence that is three times greater in females than males (0.9% versus 0.3%) . This can lead to severe medical complications that include arrested growth, hypothermia, bradycardia (slow heart rate), arrhythmias (abnormalities in the heart electrical conduction), low blood pressure, amenorrhea (missed periods), infertility, osteoporosis, heart valve abnormalities, gastroparesis, constipation and electrolyte issues, anemia, hair loss, and vitamin deficiencies .
Other psychiatric causes may include depression, body dysmorphia, and the manic phase of bipolar disorder .
Thyroid issues such as hyperthyroidism is when your thyroid is very active and leads to increased metabolism.
Usually these patient present with underweight, or rapid weight loss despite increased appetite, along with complaints of difficulty concentrating, fatigue, frequent bowel movements, visibly enlarged thyroid gland, hand tremors, heat intolerance [9–10].
Usually you will be followed by an endocrinologist who will order further testing to find the cause of your hyperthyroidism so you can manage your symptoms. Other endocrine issues can be seen with diabetes mellitus type 1, adrenal insufficiency, and pheochromocytoma .
Malabsorption disease, such as celiac disease, is usually characterized by pale, greasy, voluminous, foul-smelling stools and weight loss despite adequate food intake .
This can result in poor intake of calories from fat, protein or carbohydrates that leads to weight loss. Furthermore, these patients often will be deficient in vitamins and minerals such as folic acid, B12, and iron which can lead to anemia .
It is highly recommended you see a doctor if you have these issues because you may require testing such as a biopsy of your intestinal tissue to diagnose any malabsorption issues.
If you do indeed have celiac disease you will need to be on a gluten-free diet the rest of their life . Other causes may include peptic ulcer disease, diseases that cause malabsorption and inflammatory bowel disease (ibd).
Cancer can also lead to weight loss since the tumors themselves require large amount of caloric intake to contribute to their growth. Anorexia and weight loss are present in 15-40% of all cancer patients at diagnosis .
Malignancies (particularly gastrointestinal, lung, lymphoma, renal, and prostate cancers) often cause weight loss and patients may present pain, abdominal distention, nausea, vomiting, difficulty swallowing, early satiety due to liver and spleen enlargement or obstruction, or symptoms of malabsorption .
Infectious disease such as hiv/aids , tuberculosis , and hepatitis C  are also linked with weight loss. Although rare in more developed countries like the United States, one can be underweight from parasitic infections.
Some chronic conditions can lead to being underweight such as chronic heart failure, chronic lung disease (pulmonary cachexia syndrome), and advanced kidney disease .
Neurological conditions such as stroke, dementia, Parkinson disease, and amyotrophic lateral sclerosis (als) can lead to being underweight due to difficulty swallowing and altered mental status . Medications and illicit drug use such as alcohol, tobacco, cocaine, amphetamines, and withdrawal from chronic marijuana use .
The bottom line is that while there are many causes of being underweight, you should see a doctor as you may need a workup to find the underlying cause of why you are underweight.
It may be something easily manageable like diabetes mellitus, or something life threatening like cancer or a tuberculosis or hivinfection.
How does being underweight affect your health?
As discussed in detail in the previous section, there are various reasons one can be underweight and this can lead to various medical complications that can affect your health in a variety of specific way.
If you have poor food intake this can lead to arrested growth in height, electrolyte abnormalities, and irregularities of the heart rhythm and heart valve issues.
On the other hand if you have hiv this can lead to weight loss and susceptibility to a variety of infections which lead to multiple hospitalizations and early death.
Thus, one needs to keep in mind that you can have various symptoms occur at once. Other symptoms may include low blood pressure leading to fainting, slow digestive tract leading to constipation, anemia, hair loss, and vitamin deficiencies .
New research suggests that being underweight puts people at higher risk of dying. Even more surprisingly was that even when factors such as smoking, alcohol use or lung disease are considered, or adults with a chronic or terminal illness are excluded, the study found there was a connection between being underweight and an increased risk of dying earlier for both adults and even fetuses. [24–25].
The bottom line is that being underweight can have long term ramifications on shorter life expectancy . You also have impaired immune systems which increases the risk of infections [27, 28], osteoporosis  and fertility problems.
What’s the healthiest way to gain weight?
Gaining weight requires you to eat more calories than you are burning. However, you cannot just binge on pizza and ice cream. That is the wrong way! The safest and healthiest way to gain weight is to gain muscle mass not fat. This means you need to eat healthy foods and live a healthy lifestyle with exercise, and avoid drinking and smoking. The easiest way to figure out how to eat more calories than you burn is to use a calorie calculator to find your daily target, then work out a plan to make eating that many calories each day a habit.
Eat More Calories than you Burn
Weight gain is simple: You must eat more calories than you burn. You must consume more calories than recommended to maintain your normal weight.
The nih Body Weight Planner is great tool for understanding your current caloric intake and exercise and see what adjustments you need to make to achieve a goal weight in a given amount of time.
Uniquely, you can follow your bodyweight overtime so you can make adjustments after this week. Use this calculator to find your baseline current caloric intake you need to maintain your weight. Then add 500-750 calories on top of what you currently eat.
So if you are a fairly active person that eats 2,500 calories and weigh 150lbs but find your bmito be only 17, it is time to make a change.
You should strive to eat 3,000 – 3,250 calories a day to gain weight given your current activity level. Keep in mind that if you are more physically active that this will also contribute to you burning calories.
Eat More Carbohydrates and Fats
Of course, you do not want to overeat too much as this can lead to metabolic syndrome and severe obesity .
However, overall, aim to eat anywhere from 3-5 meals a day with an appropriate amount of carbohydrates and fats, that is rich in protein.
It is recommended by the Institute of Medicine recommends adults consume a minimum of 130 grams of carbohydrates each day, and 45 to 65 percent of their daily energy intake from carbohydrates .
So if you consume 2,500 calories you should consume 280 to 400 grams of carbohydrates per day, and 400 to 575 grams of carbs each day when eating a 3,500-calorie, weight-gain meal plan.
If you have trouble gaining weight, or are involved in heavy weight training, the Academy of Nutrition and Dietetics recommends you need at least 2.5 to 3.5 grams of carbohydrates per pound of body weight each day .
Yes, you can eat more fats. Again, this does not mean you should binge on cheese cake or butter. You never want to overdue the saturated fats due to risk to your cardiovascular health .
However, you should not be afraid of fats. Obviously you want to opt to eat foods with more calories .
Pick healthy fats like almonds, fish, and peanut butter . Add extra slices of cheese or drink whole milk.
You should strive for energy rich foods to increase your caloric intake. You will find that your exercise will be more efficient.
Make sure to eat at least 3 meals per day
If you find yourself full, try to eat smaller portions more frequently. Try to eat for breakfast egg whites with bread and a protein shake.
Then have a snack of fruits with nuts and yogurt. For lunch, instead of eating a salad, opt to pick something packed with protein and carbohydrates such as chicken pasta or a hamburger.
Then for lunch, be sure to eat some brown rice with chicken and vegetables. And you can eat before bed some yogurt and fruit or protein shake.
Remember, as long as you are eating lean proteins with appropriate complex carbohydrates and fat, you should gain weight without the complication of gaining too much fat.
Eat more protein
Protein is the most important nutrient you should be eating to help you gain muscle and mass. In order to gain weight, you should be eating 1-1.5g of protein per pound of bodyweight .
You need to keep in mind however that at this time protein can suppress the appetite. Protein rich sources include chicken, meat, fish, eggs, dairy, legumes, nuts and whey proteins .
Protein leads to muscle building which can help you build mass and strength . Every meal you should eat should contain protein to help you gain mass and muscle.
The more physically active you are you will require greater protein intake to repair your muscles.
Remember To Do Strength Training
There have been plenty of articles about how strength training can lead to muscle gains . You should aim to go to do weight lifting 3-4 times a week.
If you are unfamiliar with how to weight train you should consider hiring a personal trainer or going with a friend who can teach you. It is probably best for you to focus on total body lifts that incorporate large muscle groups so you can build more muscle mass.
Some of the best exercises for your legs include squats, leg press, leg extensions and leg curls. In order to build your upper body try doing bench press, deadlifts, pull-ups, rows, shoulder press, bicep curls and triceps pulldowns .
You want to limit your cardiovascular activity because this will burn calories making it difficult for you to gain mass.
If you must do cardio, and can’t live without it, try to shorten your cardio sessions. Also increase your caloric intake on days you do cardio so that you avoid burning too many calories.
You can do this!
While being underweight can lead to medical complications including shorter life expectancy, keep in mind that you can still gain weight.
Always speak with your physician to first find the underlying cause for your underweight situation.
Once the cause is found you may need both a nutritional and medical approach to safely and healthily gain weight.
Be conscious that you need to implement key strategies to gain weight such as eating more calories than you burn, and eating more meals rich in lean protein, complex carbohydrates and healthy fats.
Strive to strength train three to four times a week so that you are building lean muscle to add to your body frame.
It is advised that you discuss any weight gaining plans with your doctor first. Stay consistent and focused. With these strategies you can gain weight safely, and get leaner and stronger too!
Rajiv M Mallipudi, md, mhs is an internal medicine resident physician, personal trainer, athlete and author. He has over a decade of personal training experience and helped hundreds of clients of all levels achieve their weight loss and fitness goals. This inspired him to work as a clinical researcher at the nationally recognized Johns Hopkins Hospital Weight Management Center. During medical school he and his classmates created the health and wellness organization, medfit, which provided personal training and nutrition counseling to the medical student body. In his spare time, Dr. Mallipudi enjoys playing ice hockey, dancing, and training for his next bodybuilding and powerlifting competitions. Dr. Mallipudi serves as a contributing writer for the Diet and Fitness sections.
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