Being underweight, defined as a BMI below 18.5, can result from genetics, medical conditions, mental health factors, or simply not eating enough calories to match your body’s energy demands. For many people, the cause isn’t a single issue but a combination of factors working together. Understanding the most common reasons can help you figure out what applies to your situation and what steps to take next.
How Underweight Is Defined
The standard measure is body mass index, or BMI, which compares your weight to your height. The CDC classifies any adult with a BMI under 18.5 as underweight. For context, that’s roughly 115 pounds or less for someone who’s 5’6″. BMI isn’t perfect. It doesn’t distinguish between muscle and fat, and it doesn’t account for bone density or body frame. But it’s a useful starting point, especially if your weight has been dropping without explanation.
Your Genetics May Set a Lower Baseline
Some people are naturally lean because of the genes they inherited. Variants of a gene called FTO, one of the most studied genes linked to body composition, illustrate this clearly. People who carry certain versions of this gene tend to have lower body weight, lower BMI, less total body fat, and smaller waist and hip measurements compared to carriers of other variants. These differences hold up even when diet is accounted for.
If your parents and siblings have always been thin, and you’ve been slim your whole life without any health problems, genetics is a likely explanation. A naturally fast metabolism, meaning your body burns more calories at rest, can also run in families. This doesn’t mean you can’t gain weight, but it does mean your body may resist it more than average.
An Overactive Thyroid
Your thyroid gland controls how fast your body burns energy. When it produces too much hormone, a condition called hyperthyroidism, your resting energy expenditure shoots up. Resting energy expenditure accounts for roughly 60% of the total calories you burn in a day, so even a moderate increase can create a significant calorie deficit without any change in how much you eat.
The excess thyroid hormone ramps up fat metabolism and triggers your sympathetic nervous system, the same system responsible for your fight-or-flight response. The result is that your body burns through its energy stores faster than you can replace them. Other signs of an overactive thyroid include a rapid heartbeat, feeling hot all the time, anxiety, trembling hands, and difficulty sleeping. A simple blood test can check your thyroid levels.
Digestive Problems That Block Nutrient Absorption
You might be eating enough food and still be underweight if your body can’t properly absorb what you’re consuming. Celiac disease is one of the most common examples. When someone with celiac disease eats gluten (a protein in wheat, barley, and rye), their immune system attacks the lining of the small intestine, destroying the tiny finger-like projections called villi that are responsible for absorbing nutrients. Once those villi are damaged, your body can’t pull in enough vitamins, minerals, fats, and proteins from food, no matter how much you eat. The result is malnutrition and weight loss.
Celiac disease affects an estimated 1 in 100 people worldwide, and many go undiagnosed for years. Symptoms beyond weight loss include bloating, diarrhea, fatigue, and sometimes an itchy skin rash. Other digestive conditions that interfere with absorption include Crohn’s disease, chronic pancreatitis, and certain parasitic infections. If you’re eating well but still losing weight, or if you have persistent digestive symptoms, a malabsorption issue is worth investigating.
Undiagnosed Diabetes
Type 1 diabetes, which often appears in childhood or young adulthood, can cause rapid and dramatic weight loss before it’s diagnosed. Without enough insulin, your cells can’t use glucose from food for energy. Your body enters a catabolic state, breaking down its own fat stores and muscle tissue to keep functioning. This breakdown of fatty acids also produces ketones, acidic byproducts that accumulate in the blood and can become dangerous.
The hallmark signs are extreme thirst, frequent urination, constant hunger despite eating, and unexplained weight loss. Type 2 diabetes can also cause weight loss in some cases, though it’s more commonly associated with weight gain. If you’ve lost weight quickly and notice any of these symptoms, a blood sugar test can provide answers fast.
Stress, Anxiety, and Depression
Mental health has a direct, physical effect on appetite. During acute stress, your body releases adrenaline and noradrenaline as part of the fight-or-flight response. Noradrenaline actively suppresses appetite, which is why you may have zero interest in food during a crisis or an intensely stressful week.
Chronic stress works through a different pathway but can have the same outcome. Prolonged stress triggers a cascade of hormones, including one called CRH, which is a potent appetite suppressant. It directly influences how much food you want to eat and how your body manages its energy balance. Depression can dull your interest in food entirely, making meals feel like a chore. Anxiety can cause nausea or stomach discomfort that makes eating unpleasant. Over weeks and months, the calorie deficit from simply not eating enough adds up, and weight drops steadily.
This is one of the most underrecognized causes of being underweight. People often look for a physical explanation first, but if you’ve been going through a difficult period and your appetite has vanished, the connection is probably not a coincidence.
Not Eating Enough Calories
Sometimes the explanation is straightforward: you’re not taking in enough energy to maintain your weight. This can happen for reasons that don’t feel like a problem on a daily basis. A busy schedule that leads to skipped meals, a naturally small appetite, a restrictive diet, or simply not realizing how many calories your body needs can all leave you in a chronic deficit.
Gaining one pound of body weight requires roughly 3,500 extra calories over the course of a week, or about 500 extra calories per day. If you’re trying to gain lean muscle rather than just fat, the surplus needed is somewhat less, around 2,000 to 2,500 extra calories per week, but it still needs to be paired with resistance training. For someone who already struggles to eat enough, that kind of surplus can feel daunting. Calorie-dense foods like nuts, nut butters, avocados, olive oil, whole milk, cheese, and dried fruit can help you increase your intake without dramatically increasing the volume of food on your plate.
Medications and Substance Use
Certain medications suppress appetite or speed up metabolism as a side effect. Stimulant medications used for ADHD are well known for this. Some antidepressants, particularly when first started, can reduce appetite. Chemotherapy drugs, certain antibiotics, and medications for asthma can also contribute. If your weight loss started around the same time you began a new medication, that timing is worth noting.
Excessive caffeine, nicotine, and recreational stimulants all suppress hunger as well. Alcohol can replace meals for some people, providing calories but none of the nutrients your body needs to maintain healthy tissue.
Health Risks of Staying Underweight
Being underweight isn’t just a cosmetic concern. It carries real health consequences. Your bones need adequate nutrition and body weight to maintain their density. Prolonged low weight increases the risk of osteoporosis and fractures, particularly as you age. Your immune system also depends on proper nutrition to function. People who are chronically underweight tend to get sick more often and recover more slowly.
Other risks include hair loss, chronic fatigue, fertility problems in both men and women, and poor wound healing. In severe cases, the heart muscle can weaken. If you’ve lost more than 5% of your body weight over 6 to 12 months without trying, that’s a widely used clinical threshold for concern, even if you don’t feel sick. For older adults or anyone with existing health conditions, even a smaller drop in weight can be significant.
Figuring Out Your Specific Cause
Start by asking yourself a few questions. Has your weight always been low, or is this a recent change? If you’ve been thin your entire life, eat regularly, and feel healthy, genetics and a naturally fast metabolism are the most likely explanations. If your weight has dropped recently, think about what else has changed: new stress, new medications, digestive symptoms, mood changes, or increased thirst and urination.
A basic medical workup for unexplained low weight typically involves blood tests to check thyroid function, blood sugar, inflammatory markers, and nutrient levels. If digestive symptoms are present, testing for celiac disease or other absorption disorders may be included. For many people, the cause turns out to be a combination of a naturally smaller appetite, a stressful period, and not quite eating enough, all of which are fixable once you recognize the pattern.