What Does Underweight Mean?
Medically speaking, you're considered underweight if your BMI is below 18.5. But here's the thing - that's just a number on a chart. What matters more is understanding why your BMI is low et whether it's affecting your health.
Some people are naturally slim with a faster metabolism or smaller frame. They feel great, have energy, et their bloodwork comes back perfect. That's fine. But if you're losing weight unintentionally, feeling weak, or showing signs of malnutrition, that's a different story entirely.
Common Causes of Being Underweight
Let's talk about why someone might have a low BMI. It's rarely just "good genes" - there's usually something specific going on:
High Metabolism or Naturally Small Build
Some people genuinely burn calories faster than others. They can eat plenty et still stay thin. If you've been this way your whole life, feel healthy, et your doctor isn't concerned, you might just be one of these people. Lucky you, I guess?
Not Eating Enough
This seems obvious, but it's super common. Maybe you:
- Skip meals because you're busy
- Don't feel hungry due to stress or anxiety
- Have a poor appetite from medications
- Struggle financially to afford enough food
- Live alone et don't bother cooking proper meals
Whatever the reason, if you're not consuming enough calories consistently, your weight drops. Simple math, really.
Eating Disorders
This is serious stuff. Anorexia nervosa, bulimia, et other eating disorders can cause dangerous weight loss. These aren't about vanity or willpower - they're genuine mental health conditions that need professional treatment.
Signs include: obsessing over calories, intense fear of weight gain, distorted body image, excessive exercise, hiding food, or eating in secret. If this sounds familiar, please talk to a healthcare provider. Like, today.
Digestive Problems
Your gut might not be absorbing nutrients properly. Conditions like:
- Celiac disease (gluten damages intestines)
- Crohn's disease or ulcerative colitis (inflammatory bowel disease)
- IBS (irritable bowel syndrome)
- Food intolerances or allergies
- Chronic diarrhea or vomiting
You might be eating enough, but if your body can't process the food, you won't gain weight. Frustrating, right?
Underlying Medical Conditions
Several health issues can cause weight loss:
- Hyperthyroidism: Overactive thyroid cranks your metabolism into overdrive
- Diabetes: Especially type 1, can cause rapid weight loss
- Cancer: Many cancers cause unintentional weight loss
- Chronic infections: Like tuberculosis or HIV
- Mental health: Depression, anxiety, ADHD can all affect appetite
If you're losing weight without trying, get it checked out. Seriously.
Overtraining
Athletes et fitness enthusiasts sometimes burn way more calories than they consume. You're hitting the gym hard, running miles every day, maybe doing two-a-day workouts. But if you're not eating enough to fuel all that activity, your weight drops.
This is especially common in endurance sports, gymnastics, dance, et any sport where being lean is emphasized.
Health Risks of Being Underweight
Okay, let's be real about what can happen when you're significantly underweight for too long:
Weakened Immune System
When your body doesn't have enough fuel, your immune system suffers. You catch every cold going around, infections last longer, et wounds heal slowly. Your body is literally running on empty, so it can't mount a proper defense.
Fertility Issues
For women especially, low body weight can mess with hormones. Your period might become irregular or stop completely (amenorrhea). This isn't just about periods - it's a sign your body doesn't have enough resources to assistance a pregnancy, so it shuts down reproductive functions.
Men can experience low testosterone, reduced sperm count, et decreased libido too.
Bone Loss et Osteoporosis
Your bones need proper nutrition to stay strong. Being underweight - especially combined with low calcium et vitamin D - can lead to osteoporosis, where bones become brittle et break easily. This isn't just an old person problem. Athletes with low BMI can develop stress fractures in their 20s.
Anemia et Fatigue
Not eating enough means you're probably not getting enough iron, B12, et folate. Result? Anemia. You feel exhausted all the time, get dizzy standing up, can't concentrate, et look pale. Everything feels harder than it should.
Hair Loss et Brittle Nails
When your body is short on nutrients, it priorities vital organs. Hair et nails? Not vital. So they get the leftovers, which means hair falls out, nails break easily, et skin looks dull. Not fun.
Heart Problems
In severe cases, being underweight can affect your heart. It might beat irregularly, blood pressure drops too low, et in extreme malnutrition, the heart muscle itself can weaken. This is life-threatening territory.
Healthy Weight Gain Strategies
So you need to gain weight. Great! But let's do this smartly, not by demolishing tubs of ice cream (though a little ice cream won't hurt).
1. Eat More Frequently
If you struggle to eat large meals, eat smaller meals more often. Aim for 5-6 times a day instead of 3. This is way easier than forcing yourself to eat huge portions when you're not hungry.
Set reminders on your phone if you need to. Seriously, "Time to eat" alarms work.
2. Choose Nutrient-Dense, Calorie-Rich Foods
You want foods that pack lots of calories et nutrition into small servings. Think:
- Nuts et nut butters: Almonds, cashews, peanut butter - high in calories et healthy fats
- Avocados: Loaded with healthy fats et calories
- Whole grains: Brown rice, whole wheat pasta, quinoa, oatmeal
- Fatty fish: Salmon, mackerel, sardines for protein et omega-3s
- Full-fat dairy: Whole milk, cheese, yogurt (if you tolerate dairy)
- Dried fruits: Raisins, dates, apricots - concentrated calories
- Healthy oils: Olive oil, coconut oil - add to cooking
Drizzle olive oil on vegetables. Add nuts to salads. Put nut butter on fruit. Little additions add up.
3. Drink Your Calories
Sometimes it's easier to drink calories than eat them. Try:
- Smoothies with fruit, yogurt, protein powder, nut butter
- Whole milk instead of water or skim milk
- 100% fruit juices (not soda - that's empty calories)
- Protein shakes or meal replacement drinks
Don't rely solely on liquid calories, but they can definitely help.
4. Strength Training
This might sound counterintuitive, but lifting weights helps you gain healthy weight (muscle) rather than just fat. You'll look better, feel stronger, et improve your bone density. Win-win-win.
Focus on compound exercises: squats, deadlifts, bench press, rows. Three times a week is plenty. And eat extra to fuel the muscle growth.
5. Limit Cardio (For Now)
If you're trying to gain weight, maybe dial back the long runs or hour-long cycling sessions. Cardio burns calories you're trying to keep. A little cardio for heart health is fine, but don't overdo it.
6. Track Your Intake
Use an app like MyFitnessPal to see how much you're actually eating. Many underweight people think they eat plenty, but when they track it, they're shocked how little it actually is. Awareness is powerful.
7. Address Underlying Issues
If stress kills your appetite, work on stress management. If medication suppresses hunger, talk to your doctor about alternatives. If digestive issues are the problem, get them treated. You can't out-eat an underlying medical condition.
When to See a Doctor
You should definitely talk to a healthcare provider if:
- Your BMI is under 18.5 et you're experiencing symptoms like fatigue, weakness, or frequent illness
- You've lost weight unintentionally (especially 10+ pounds in 6 months)
- Your period has stopped (for women)
- You're struggling with an eating disorder or disordered eating thoughts
- You've tried to gain weight for several months with no success
- You have digestive symptoms like chronic diarrhea, bloating, or pain
- You're concerned about your health for any reason
A doctor can run blood tests, check for underlying conditions, et refer you to specialists if needed. A registered dietitian can create a personalized meal plan. Don't try to handle serious health issues on your own.
The Bottom Line
Being underweight isn't automatically unhealthy, but it can be. The key is understanding why your BMI is low et whether it's affecting your health negatively.
If you need to gain weight, do it gradually et healthily. Aim for 0.5-1 pound per week through nutrient-dense foods et strength training. Don't just eat junk food to hit a number - build actual health.
And honestly? If you're struggling, get professional help. There's no shame in working with a doctor, dietitian, or therapist to reach a healthy weight. Your health matters.