Nutrition Diet: How to gain weight severely underweight?

October 13, 2025 •

4 min read

Being severely underweight can lead to serious health issues like weakened immunity, osteoporosis, and malnutrition. This comprehensive guide addresses the vital question: How to gain weight severely underweight? We'll explore safe, healthy strategies to help you reach a healthier body mass.

Quick Summary

Gaining weight healthily when severely underweight involves creating a safe caloric surplus with nutrient-dense foods, eating smaller, more frequent meals, and incorporating strength training. Professional guidance is essential to ensure a balanced, effective plan.

Key Points

Calorie Surplus: To gain weight, consume 300 to 500 extra calories per day from healthy, nutrient-dense foods.
Frequent Meals: Combat a small appetite by eating 5-6 smaller meals and snacks throughout the day instead of large, overwhelming portions.
Nutrient-Dense Choices: Prioritize foods rich in healthy fats, protein, and complex carbohydrates, such as nuts, avocados, whole-grain bread, and lean meats.
Incorporate Strength Training: Combine your diet with resistance exercises like weightlifting to build healthy muscle mass, not just body fat.
Smart Snacking: Opt for easy, high-calorie snacks like trail mix, protein bars, or crackers with nut butter to boost your daily intake.
Professional Guidance: Always consult a doctor or registered dietitian before beginning a weight gain program, especially if severely underweight, to rule out medical issues.
High-Calorie Shakes: Use nutrient-rich smoothies with ingredients like whole milk, nut butter, and protein powder to increase calorie intake without feeling overly full.

Understanding Severe Underweight

Severe underweight is a medical concern characterized by a body mass index (BMI) below 18.5, which can lead to a host of health problems. Beyond just aesthetics, being significantly underweight can harm your body in numerous ways, from weakening your immune system and increasing susceptibility to infections to causing anemia, osteoporosis, and even infertility. For individuals struggling with low body weight, addressing the issue is critical for long-term health and well-being. Before embarking on any weight gain journey, it is crucial to consult a healthcare provider to rule out any underlying medical conditions that might be hindering weight gain. Only after a professional medical assessment should you begin a targeted and safe nutritional plan.

The Foundation: Achieving a Calorie Surplus

To gain weight, you must consume more calories than your body expends. This fundamental principle is known as a caloric surplus. For severely underweight individuals, the goal is not to binge on junk food, but rather to increase calorie intake in a healthy, controlled manner. A safe and sustainable approach involves adding around 300 to 500 extra calories per day to your typical intake. For those aiming for more rapid, supervised weight gain, a daily surplus of up to 1,000 calories might be recommended. Tracking your calorie intake over a few weeks can be helpful to understand your current consumption and make appropriate adjustments. Online apps and tools can assist in this process.

Prioritizing Nutrient-Dense Foods

The key to a healthy weight gain is to choose foods that are rich in nutrients, not just empty calories from sugar and unhealthy fats. Focus on nutrient-dense foods that provide essential vitamins, minerals, healthy fats, and quality protein. Some excellent examples include:

Nuts and Seeds: Almonds, walnuts, macadamia nuts, peanuts, and sunflower seeds are packed with calories and healthy fats.
Healthy Fats: Incorporate avocados, olive oil, and nut butters into your meals.
High-Calorie Dairy: Full-fat yogurt, cheese, and milk are great sources of protein and calories.
Protein Sources: Include eggs, lean meats (or fattier cuts), fish, legumes, and protein powders.
Complex Carbohydrates: Whole grains like oats, brown rice, and quinoa, along with starchy tubers like potatoes and sweet potatoes, provide sustained energy.

Practical Tips for Increasing Calories

Enrich Your Meals: Add grated cheese to soups and mashed potatoes, sprinkle nuts and seeds on cereal or salads, and use healthy oils and butter generously when cooking.
High-Calorie Drinks: Incorporate milkshakes or smoothies made with full-fat milk, yogurt, and nut butter.
Larger Portions: Gradually increase your portion sizes at main meals to boost overall intake.

Timing and Frequency: Eating More Often

For many severely underweight people, a low appetite can be a major obstacle. Eating three large meals might seem unappealing and overwhelming. The solution is to eat smaller, more frequent meals and snacks throughout the day, every 3 to 5 hours. This approach helps maintain a consistent calorie intake without overstraining your stomach. Having nutrient-dense snacks readily available is a great strategy. Examples include trail mix, yogurt with fruit, or crackers with peanut butter.

Building Muscle with Strength Training

To ensure that the weight you gain is healthy muscle mass rather than just body fat, incorporating strength training is crucial. Resistance exercises stimulate muscle growth, and muscle weighs more than fat, contributing to healthy weight gain. Weightlifting is highly effective, but bodyweight exercises like push-ups and squats, or even yoga, can also help build muscle. Avoid excessive cardiovascular exercise, as it burns calories that you are trying to retain. Aim for strength training 3-4 days a week.

The Role of Supplements

While whole foods should form the foundation of your diet, supplements can be useful for those who find it challenging to meet their calorie goals from food alone.

Protein Shakes: Protein powders can be added to milkshakes and smoothies to significantly increase protein and calorie content. Whey and casein protein are common choices.
Mass Gainers: These supplements are formulated with high calories and a blend of carbs and protein. However, some can be poorly balanced, so it's important to choose high-quality products and not rely on them exclusively.
Creatine: This supplement can provide additional energy for muscles, helping you lift more and build mass.

Potential Obstacles and Solutions

Low Appetite: Eating smaller, frequent meals and using high-calorie liquids like smoothies can help. Engaging in light exercise, like a short walk, can also help stimulate appetite.
Fast Metabolism: Some individuals have a naturally high metabolism, making weight gain difficult. This can be overcome with a consistently higher caloric intake and careful tracking to ensure you are consuming enough.
Underlying Medical Conditions: Unintentional weight loss or difficulty gaining weight can be a sign of a more serious health issue, such as a thyroid problem or digestive disorder. A doctor's visit is essential to address any such conditions.

Comparison of High-Calorie Nutrients


Nutrient Type	Source Examples	Calories per Gram	Benefits for Weight Gain
Healthy Fats	Avocados, nuts, olive oil, nut butters	~9	High-density calories, essential for hormone production and nutrient absorption.
Protein	Meat, eggs, dairy, legumes, fish	~4	Builds and repairs muscle tissue, crucial for healthy weight gain.
Complex Carbohydrates	Brown rice, whole grains, potatoes	~4	Sustained energy source for exercise and daily activities.
Free Sugars	Candy, soda, processed snacks	~4	Provides quick energy, but is nutrient-poor and should be limited.

Conclusion: A Balanced, Patient Approach

Addressing severe underweight is a journey that requires patience, consistency, and a strategic approach. It is about more than just eating; it's about providing your body with the right combination of high-calorie, nutrient-dense foods, coupled with strength training to build muscle. Always prioritize healthy, whole foods over processed junk food, and use supplements judiciously. Most importantly, consult with a healthcare professional to ensure there are no underlying health issues and to receive personalized guidance. By following these principles, you can safely and effectively move towards a healthier body weight and better overall health.

Learn more about creating a balanced and healthy eating pattern from resources like The Eatwell Guide on the NHS website.

Frequently Asked Questions

Being severely underweight can lead to numerous health problems, including weakened immune function, osteoporosis (thinning of bones), anemia, irregular menstruation, fertility issues, and potentially more serious conditions.

If you have a small appetite, focus on eating smaller, more frequent meals and snacks throughout the day. Opt for calorie-dense foods that don't take up much volume, and drink high-calorie smoothies or milkshakes between meals to add calories without feeling too full.

The healthiest way to gain weight is by building muscle mass, not just adding body fat. This is achieved by combining a caloric surplus with regular strength training. Gaining weight through high-calorie, nutrient-dense foods helps build muscle rather than unhealthy fat stores.

Good examples include nuts, seeds, avocados, nut butters, full-fat dairy products like Greek yogurt and cheese, eggs, fatty fish like salmon, whole grains, and starchy vegetables such as potatoes and sweet potatoes.

Exercise is very important, specifically strength training. Lifting weights or doing resistance exercises helps build muscle mass, ensuring your weight gain is healthy and not just fat. Light cardio can also stimulate appetite, but excessive cardio should be avoided.

Mass gainer supplements can be a convenient way to increase calorie intake, but they should not be your only source of calories. Focus primarily on whole, nutrient-dense foods and use supplements as a complement to your diet. Consult a dietitian for guidance on suitable options.

If you're consistently eating in a caloric surplus and doing strength training but not gaining weight, you should consult a doctor. There might be an underlying medical condition, like a thyroid issue or malabsorption disorder, that needs to be addressed.