How can I gain weight if I am underweight? A healthy nutrition guide

October 7, 2025 •

4 min read

Approximately 1.9 billion adults worldwide are overweight, but for some, being underweight with a BMI below 18.5 presents unique health challenges, from weakened immunity to fatigue. For those asking, 'How can I gain weight if I am underweight?', the solution lies in a strategic nutrition diet focused on a consistent calorie surplus and nutrient-dense foods.

Quick Summary

Gaining weight healthily involves consistently consuming more calories than you burn. This is best achieved by focusing on nutrient-rich, energy-dense foods and incorporating strength training to build muscle mass, ensuring the weight gain is healthy and sustainable.

Key Points

Calorie Surplus is Essential: Consistently eat more calories than you burn, ideally 300-500 extra per day, by focusing on nutrient-dense foods, not junk food.
Prioritize Nutrient-Dense Foods: Opt for high-calorie, healthy options like nuts, full-fat dairy, avocados, and whole grains to maximize nutrition while increasing calories.
Incorporate Strength Training: Pair your increased calorie intake with resistance training, such as weightlifting, to build muscle mass rather than unhealthy fat.
Eat Smaller, More Frequent Meals: Instead of forcing large meals, break up your daily intake into smaller, more frequent meals and snacks to boost calories without discomfort.
Track Your Progress Holistically: Don't rely only on the scale; use body measurements, progress photos, and monitor strength gains to get a comprehensive view of your success.
Avoid Excessive Cardio: Limit prolonged cardio sessions which burn calories you're trying to save for weight gain. Focus more on building muscle through strength training.
Ensure Adequate Rest: Sleep and recovery are crucial for muscle growth. Aim for 7-9 hours of quality sleep per night.

The Importance of a Calorie Surplus for Healthy Weight Gain

To gain weight, you must create a consistent calorie surplus, meaning you consume more calories than your body expends each day. The key is to do this in a controlled, healthy manner to ensure you gain a balanced combination of muscle and subcutaneous fat, not unhealthy belly fat. Instead of relying on processed junk food, focus on wholesome, nutrient-dense foods rich in healthy fats, proteins, and complex carbohydrates. For a slow and steady gain, aiming for an extra 300 to 500 calories per day is often recommended, though a healthcare provider can help determine the right target for you.

Prioritizing Nutrient-Dense Foods

Not all calories are created equal. Filling up on empty calories can lead to unhealthy fat gain and leave you with nutrient deficiencies. Here are some nutrient-dense foods to incorporate into your diet:

Nuts and Nut Butters: Calorie-dense and packed with healthy fats, protein, and minerals. Almonds, walnuts, and peanut butter are excellent choices.
Full-Fat Dairy: Whole milk, full-fat Greek yogurt, and cheese provide a good balance of protein, carbs, and fat.
Energy-Dense Fruits and Dried Fruits: Bananas, avocados, and dried fruits like raisins and dates offer concentrated calories and are easy to snack on.
Healthy Fats and Oils: Add extra virgin olive oil to salads or avocado oil to your cooking for an easy calorie boost.
Whole Grains and Starches: Brown rice, oats, quinoa, potatoes, and sweet potatoes are vital for sustained energy and calories.
Lean Protein Sources: Include lean meats, chicken, fish, eggs, and legumes to support muscle growth.

The Role of Macronutrients in Your Diet

Macronutrients—protein, carbohydrates, and fats—are crucial for weight gain, each playing a specific role.

Protein: Essential for muscle repair and growth. Aim for a consistent intake spread throughout the day to maximize its effects.
Carbohydrates: Provide the energy needed for both daily activities and intense workouts. Opt for complex carbs to fuel your body effectively.
Fats: As the most calorie-dense macronutrient, healthy fats are vital for increasing your overall calorie intake without requiring a large volume of food.

Comparison of Healthy vs. Unhealthy Weight Gain

To illustrate the right approach, consider this comparison:


Feature	Healthy Weight Gain	Unhealthy Weight Gain
Food Focus	Nutrient-dense whole foods (nuts, whole grains, lean protein)	Calorie-rich junk food and processed snacks (chips, sweets)
Calorie Source	Balanced mix of protein, carbs, and healthy fats	Primarily sugar and unhealthy saturated fats
Body Composition	Balanced increase in lean muscle mass and fat	Predominantly increase in body fat, especially visceral fat
Physical Activity	Strength and resistance training prioritized	Often involves too much cardio or no exercise, hindering muscle growth
Health Impact	Improved energy, bone density, and overall well-being	Increased risk of chronic diseases like type 2 diabetes and heart disease

Combining Strength Training with Nutrition

Resistance training is a non-negotiable component of healthy weight gain. Simply eating more calories without exercise will lead to storing excess energy as fat. Strength training stimulates muscle hypertrophy, prompting your body to use the calorie surplus to build muscle mass.

Examples of effective strength training exercises include:

Squats
Deadlifts
Bench presses
Overhead presses
Pull-ups and push-ups

Aim for two to three intense sessions per week, allowing your muscles adequate time for recovery between workouts.

Optimizing Your Weight Gain Lifestyle

Beyond diet and exercise, a few lifestyle adjustments can help you achieve your goals.

Eat Frequent Meals: Instead of three large meals, try eating three moderate meals and two to three substantial, nutrient-dense snacks throughout the day. This helps increase your overall calorie intake without feeling overly full.
Stay Hydrated, Strategically: Drink fluids between meals rather than with them to avoid feeling full too quickly and leaving less room for food.
Get Enough Sleep: Adequate rest is crucial for muscle repair and growth. Aim for 7–9 hours of sleep per night to support your body's recovery process.
Manage Stress: High stress levels can affect appetite and metabolism. Techniques like yoga or meditation can help.

Tracking Your Progress Effectively

Monitoring your progress helps maintain motivation and ensures you're on the right track. Don't rely solely on the scale, as it doesn't differentiate between muscle and fat. Consider these methods:

Consistent Weigh-ins: Weigh yourself regularly at the same time and day of the week to see long-term trends.
Body Measurements: Use a tape measure to track changes in your arms, chest, waist, and thighs.
Progress Photos: Take photos from the front, side, and back every few weeks to visually track your physique changes.
Strength Gains: Monitor improvements in your workouts, such as lifting heavier weights or performing more reps.
Energy Levels and Well-being: Notice increases in energy, better mood, and improved performance outside of the gym.

Conclusion

Healthy weight gain for underweight individuals is a strategic journey centered on a balanced, calorie-surplus diet and consistent strength training. By prioritizing nutrient-dense foods, optimizing macronutrient intake, and making smart lifestyle choices, you can achieve your goals sustainably. Patience is key, as steady progress is healthier and more enduring than rapid, unhealthy weight gain. Consulting a healthcare provider or a registered dietitian is always recommended to ensure your plan is safe and tailored to your specific needs.

To learn more about healthy eating, you can explore resources like the NHS's guide on healthy ways to gain weight.

Frequently Asked Questions

While eating junk food can increase your calorie intake, it will not contribute to healthy weight gain. It often leads to storing unhealthy fat, particularly around the abdomen, and provides few essential nutrients for muscle building and overall health.

For building muscle, adequate protein is important, but excess protein intake is not beneficial beyond a certain point. A moderate amount, such as 1.6 to 2.2 grams per kilogram of body weight per day, is recommended, especially when combined with strength training.

Yes, drinking water right before meals can fill your stomach and reduce your appetite, making it harder to consume the necessary calories for weight gain. It's better to drink fluids between meals.

Nutrient-dense snacks are key. Good options include homemade protein smoothies with whole milk, fruit, and nut butter; a handful of nuts and dried fruit; or avocado toast.

You don't need to avoid cardio entirely, as it is important for heart health. However, excessive cardio can burn a high number of calories that could otherwise contribute to weight gain. Focus more on resistance training, and limit cardio sessions to shorter durations.

Healthy weight gain is a gradual process. Rapid weight gain is often unsustainable and unhealthy. A realistic goal is to gain around 0.5 to 1 pound (0.2 to 0.5 kg) per week through consistent effort and diet adjustments.

Supplements like protein powder or mass gainers can be helpful additions, but they should not replace a balanced, whole-foods diet. Always prioritize nutrient-dense meals and use supplements strategically.