What Should an Underweight Person Eat? A Guide to Healthy Weight Gain

October 8, 2025 •

5 min read

According to the World Health Organization, more than 75% of the world’s population lives in countries where overweight and obesity kill more people than underweight. However, being underweight carries its own set of health risks, and understanding what should an underweight person eat is crucial for safe and effective weight gain. This guide outlines a nutritional approach to help you reach a healthy body weight.

Quick Summary

This article details a diet plan for healthy weight gain, focusing on a calorie surplus achieved through nutrient-dense foods. It provides specific food recommendations across different groups and highlights strategic eating patterns to increase calorie intake safely and effectively. The importance of combining nutrition with exercise is also addressed.

Key Points

Increase Calorie Intake Safely: Achieve a calorie surplus of 300-500 extra calories per day by consuming nutrient-dense foods, not junk food.
Eat More Frequently: Instead of three large meals, try eating five to six smaller, nutritious meals and snacks throughout the day.
Prioritize High-Calorie, Nutritious Foods: Incorporate sources like nuts, avocados, full-fat dairy, dried fruit, and healthy oils to boost calorie intake.
Embrace Protein and Strength Training: Ensure adequate protein intake and combine with resistance exercise to build lean muscle mass, not just fat.
Utilize High-Calorie Beverages: Drink smoothies, shakes, or whole milk between meals to add calories without feeling overly full.
Seek Professional Medical Advice: Consult a doctor or dietitian to rule out underlying issues and create a personalized, healthy weight gain plan.

The Foundation of Healthy Weight Gain

For an underweight individual, the fundamental principle of weight gain is to achieve a consistent calorie surplus. This means consuming more calories than your body expends daily. The goal is not just to gain weight, but to gain it in a healthy way, prioritizing lean muscle mass over excessive body fat. A common and healthy target for a gradual increase is adding an extra 300 to 500 calories per day, but individual needs can vary significantly based on metabolism, activity level, and underlying health conditions. For this reason, consulting a healthcare provider or a registered dietitian is always the recommended first step.

Crucially, this extra energy should come from nutrient-dense, high-calorie foods, not from empty calories found in junk food, sugary snacks, and fried foods. Focusing on nutritious sources ensures you are getting the vitamins, minerals, and other essential nutrients your body needs to function optimally while gaining weight. Combining this calorie surplus with a regular resistance training program is vital for directing the weight gain towards muscle development.

Calorie-Dense and Nutrient-Rich Food Groups

To build a diet for healthy weight gain, focus on incorporating a variety of foods from the following categories:

Protein Sources

Protein is the building block of muscle, making it essential for weight gain. High-quality protein sources include:

Meat: Opt for red meat like steak for protein and iron, or fattier cuts for more calories. Lean meats like chicken and turkey are also excellent sources.
Fish: Fatty fish like salmon and sardines provide a great combination of protein and heart-healthy omega-3 fatty acids.
Eggs: Whole eggs are an affordable and versatile source of high-quality protein and fats.
Legumes: Beans, lentils, and chickpeas are excellent plant-based sources of protein and fiber.
Protein Powders: Supplements like whey or plant-based protein powders can help boost intake, especially in shakes.

Healthy Fats and Oils

Fats are the most calorie-dense macronutrient, providing 9 calories per gram. Healthy fats are a cornerstone of any weight gain plan:

Nuts and Seeds: Almonds, walnuts, macadamia nuts, pumpkin seeds, and chia seeds are packed with calories and nutrients. They are perfect for snacking or adding to meals.
Nut Butters: Peanut, almond, and cashew butters offer a convenient way to add calories to toast, smoothies, and snacks.
Avocados: A large avocado contains around 365 calories and is rich in heart-healthy monounsaturated fats.
Oils: Olive oil, coconut oil, and avocado oil are easy to add to cooking, salads, and sauces for a calorie boost.

Carbohydrates

Carbohydrates are your body's primary source of energy. Choose complex, unrefined carbohydrates over simple sugars:

Whole Grains: Oats, brown rice, quinoa, and whole-grain bread are excellent sources of sustained energy and fiber.
Potatoes and Root Vegetables: Sweet potatoes, yams, and potatoes are starchy, calorie-dense carbs that are easy to incorporate into meals.
Dried and Calorie-Dense Fruit: Dates, raisins, apricots, and bananas are higher in calories and natural sugars, making them perfect additions to snacks or smoothies.

Dairy and Alternatives

Full-fat dairy provides valuable protein, calcium, and fats, while fortified alternatives offer good nutrient profiles:

Full-Fat Milk and Yogurt: Switching from skim or low-fat options to whole milk and full-fat Greek yogurt is a simple way to increase calories.
Cheese: A versatile, calorie-dense food that can be added to many savory dishes.

Strategic Eating Patterns and Meal Enhancement

Eat More Frequently

Instead of feeling overwhelmed by three large meals, many underweight people benefit from eating five to six smaller, more frequent meals throughout the day. This can make increasing your calorie intake more manageable. Plan for three main meals with two or three nutritious snacks in between.

Fortify Your Food

Boost the calorie count of your existing meals without dramatically increasing portion sizes. Some simple tricks include:

Add a tablespoon of olive oil or nut butter to smoothies.
Stir powdered milk or cheese into mashed potatoes, soups, and casseroles.
Sprinkle nuts, seeds, or granola on yogurt, oatmeal, or salads.
Use full-fat dairy products in sauces, drinks, and recipes.

Drink Your Calories

High-calorie liquids are often easier to consume than solid food when you have a small appetite. Consider making your own nutrient-dense shakes or smoothies by blending:

Whole milk or full-fat yogurt
Fruits like bananas and mangoes
Nut butters or seeds (e.g., chia, flax)
Protein powder
A handful of oats

Comparison of Food Choices for Calorie Density


Food Item	Lower-Calorie Option	Higher-Calorie Option
Milk	Skim or low-fat milk	Whole milk, flavored milk
Yogurt	Non-fat or low-fat yogurt	Full-fat Greek yogurt, topped with granola and nuts
Snacks	Air-popped popcorn	Trail mix with nuts, seeds, and dried fruit
Dressings	Low-fat or fat-free dressing	Oil-based vinaigrette, creamy dressing
Cereals	Rice Krispies	Granola with dried fruit and nuts

What to Avoid for Healthy Weight Gain

While the goal is to increase calories, not all calories are created equal. Filling up on low-nutrient, high-sugar, or high-salt foods can lead to unhealthy fat accumulation and other health problems. Limit your intake of:

Sugary drinks: Soda, sweetened tea, and excessive fruit juice provide empty calories.
Fried foods: While high in calories, they are also high in unhealthy fats.
Sweets and pastries: These offer little nutritional value beyond calories.
Diet or low-fat products: These have fewer calories and nutrients than their full-fat counterparts.
Filling up on water before meals: Drinking water can suppress appetite; focus on nutrient-dense beverages instead.

Conclusion: Consistency and Professional Guidance are Key

Gaining weight healthily requires a strategic and consistent approach. It’s a process that takes time, patience, and a focus on nutrient-dense foods combined with an appropriate exercise regimen, particularly strength training. By adopting frequent, fortified meals and prioritizing healthy calorie sources, you can work towards achieving a healthy body weight and improving your overall health. Remember, before beginning any weight gain program, it is crucial to consult with a doctor or registered dietitian. They can help rule out any underlying medical causes for being underweight and create a personalized plan tailored to your specific needs.

For further information on healthy eating, you can explore resources from the National Health Service (NHS), a trusted source on nutrition and weight management: https://www.nhs.uk/live-well/healthy-weight/managing-your-weight/healthy-ways-to-gain-weight/

Frequently Asked Questions

Focus on calorie-dense, nutrient-rich foods that take up less volume, such as nuts, seeds, dried fruits, nut butters, and full-fat dairy. You can also drink your calories with smoothies and shakes.

No, it is not recommended to rely on junk food for weight gain. While high in calories, these foods often lack essential nutrients and can lead to unhealthy fat gain and other health issues. Prioritize nutrient-dense whole foods instead.

For building muscle, adequate protein intake is crucial. While specific needs vary, incorporating high-quality protein sources like meat, fish, eggs, and legumes into every meal is a good strategy when combined with resistance training.

Weight gainer shakes can be a convenient way to increase calorie and protein intake, especially for those who struggle to eat enough solid food. However, they are not strictly necessary and can be replaced by homemade nutrient-dense smoothies.

Exercise, particularly strength or resistance training, is vital for ensuring that weight gain is primarily muscle mass rather than body fat. It also helps stimulate your appetite.

Good options include a handful of nuts or trail mix, a banana with peanut butter, full-fat Greek yogurt with granola and fruit, or cottage cheese.

It's important to consult a doctor before starting any weight gain plan to rule out underlying medical conditions such as a fast metabolism, thyroid issues, or digestive problems.