What’s the Best Way to Gain Weight if You’re Underweight? (For Older Adults + General Guidance)

If you’re struggling with being underweight—or you’ve noticed unexpected weight loss—your first instinct might be to simply “eat more.” While that advice can help, the best way to gain weight depends on two things:

Your calorie and protein intake (the “fuel” problem)
The reason your weight is low (the “cause” problem)

In this long, practical, evidence-based guide, you’ll learn what underweight means medically, the safest strategies to gain weight in a healthy way, meal ideas that work for real life, and when to seek medical help. You’ll also find links to scientific and medical sources throughout the article.

1) How Underweight Is Defined (BMI + Clinical Context)

Clinically, underweight is commonly defined as a BMI under 18.5. (cdc.gov)

That said, BMI is a screening tool—not a complete explanation of why you’re underweight. If you’re older, BMI may be less informative because body composition changes with age (more fat changes, and muscle loss can occur even at “normal” weight).

Practical takeaway: If your weight is persistently low, or if you’ve lost weight unintentionally, it’s worth addressing both intake and underlying causes.

2) Why You Might Be Underweight (Common Causes)

There are many reasons people struggle to reach a healthier weight. Some are purely nutritional; others require medical evaluation.

Common causes include:

Not eating enough calories (low appetite, busy schedule, difficulty preparing meals)
Higher calorie needs (some active lifestyles)
Recovery from illness
Age-related appetite changes
Dental problems or difficulty chewing/swallowing
Gastrointestinal issues (malabsorption, chronic diarrhea, inflammation)
Hormonal or metabolic conditions (e.g., thyroid disorders)
Medication side effects
Stress, anxiety, or depression
Substance-related factors (tobacco, stimulant use, etc.)

Because of this variety, the best plan is individualized.

3) When to See a Doctor or Dietitian

If your low weight is unexpected, worsening, or comes with other symptoms, it’s safest to get medical guidance.

Consider contacting a clinician if you have:

Unintentional weight loss
Ongoing loss of appetite
Frequent GI symptoms (vomiting, chronic diarrhea, persistent abdominal pain)
Difficulty swallowing
Fatigue, fever, night sweats, or other systemic symptoms
Any signs that suggest a nutrient absorption issue

A clinician or registered dietitian (RD) can help you identify the likely cause and create a calorie/protein plan that matches your needs.

Why this matters: If your weight is low due to an underlying medical issue, food strategies alone may not fully solve it.

4) The Best Evidence-Based Ways to Gain Weight

1) Increase Calories—Without Overwhelming Your Appetite

For many underweight adults, the biggest obstacle is not “what to eat,” but how to fit more calories into the day.

A proven approach is to:

Eat more often (example: 5–6 smaller meals/snacks rather than 2–3 big ones)
Use consistency: aim for a routine even if appetite is low
Track progress: use weekly weight or measurements rather than daily swings

Goal mindset: Build a small calorie surplus over time.

2) Choose Calorie-Dense, Nutrient-Rich Foods

Healthy weight gain is easiest when calories come with nutrients.

Prioritize foods like:

Nut butters, nuts, trail mix
Avocado
Olive oil and other healthy fats
Dairy (milk, yogurt, cheese) or lactose-free alternatives
Whole grains (rice, oats, quinoa, whole-grain bread)
Eggs, lean meats, fish
Beans and lentils (calories + protein + fiber)

This approach helps reduce the chance that you gain weight mostly from low-nutrient “empty” calories.

3) Use “Add-Ons” to Boost Calories and Protein

Add-ons let you raise calories without increasing meal volume too much—especially helpful for older adults whose appetite may be smaller.

Examples:

Add cheese to eggs, pasta, potatoes, casseroles
Spread nut butter on toast, bagels, oatmeal, bananas
Stir powdered milk into mashed potatoes or soups (if tolerated)
Add olive oil to rice, pasta, or cooked vegetables
Use Greek yogurt as a topping or ingredient

4) Use Smoothies and Shakes Strategically

If solid food feels like a chore, smoothies can be a practical way to meet calorie and protein goals.

A simple calorie/protein smoothie template:

Milk or fortified soy milk
Greek yogurt (or protein-rich alternative)
Peanut butter or nut butter
Banana (or berries)
Oats (optional for extra calories and fiber)

Important: Smoothies should typically supplement, not completely replace meals—otherwise, total intake may not rise as expected.

5) Time Drinks Properly (So They Don’t Replace Meals)

Many people feel full faster if they drink right before or during meals.

Practical tactic:

Keep liquids moderate around meals
Drink water between meals
If using high-calorie drinks (smoothies/shakes), schedule them intentionally—often as snacks rather than immediately before eating

The goal is to avoid “accidental volume filling.”

6) Strength Training for Lean Mass

When underweight adults try to gain weight, a key health goal is often to gain lean mass (muscle), not only fat.

Strength training also helps maintain function and can support healthier body composition with age.

For older adults, the CDC highlights weekly activity targets and includes muscle-strengthening activities at least 2 days per week as part of the general physical activity guidance. (cdc.gov)

Protein + resistance training is especially relevant for aging muscle and sarcopenia prevention discussions; nutrition guidance often emphasizes adequate protein alongside resistance training. (pmc.ncbi.nlm.nih.gov)

(If you have osteoporosis, joint issues, or medical limitations, ask a clinician or physical therapist for safe exercise options.)

5) A Simple, Repeatable Weight-Gain Meal Plan (1 Day + Variations)

Here’s a realistic 1-day example you can adapt:

Breakfast

Oatmeal cooked with milk + peanut butter + banana
Optional: Greek yogurt on the side

Snack 1

Trail mix (nuts + dried fruit)
or cheese + whole-grain crackers

Lunch

Rice bowl: beans or chicken + olive oil dressing + avocado
Add: a side of yogurt or a glass of milk

Snack 2

Smoothie: milk + Greek yogurt + nut butter + oats + berries

Dinner

Pasta with lentils or lean meat
Add olive oil to vegetables
Optional: dessert like yogurt or a small portion of ice cream if tolerated

Pre-bed (if you’re still not meeting calories)

Peanut butter toast
or warm milk / yogurt

Variation ideas (rotate to avoid boredom):

Eggs + whole-grain toast + olive oil sautéed vegetables
Lentil soup + bread + cheese
Salmon (or other protein) + potatoes + olive oil
Yogurt parfaits with nuts and granola

6) Supplements: Helpful in Some Cases—Not a Shortcut

Supplements can help with calorie/protein gaps in some situations, but they’re not a substitute for addressing underlying causes.

Potential supplement categories:

Oral nutrition supplements (high-calorie/protein shakes)
Protein powders (helpful if you can’t meet protein from food)
Vitamin D if you’re deficient or at risk
B12 especially if you follow a plant-heavy diet and have low intake
Iron only if deficiency is confirmed (or strongly suspected clinically)

If considering supplements, it’s smart to discuss them with a clinician—especially for older adults, because medication interactions and deficiency causes matter.

The FDA explains key rules around dietary supplements and that they are not approved like drugs to treat disease. (fda.gov)
For vitamin D background, NIH Office of Dietary Supplements materials are commonly used by clinicians. (mayoclinic.org)

7) How Fast Should You Gain Weight?

A common clinical goal is slow, steady gain, often around 0.25–0.5 lb (0.1–0.25 kg) per week for many adults—though your clinician may adjust based on health status.

Why slow matters: rapid gain can sometimes come with GI discomfort, poor food choices, or unaddressed medical problems.

8) Common Mistakes That Prevent Weight Gain

Increasing calories without protein
Relying only on low-nutrient “junk” calories (can cause GI upset or poor nutrient status)
Using smoothies as a replacement for all meals (instead of a strategy to increase total daily intake)
Drinking large amounts immediately before eating (reducing appetite)
Not addressing underlying causes (thyroid, GI issues, dental problems, mental health)
Giving up too early (weight gain can take weeks, not days)

9) FAQ (Older Adults & Underweight)

Q1: Is BMI the only way to tell if I’m underweight?

No. BMI < 18.5 is a common clinical cutoff, but clinicians also consider symptoms, body composition, and unintentional weight loss. (cdc.gov)

Q2: What if I feel full quickly?

Try smaller more frequent meals, add calorie-dense add-ons, and consider protein-focused smoothies as snacks.

Q3: Can strength training help me gain weight?

It can help you build muscle, which improves body composition and overall health—especially when paired with adequate calories and protein. (cdc.gov)

Q4: Are weight-gain supplements always necessary?

No. Many people can gain weight with food strategies alone. Supplements can help when appetite is low or food intake is limited.

Q5: When is low weight an emergency?

Seek urgent care if you have severe symptoms such as chest pain, trouble breathing, persistent vomiting, blood in stool, severe dehydration, or rapidly worsening weight loss.

10) Key Takeaways

Underweight is commonly defined as BMI < 18.5. (cdc.gov)
The best weight-gain plan addresses both calories and the reason your weight is low.
Use small frequent meals, calorie-dense nutrient-rich foods, and add-ons.
Use smoothies strategically (as a supplement).
Consider strength training for healthier lean mass gain. (cdc.gov)
Get medical help if your weight loss is unexpected or accompanied by symptoms.

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