Deficitly
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How do you gain weight with a fast metabolism?
Eat about 300–500 calories a day above your real maintenance, mostly from calorie-dense foods, and lift weights 3 times a week so the weight you gain is mostly muscle. A "fast metabolism" is rarely a broken thermostat: resting burn varies by only about ±10% between people of the same size. Usually it's a high daily burn from moving a lot, plus eating less than it feels like. A slim 25-year-old man of 63 kg (139 lb) burns about 2,100 calories at 4,000 steps a day but 2,360 at 14,000.
Do you really have a fast metabolism?
Resting metabolism mostly follows body size: lean mass explains most of the difference between people, and what's left is about ±10%, so ±160 calories a day for the man above (resting burn 1,640). The bigger differences come from movement. Some people automatically fidget, stand and walk more when they eat more: in an overfeeding study, the extra daily movement of the most active people burned up to about 700 calories a day, so they gained almost no fat. That's real, and it means you need to eat more than a standard calculator suggests. Check yours with the BMR calculator.
The other half is intake. Thin people often remember the big meals and forget the skipped ones. Track a normal week before deciding you "eat loads". To measure your real burn instead of guessing, use the box on signs of a fast or slow metabolism.
How many calories to gain weight
| Person | Maint. | Slow+0.25%/wk | Faster+0.5%/wk |
|---|---|---|---|
| Woman, 48 kg 106 lb165 cm 5′5″ | 1,640 | 1,770 | 1,910 |
| Woman, 53 kg 117 lb168 cm 5′6″ | 1,740 | 1,890 | 2,040 |
| Man, 60 kg 132 lb178 cm 5′10″ | 2,140 | 2,310 | 2,470 |
| Man, 66 kg 146 lb183 cm 6′0″ | 2,280 | 2,460 | 2,640 |
Start from your maintenance with your real steps and workouts (maintenance calorie calculator), add the surplus, and adjust by the scale. The textbook surplus for a hardgainer is often too small at first, because daily movement rises as you eat more. The weight gain calculator does both steps, with a goal date.
Calorie-dense foods that make it easier
| Food | Per serving | Per 100 g |
|---|---|---|
| Olive oil1 tbsp (13.5 g) | 120 | 885 |
| Peanut butter2 tbsp (32 g) | 190 | 600 |
| Almonds1 oz (28 g) | 160 | 580 |
| Avocado½ avocado (68 g) | 110 | 160 |
| Hummus2 tbsp (30 g) | 70 | 235 |
| Oats, dry½ cup (40 g) | 150 | 380 |
| Salmon, farmed, cooked100 g (3.5 oz) | 205 | 205 |
| Whole-wheat pasta, cooked1 cup (140 g) | 210 | 150 |
| White rice, cooked1 cup (158 g) | 205 | 130 |
| Banana1 medium (118 g) | 105 | 90 |
| Milk, 1%1 cup (244 g) | 100 | 40 |
| Honey1 tsp (7 g) | 20 | 305 |
Fats are the easiest calories to add without feeling stuffed: a tablespoon of oil on rice or vegetables is about 120 calories you barely notice. Drinks help too: milk and smoothies are less filling than the same calories as food.
Easy ways to add 500 calories a day
- One day's extras: 1 tbsp olive oil + 2 tbsp peanut butter + 1 medium banana + 1 cup milk, 1% = about 520 calories.
- Eat four times a day instead of three, with a protein source each time.
- Bigger portions of the starch at dinner: an extra cup of rice is about 205 calories.
- Nuts and peanut butter as snacks, oats cooked in milk for breakfast.
Lift so the weight is muscle
A surplus without training adds mostly fat. Three full-body sessions a week with progressively heavier weights, plus about 1.6–2.2 g of protein per kg (protein to build muscle), turn the surplus into muscle. Keep the surplus modest: a lean bulk of 0.25–0.5% of your weight a week (calories to build muscle). Bulk or cut first? See bulking vs cutting. More on eating enough: how to gain weight in a healthy way.
When being thin is a medical question
See a doctor if you're losing weight without trying, if your BMI is under 18.5 and you can't gain, or if you have a racing heart, heat intolerance, digestive problems or tiredness. An overactive thyroid, coeliac disease and other conditions can make gaining weight hard, and they need treatment rather than more food.
How to tell if it's working
Weigh yourself a few times a week and compare weekly averages: you want about 0.25–0.5% of your weight a week. Daily weight swings with water and food (why your weight fluctuates), so judge after 2–3 weeks and add 200 calories if the trend is flat.
Sources
- Johnstone AM, Murison SD, Duncan JS, Rance KA, Speakman JR. Factors influencing variation in basal metabolic rate include fat-free mass, fat mass, age, and circulating thyroxine but not sex, circulating leptin, or triiodothyronine. Am J Clin Nutr 2005;82(5):941–948.
- Levine JA, Eberhardt NL, Jensen MD. Role of nonexercise activity thermogenesis in resistance to fat gain in humans. Science 1999;283(5399):212–214.
- U.S. Department of Agriculture, Agricultural Research Service. FoodData Central, SR Legacy (2018). Food IDs in the table notes.