How to gain weight healthily
Add roughly 300 to 500 kcal a day to what you already eat[2], lift weights two or three times a week, and keep protein near the useful ceiling of about 1.6 g per kilogram of body weight a day[5]. Gaining faster than that mostly adds fat rather than muscle[3]. If the weight came off on its own, see a doctor before changing your food.
On this page
First make sure it is weight you are supposed to be gaining
Two different people ask this question. One has always been thin, eats what feels like plenty, and wants more muscle on a light frame. The other used to weigh more and does not know where the weight went. Only the first of those is a nutrition problem.
Weight that leaves on its own — especially alongside fatigue, a change in appetite, loose stools, night sweats or unusual thirst — is a reason to see a doctor before you change anything you eat. An overactive thyroid, poorly controlled diabetes and coeliac disease all present as “I eat normally and keep losing weight”, and no meal plan treats any of them. Health services call an adult underweight below a body mass index of 18.5[1]; if you are under that line, the first appointment is with a clinician, not with a bigger breakfast. What a BMI number does and does not tell you is covered in the guide on what BMI tells you, and what it doesn't.
How big the surplus should be
Gaining is the same arithmetic as losing, run in the other direction: eat slightly more than you burn, consistently, for long enough to matter. The figure health services suggest is an extra 300 to 500 kcal a day on top of what you normally eat[2] — a sandwich and a glass of milk, not a second dinner. If you do not know what your normal day already adds up to, the guide on how your daily calories are calculated is the starting point, because a surplus is defined against maintenance and nothing else.
The temptation is to double it and arrive sooner. The evidence says that mostly buys fat. When elite athletes were coached to gain at about 0.7% of body weight a week, they put on more total weight than athletes left to eat as they pleased, but their fat mass rose by 15% against 3% in the free-eating group, and the gain in lean mass — 1.7 kg against 1.2 kg — was not statistically different[3]. A review of surpluses across the 5 to 15% range reached the same conclusion: raising the surplus raises the rate at which fat accumulates, not the rate at which muscle is built[4].
So a slow gain is not a timid version of a fast one. It is a different result. If your weekly average weight is climbing at all and your waist is not, the surplus is the right size.
Without resistance training, a surplus is only a surplus
Extra energy does not choose where to go. What decides whether it is stored as muscle or as fat is whether the muscle has been given a reason to grow, and that reason is resistance training that gets harder over time. Two or three sessions a week covering the whole body is enough to start; the surplus is what makes the training productive, and the training is what makes the surplus worth eating.
Protein is the other half of that. Pooled data from 49 trials found that total intakes beyond roughly 1.6 g per kilogram of body weight a day produced no further gain in fat-free mass[5], which means the useful range is reachable from food — eggs, dairy, chicken, fish, meat, pulses — and does not require a shelf of supplements. The guide on how much protein you actually need walks through the arithmetic for your own body weight.
Eating more when your appetite will not cooperate
Most people who struggle to gain are not lazy about food; they fill up early. The fix is energy density rather than volume. Cook with more olive oil, tahina or ghee. Put tahina on fuul and labneh on bread. Keep dates, nuts and dried fruit where you will see them. Choose full-fat milk and laban over skimmed. Drink your extra energy when you cannot chew it — milk, a fruit-and-yoghurt smoothie, karkade with milk — because liquids leave the stomach faster than salad does.
Then change the order and the timing rather than the size of meals. Four or five smaller eating occasions beat three you cannot finish. Eat the protein and the starch before the soup and the salad. And treat the small addition as non-negotiable on the days you are busy, because the whole thing is decided by the average week, not by the big plate on Friday.
What to track, and how to tell muscle from fat
Track four things: the weekly average of your morning weight, your waist at the navel, what you are lifting, and how your clothes fit. Weight alone cannot tell you what you gained. Weight rising while the waist holds steady and the lifts go up is muscle. Weight and waist rising together means the surplus is too large — trim it, do not abandon it.
Counting the extra food is also the only honest way to know whether you ate more at all, and “I ate loads” is the most over-reported sentence in nutrition. Logging the day as you go is the whole job, and if typing it out is what stops you, a food photo calorie counter turns a plate into a logged meal in seconds.
Mistakes that cost months
Judging the plan in a fortnight. Eating a careful surplus on weekdays and resetting it at the weekend. Replacing real meals with mass-gainer powder, which crowds out food instead of adding to it. Training hard and eating at maintenance, which produces soreness and no mass. Eating well and not training, which produces fat. And reading a single morning's number as progress in either direction, when a salty dinner moves the scale more than a good week of eating does.
None of this is fast. A gain you can see in the mirror takes months of a small surplus, hard training and enough protein, and that is as good as the process gets for anyone.
Sources
- World Health Organization. Obesity: preventing and managing the global epidemic. WHO Technical Report Series 894, 2000 — BMI category thresholds. ↩
- National Health Service (UK). Advice for underweight adults. ↩
- Garthe I, Raastad T, Refsnes PE, Sundgot-Borgen J. Effect of nutritional intervention on body composition and performance in elite athletes. European Journal of Sport Science 2013;13(3):295–303. ↩
- Slater GJ, Dieter BP, Marsh DJ, Helms ER, Shaw G, Iraki J. Is an energy surplus required to maximize skeletal muscle hypertrophy associated with resistance training? Frontiers in Nutrition 2019;6:131. ↩
- Morton RW, Murphy KT, McKellar SR, et al. A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. British Journal of Sports Medicine 2018;52(6):376–384. ↩