I can't put on weight. What should I try, and when is it a doctor's question?

Three pages in this library bear on this question, and only one is about it directly. Write down a week of food when the weight won’t rise is the page for an adult who eats what feels like a lot, lifts or means to, and sees the scale stay where it was. Rest days are part of the program was written about training, not about gaining, and Calories per dollar about the weeks when money limits the food. Each is used here only for what it says.

If your weight will not rise, write down everything you eat and drink for a week and compare it with what you thought you were eating; if it is clearly more and the weight still does not move, see a doctor. If you have lost weight suddenly or without trying, have had an eating disorder, or counting food or eating more makes you anxious, skip the list and start with a doctor; the numbers are just below. Many people say the arithmetic cannot be dodged: weight goes up only when you take in more than you burn, and what feels like a lot of food is often less than your body needs. One person says people who struggle to gain often underestimate what they need, and that the intake has to be written down rather than guessed. Some people point out that lifting widens the gap, because training raises what you burn, so lifting on your usual diet can leave you flat or lighter.

Write each thing down when you eat it, not at night from memory, on paper or in a free app. One person notes that most calorie-tracking apps charge, and that free ones exist. Weigh yourself on the first morning and the last, at the same time of day. If your weight stayed about the same across the week, what you wrote down is about what your body burns on your current routine. If the list shows less than you believed, you have found the gap, and it costs nothing to fix. If it shows a lot and the scale is still flat, keep the list: it is exactly what a doctor will want to see. If none of the doctor-first lines below is true for you, the NHS says adults could try gaining weight gradually by adding around 300 to 500 extra calories a day to what they eat now, with smaller meals more often, healthy snacks in between, and high-calorie drinks such as milkshakes between meals. The page gives no target of its own, because the one figure people give, 3,000 calories or more a day, comes from one person, and the right amount depends on your health, your size and what you already eat.

This is for adults, and some readers should start with a doctor and not a list. If you have lost weight suddenly, or lost weight without changing what you eat or how you move, see a doctor before you start any plan: the NHS says to speak to a GP in both cases, and also if you use control of food to cope with feelings. If you have had an eating disorder, or counting food or eating more makes you anxious, a doctor comes first and the tracking does not suit you. In the UK, the Beat helpline is free on 0808 801 0677 (England; 3pm to 8pm, Monday to Friday); in the US, the ANAD helpline is on (888) 375-7767 (Monday to Friday, 9am to 9pm Central). If you are in crisis, call or text 988 in the US or call Samaritans on 116 123 in the UK. If eating more makes you vomit, stop pushing and see a doctor instead of forcing it; vomiting can have a medical cause or be a sign of an eating disorder, and the helplines above are for that. If you are here because someone told you your body is wrong, this is the wrong tool: some people warn that forcing yourself to eat or train only to win a partner’s approval can make anxiety and self-esteem worse.

Getting the extra in. Some people suggest cheap, dense staples such as oats, eggs, whole milk, peanut butter, rice and potatoes over pricier “health foods”. One person adds that dry beans bought in bulk and cooked in a big pot are cheap and filling, and another that a slow cooker turns tough cheap meat into food that needs little effort. One person suggests cooking big batches and freezing portions, so there is always something ready to eat when you are hungry. Prices cut both ways: one person notes that bulk buying needs spare money up front, and another that ground beef prices swing a lot. One person argues that fast food beat ingredients for calories per dollar, and another argues that potatoes, oats and milk win on cost and on health. Local prices decide that one. If solid food fills you up too soon, some people suggest drinks and extra fat: milk, home-made shakes, a drizzle of oil on a meal. On sugary food, two people split. One says sugar is fine because the goal is calories. Another says sugary junk is poor advice because you want lean weight, not diabetes risk. The NHS talks about adding healthy calories. Two small warnings: one person says a sudden pile of beans can bring a lot of wind, so start with small portions, and one person says lactose intolerance may call for a dairy alternative or a lactase supplement.

If money is what limits the food, price it once. One person answered it with a spreadsheet: calories per dollar and protein per dollar for the foods at a store near them. What that arithmetic reliably finds is the old, boring, worldwide answer: rice, beans, oats, eggs, flour, potatoes, a little oil. Calories per dollar carries three warnings with the formula. It ignores whether food is healthy. It ignores nutrients entirely, so fruit and vegetables score low and still matter. And calorie-dense is not the same as filling, so the cheapest calories can cost you back in hunger. It is a survival tool for the thinnest weeks, never a diet, and the page says that if the thin weeks have stopped being occasional, the food-help doors are the real fix; the answer on eating well on almost no money covers food banks and pantries.

Lifting, so the weight is not only fat. Many people say that to make the extra weight muscle rather than fat you need resistance training alongside the extra food, with big lifts or slowly adding load. The NHS says strength training builds muscle and can improve your appetite. For adults, the NHS and the CDC both set strengthening activity on at least 2 days a week. One person says 2 to 3 sessions a week is enough for a beginner; another says 3 to 4 sessions on the big lifts. One person says eating lots of carbohydrate without moving leads to extra fat rather than muscle, so some movement matters, and one person says poor sleep is often overlooked next to food and training. Some people warn that bad technique hurts you, and say to ask for help or watch instructional videos before the weight gets heavy. One person warns that training too hard or too often as a beginner leads to quick burnout, and one that hard cardio and group classes can leave a new lifter vomiting or miserable enough to quit. You do not need a gym to start: some people say a mat, a pair of dumbbells and resistance bands are enough at the start, and one person suggests a backpack of books or milk jugs as weights, while another warns a backpack of books may strain the back and neck.

Rest days are part of the program is the counterweight to training every day. One person expected daily workouts to speed everything up and found exhaustion instead; they cut to four set days a week, kept the off days for stretching and easy walks, and got stronger. The page puts it that training is the request and recovery is the construction: work a muscle and you have torn it down a little, and the growing back stronger happens between sessions. A martial artist on the same page adds a checklist: scale the exercise to what you can do with full, clean form, rotate what you work so each muscle group gets its recovery day, and measure against yesterday’s you and not the strongest person in the room. The page puts the off days on the calendar with the same seriousness as the workouts, and warns that rest is recovery and not an exit ramp (the people behind it still trained most days of the week): if a rest day keeps stretching into a rest week, the fix is the calendar, and soreness that feels like injury rather than effort deserves an actual pause.

When you lift, stop if you get chest pain or tightness, pain that spreads to your arm, neck, jaw or back, shortness of breath, or you feel faint or light-headed with a cold sweat: these can be signs of a heart attack, and the list is longer (feeling or being sick is another). Call 911 in the US or 999 in the UK (your local emergency number elsewhere) and do not drive yourself; the NHS says not to drive yourself to A&E, and the CDC says to call 911 right away.

If the list clearly shows you eat plenty and the weight still does not move, that is a doctor’s question. Some people say that if weight will not rise despite a high intake, or you are bloated and very thin, a doctor should look for a medical cause, such as malnutrition, a digestive-enzyme problem or a hormone imbalance. Some name an overactive thyroid; one person adds vitamin deficiencies and another diabetes. Some people say a check-up, with blood tests, should come before assuming the problem is only food. One person puts the wait at a month of clearly eating more with no gain before seeing a specialist. Another person’s view is that it is a solvable matter of eating enough and tracking it. Both can be true of different people; a week or more of written food, plus a check-up, is how you tell which one you are. If cost is the wall in the US, the clinic finder from the Health Resources and Services Administration lists health centers near you that give medical, dental and mental health care on a sliding scale based on what you can pay.

The US Centers for Disease Control and Prevention puts an adult BMI under 18.5 in the underweight range and calls BMI a screening measure that should be read with other factors, not a verdict on its own. One person points out that BMI can misjudge people with a lot of muscle or a large frame, and a doctor can tell you what your own number means. Some people say gaining is slow, and that genetics changes how quickly you fill out. One person says a beginner may see change in the first three months and more after six. One person says some people have a naturally fast metabolism or a small frame and may not reach “normal” weights easily; if that sounds like you, a doctor can say what a good target is for your body.

Who this is not for. If you are at a healthy weight or heavier, “eat more” may be the wrong direction for you. Advice given to a heavier person asking how to build muscle did not agree: one argued for eating less first, one that the first year of lifting should be a bulk because you have little muscle, one for just eating well and deciding after a few months. A doctor is the one to ask. Apart from the NHS figure above, this answer gives no protein target, calorie total or meal plan, because people gave different ones and the right amount depends on your health; a doctor or a dietitian can tell you. It does not weigh supplements or protein powders either. The NHS advice for adding protein is ordinary food: beans, pulses, fish, eggs and lean meat.