The supplements with the strongest evidence for supporting weight gain are protein powders and creatine monohydrate, both backed by decades of research showing measurable increases in body mass when paired with resistance training. Other options like HMB and certain vitamins can play supporting roles, but no supplement replaces the fundamental requirement of eating more calories than you burn.
Protein Powder: The Foundation
Protein supplements are the most straightforward tool for weight gain because they make it easier to hit the daily intake your muscles need to grow. For people actively weight training, research consistently recommends 1.2 to 2.0 grams of protein per kilogram of body weight each day. For a 150-pound person, that translates to roughly 82 to 136 grams of protein daily. If you’re struggling to get that from food alone, a couple of shakes can close the gap without requiring you to eat another full meal.
Whey protein is the most popular choice because it digests quickly and delivers all nine essential amino acids your muscles need to rebuild after exercise. This matters more than most people realize. Research from King’s College London found that supplements containing all nine essential amino acids stimulate a muscle-building response roughly twice as strong as supplements with only branched-chain amino acids (BCAAs). So if you’re choosing between a BCAA supplement and a complete protein source like whey, the complete protein wins every time.
Casein protein, the other major protein found in milk, digests much more slowly than whey. After you drink whey, muscle protein synthesis peaks about 60 minutes later. With casein, that peak is delayed to around 120 minutes. This slow-release quality makes casein especially useful before bed. Studies show that consuming around 40 grams of casein about 30 minutes before sleep significantly increases amino acid levels in the blood overnight, keeping your body in a muscle-building state rather than a muscle-breaking-down state while you sleep. If you train in the evening, a casein shake before bed is one of the simplest upgrades you can make.
Creatine Monohydrate
Creatine is the most studied sports supplement in existence, and it reliably adds body weight through two mechanisms. First, creatine pulls water into your muscle cells. Because creatine enters muscles through a sodium-dependent transporter, the rapid increase in muscle creatine (especially during a loading phase) changes the osmotic balance inside cells, causing them to retain water. This can add noticeable weight within the first week.
In one controlled study, participants who took creatine for just seven days (without exercising) gained about 0.5 kg (roughly 1 pound) more lean body mass than the placebo group. Over the longer term, a broader review of healthy adults aged 18 to 47 found that creatine supplementation combined with resistance training increased lean body mass by about 1.1 kg (2.4 pounds) more than training alone. That initial water weight isn’t “fake” weight in any negative sense. Fuller, more hydrated muscle cells also appear to support better training performance, which leads to more actual muscle growth over time.
The standard approach is 3 to 5 grams of creatine monohydrate per day, taken consistently. Some people start with a loading phase of 20 grams per day for five to seven days to saturate their muscles faster, but this isn’t required. Daily low-dose supplementation reaches the same saturation point within a few weeks.
HMB for Preserving and Building Lean Mass
HMB is a compound your body naturally produces from the amino acid leucine, but in very small amounts. You’d need to consume about 60 grams of leucine (an impractical amount) to produce just 3 grams of HMB, which is why supplementation makes sense for people trying to gain weight.
In early research on weight-training adults, those taking 3 grams of HMB daily gained 1.2 kg of lean body mass over three weeks, compared to just 0.4 kg in the group taking nothing. The standard protocol used across studies is 1 gram taken three times per day, totaling 3 grams. HMB works primarily by reducing muscle protein breakdown, which means your body holds onto more of the muscle you build during training. This makes it particularly useful if you’re someone who tends to lose progress during rest days or periods when your calorie intake dips.
Mass Gainers and Calorie Density
Mass gainer shakes are simply protein powders blended with carbohydrates and fats to pack 500 to 1,200 calories into a single serving. They don’t contain any magic ingredient. Their value is purely practical: if you have a small appetite or a fast metabolism, drinking calories is significantly easier than chewing them. A single mass gainer shake can match the calorie content of an entire meal.
The quality varies widely between brands. Look for products where most of the protein comes from whey or casein rather than soy, and where the carbohydrate source isn’t just maltodextrin (a cheap sugar that spikes blood sugar rapidly). You can also make your own by blending whey protein with oats, peanut butter, banana, and whole milk, giving you more control over ingredients and cost.
Zinc and Appetite
If your main barrier to gaining weight is a lack of hunger, a zinc deficiency could be part of the problem. Zinc deficiency is well documented to cause anorexia (loss of appetite), growth problems, and taste disorders. Animal research has shown that oral zinc rapidly stimulates food intake by activating hunger-signaling pathways in the brain, specifically increasing the expression of neuropeptide Y and orexin, two chemicals that drive the urge to eat.
This doesn’t mean megadosing zinc will turn you into an eating machine. The appetite-stimulating effect is most pronounced in people who are actually deficient. If you suspect low zinc levels (common signs include poor appetite, frequent colds, slow wound healing, and dulled taste), a standard zinc supplement of 15 to 30 mg per day can help restore normal hunger signals. Foods rich in zinc include red meat, oysters, pumpkin seeds, and chickpeas.
Herbal Appetite Stimulants
Gentian root is one of the few herbal options with a long tradition of use as an appetite stimulant, and its mechanism is straightforward. Bitter compounds in the root, particularly gentiopicrin and amarogentin, stimulate digestive secretions and increase the sensation of hunger. Gentian extracts are typically taken as a tincture before meals. You’ll find gentian sold in liquor stores as “bitters” for exactly this reason. While the evidence is largely traditional rather than from large clinical trials, many people who struggle with low appetite find bitter herbs helpful as a mealtime trigger.
What Doesn’t Work as Well as Marketed
BCAA supplements are one of the most overhyped products in the weight gain space. While they do contain three important amino acids (leucine, isoleucine, and valine), they lack the other six essential amino acids your muscles need. Research directly comparing the two found that complete protein sources produce double the muscle-building response of BCAAs alone. If you’re already taking whey or eating enough protein-rich food, adding BCAAs on top provides little additional benefit.
Fish oil is another supplement often recommended for muscle building, but the evidence in healthy young adults is weak. A controlled trial giving young men 5 grams of fish oil daily for eight weeks found no significant increase in muscle protein synthesis compared to a placebo, either at rest, after eating, or after exercise combined with eating. Some studies have shown benefits in elderly populations, where muscle maintenance becomes harder, but for younger adults actively trying to gain weight, fish oil isn’t a priority. It has other health benefits, but accelerating muscle growth isn’t reliably one of them.
Safety at Higher Intakes
A year-long randomized trial found that protein-enriched meal replacement plans did not adversely affect liver function, kidney function, or bone density in healthy adults, as long as protein intake stayed within the recommended range of 10 to 35 percent of total calories. No participants experienced severe adverse events. That said, the researchers cautioned against applying these results to extremely high protein diets popular with some athletes, particularly those that rely heavily on pure protein supplements or high-fat animal meats, which could introduce other risks.
Creatine monohydrate has an excellent safety profile in studies lasting up to five years, with no consistent evidence of kidney or liver damage in healthy individuals. People with pre-existing kidney conditions should approach high-protein and creatine regimens more cautiously. For everyone else, the main side effects of aggressive supplementation for weight gain tend to be digestive: bloating, gas, or discomfort, usually resolved by adjusting timing or splitting doses throughout the day.