Researchers tested whether a special probiotic supplement could help underweight adults gain weight when combined with eating extra calories. Over 8 weeks, 95 underweight adults in Iran either took probiotics or a placebo while eating an extra 500 calories daily. The group taking probiotics gained more weight and had better appetite than the placebo group. This suggests that certain beneficial bacteria in the gut might help the body use food more effectively for weight gain, though more research is needed to confirm these findings work for everyone.

The Quick Take

  • What they studied: Whether a probiotic supplement (beneficial bacteria) combined with eating extra calories could help underweight adults gain weight more effectively
  • Who participated: 95 underweight adults aged 18-60 years from Mashhad, Iran. Participants were randomly assigned to receive either probiotics or a placebo (fake pill) while following a diet with 500 extra calories per day
  • Key finding: After 8 weeks, adults taking probiotics gained about 2.4 kg (5.3 pounds) compared to only 0.4 kg (0.9 pounds) in the placebo group. The probiotic group also reported feeling hungrier and ate more calories
  • What it means for you: If you’re underweight and struggling to gain weight despite eating more, probiotics might help your body use food better. However, this is one study in one location, so talk to your doctor before starting probiotics to make sure they’re right for your situation

The Research Details

This was a randomized controlled trial, which is one of the strongest types of scientific studies. Researchers divided 95 underweight adults into two groups randomly. One group received capsules containing three types of beneficial bacteria (probiotics) at a dose of 1 billion organisms per capsule, while the other group received identical-looking capsules with no active ingredients (placebo). Neither the participants nor the researchers knew who got which treatment until the study ended—this is called “double-blind” and helps prevent bias. Both groups followed the same diet plan that included 500 extra calories per day for 8 weeks.

The study measured several things: body weight, BMI (a measure of body size), how hungry people felt using simple rating scales, how many calories people actually ate, and even stool consistency. These measurements were taken at the beginning and end of the 8-week period.

This approach is important because it allows researchers to see if the probiotics themselves caused the weight gain, rather than other factors like diet changes alone. The double-blind design is especially valuable because it prevents people from expecting to gain weight (which could influence their eating) or researchers from unconsciously treating groups differently.

Understanding how gut bacteria affect weight gain is important because underweight adults face serious health problems including weakened immunity, higher risk of infections, and reduced quality of life. Current weight-gain strategies often rely only on eating more calories, which doesn’t work well for everyone. If probiotics can help, it offers a new tool to help underweight people gain weight more effectively and safely

This study has several strengths: it was randomized (reducing bias), double-blind (preventing expectations from affecting results), and placebo-controlled (showing the probiotics worked better than just eating more). The sample size of 95 people is reasonable for this type of study. However, the study was conducted in one location in Iran, so results might differ in other populations. The study lasted only 8 weeks, so we don’t know if benefits continue longer. The journal is peer-reviewed, meaning other experts checked the work before publication

What the Results Show

The probiotic group gained significantly more weight than the placebo group. On average, people taking probiotics gained 2.41 kg (about 5.3 pounds) over 8 weeks, while those taking placebo gained only 0.44 kg (about 1 pound). This difference was statistically significant, meaning it’s very unlikely to have happened by chance.

Body Mass Index (BMI), which measures whether someone is at a healthy weight for their height, also improved more in the probiotic group. The probiotic group’s BMI increased by about 0.87 points compared to 0.16 points in the placebo group.

Appetite ratings told an interesting story. People taking probiotics reported feeling significantly hungrier throughout the study compared to the placebo group. This increased appetite likely helped them eat more calories, which supported the weight gain.

Calorie intake also differed between groups. The probiotic group consumed more total calories during the study period, which makes sense given their increased appetite. This suggests the probiotics may have influenced how the body signals hunger, making it easier for underweight people to eat enough to gain weight.

The study also looked at stool consistency, which can indicate digestive health. While the abstract doesn’t provide detailed results on this measure, tracking digestive changes is important because probiotics are known to affect the digestive system. The researchers measured this to ensure the probiotics were working in the gut as intended and to check for any negative effects on digestion

Previous research has shown that gut bacteria influence how our bodies process food and regulate appetite. This study builds on that knowledge by testing whether specific probiotic strains could help underweight adults gain weight. The three bacterial strains used (Lacticaseibacillus rhamnosus GG, Lactobacillus acidophilus, and Lacticaseibacillus casei) have been studied before and are known to be safe. However, this appears to be one of the first studies specifically testing these probiotics for weight gain in underweight adults, making it a novel contribution to the field

Several limitations should be considered. First, the study only lasted 8 weeks, so we don’t know if the benefits continue beyond that timeframe or if they’re permanent. Second, all participants were from one city in Iran, so results might be different for people from other regions or ethnic backgrounds. Third, the study didn’t track what happened after people stopped taking probiotics—did they keep the weight on? Fourth, the study didn’t examine which specific probiotic strain was most effective, so we can’t say if all three bacteria contributed equally. Finally, 95 people is a moderate sample size; larger studies might reveal different results or identify which types of underweight people benefit most

The Bottom Line

If you’re underweight and struggling to gain weight despite eating more, probiotics combined with a calorie-surplus diet may help (moderate confidence level based on this single study). The most practical approach would be: (1) Eat 500 extra calories daily from nutritious foods, (2) Consider adding a probiotic supplement containing the strains tested in this study, (3) Give it at least 8 weeks to see results, (4) Track your weight weekly. However, always consult your doctor before starting probiotics, especially if you have digestive issues or take medications

This research is most relevant for underweight adults (BMI under 18.5) who want to gain weight and have struggled with traditional approaches. It may be particularly helpful for people with poor appetite or those recovering from illness. People with healthy weights should not use this approach. Those with certain digestive conditions, compromised immune systems, or taking specific medications should check with their doctor first before using probiotics

Based on this study, you might expect to see noticeable weight gain within 4-8 weeks of starting probiotics plus eating extra calories. However, individual results vary. Some people may see changes faster, while others might need longer. The study measured results at 8 weeks, so that’s a reasonable timeframe to evaluate whether this approach is working for you

Want to Apply This Research?

  • Log daily probiotic intake (yes/no) and weekly body weight measurements. Also track daily calorie intake and appetite ratings (1-10 scale) to see if appetite increases over time, which would indicate the probiotics are working
  • Set a daily reminder to take your probiotic supplement at the same time each day (consistency matters for probiotics). Pair this with a specific eating goal: add one high-calorie, nutritious snack daily (like nuts, avocado, or protein smoothies) to reach your 500-calorie surplus target
  • Create a simple weekly tracking dashboard showing: (1) probiotic adherence percentage, (2) weight trend line, (3) average appetite rating, (4) average daily calorie intake. Review this monthly to see if you’re on track. If weight isn’t increasing after 4 weeks, discuss with your doctor whether to continue or adjust the approach

This research summary is for educational purposes only and should not replace professional medical advice. Probiotics may not be appropriate for everyone, particularly those with compromised immune systems, severe digestive conditions, or those taking certain medications. Before starting any probiotic supplement or significantly changing your diet, consult with your healthcare provider or registered dietitian. Results from this single study may not apply to all populations. Individual responses to probiotics vary, and weight gain requires a comprehensive approach including adequate nutrition, medical evaluation, and professional guidance.