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New year, new supplement routine? If you’ve started 2018 with a diet or exercise plan that involves vitamins, herbs, shakes, or pills, the National Institutes of Health (NIH) has a message for you: Check the research first. Not all weight-loss supplements or fitness supplements have been shown to work, experts say, and some may even be dangerous.

To help out curious consumers, the NIH’s Office of Dietary Supplements (ODS) has created two easy-to-use factsheets. Available on the ODS website, the factsheets lay out the effectiveness and safety of many different supplement products, and the ingredients within them.

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“It’s January and people are making resolutions about their overall health and weight and fitness,” says Paul Thomas, nutrition scientist for the ODS and author of the new factsheets. “We thought it was a nice time to emphasize that we have these factsheets that may be useful for people who are considering whether or not these supplements may be of value for achieving their goals.”

One of the factsheets, titled Dietary Supplements for Exercise and Athletic Performance, covers more than 20 common supplement ingredients that have been purported to improve strength or endurance, increase exercise efficiency, achieve a fitness goal more quickly, or increase tolerance for intense training. Products containing these ingredients are sometimes called ergogenic aids, and are often sold in drugstores, organic food stores, and fitness clubs.

Research shows that some of these ingredients may be helpful in specific circumstances. Creatine, for example, might help with short bursts of high-intensity activity (like weight lifting), but not for endurance efforts (like distance running). Drinking beetroot juice might improve aerobic exercise performance, but it’s not known whether supplements containing beetroot powder have the same effect.

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Caffeine is another ingredient that gets a cautious thumbs up. “Sports-medicine experts agree that caffeine can help you exercise at the same intensity level for longer and reduce feelings of fatigue,” the factsheet states. Intake of up to 400 or 500 mg a day is safe for most adults, and experts recommend taking 2 to 6 milligrams per kilogram of body weight, 15 to 60 minutes before exercise.

For other ingredients, there’s limited or no science to back up their fitness-related claims. Antioxidants, for example, may be good for overall health, but no studies have found that they play a role in athletic performance. And ingredients like Tribulus terrestris and deer antler velvet—marketed for muscle building and male virility—haven’t even been studied enough to know whether they’re truly safe.

“You may be surprised to learn that makers of performance supplements usually don’t carry out studies in people to find out whether their products really work and are safe,” the factsheet states. And when human studies do happen, they often include only a small number of young and healthy people—often just men—for only a few days or weeks.

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Other ingredients on the factsheet include tart cherry juice, amino acids, ginseng, iron, and protein—which you’re probably already eating enough of, the evidence suggests. The bottom line? Only a few of these have been shown to really improve exercise and athletic performance, the NIH says, and even those should only be used by athletes who are already eating a good diet and training properly.

The other new factsheet includes supplements marketed for weight loss—a category Americans spend more than $2 billion a year on. “Sellers of these supplements might claim that their products help you lose weight by blocking the absorption of fat or carbohydrates, curbing your appetite, or speeding up your metabolism,” the factsheet states. But there’s little evidence that they work, say Thomas. Plus, they can be expensive and can interact with prescription drugs, and some may even be harmful.

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A few ingredients on the factsheet, like chromium, might help you lose a very small amount of body fat, studies show. Drinking green tea is also safe for most people, and may help them lose a small amount of weight—although green-tea extract pills have been linked to liver damage. (Drinking a lot of green tea can also interact with statin medications, a recent study found.)

But some, like bitter orange, raspberry ketone, and hoodia, have not been studied enough to prove that they’re safe. Others, like beta-glucans and garcinia, seem to be safe when taken as directed, but have not been shown to have any effect on weight loss. Many of these ingredients can also cause unpleasant side effects, like flatulence or diarrhea, nausea or vomiting, headache, and stomach cramps.

The NIH doesn’t want people to stop taking supplements entirely, and Thomas says that supplements, for the most part, are safe to take as directed. But he does want to remind consumers of the uncertainty that exists in the industry, and stresses that pills and powders are not a magic solution to getting in shape.

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“These supplements cannot and are not even marketed to take the place of the standard types of things we know we should be doing for our overall health, fitness, and weight loss,” he says. “Those include eating a very good diet and getting regular physical activity. The question becomes whether any of these supplements possibly add to that, or could possibly be an additional kind of aid.”

If you’re thinking about taking a performance or weight-loss supplement, the factsheet states, talk to your doctor. Talking to a health expert is especially important if you’re a teenager, have any medical conditions, or are currently taking any medications or other supplements.