Supplements for Ultrarunners: What the Science Says

The supplement aisle is full of big promises. Very few of them hold up. In 2018, a panel of experts for the International Olympic Committee (IOC) found good evidence for only a few supplements. They said supplements can play only a small part in an athlete's diet, and that you should get a full nutrition check before you start taking anything.¹

This guide covers the supplements ultrarunners ask about most: iron, protein, caffeine, vitamin D, and creatine. It also covers laxatives like MiraLAX. For each one, here is what the research shows and what to do about it.

The Short Version

Supplement	Worth It?	Bottom Line
Iron	Only if a test shows you're low	Get your ferritin checked. Don't take iron "just in case."
Protein	Yes, mostly from food	1.6 to 2.1 g per kg per day. Powder is optional.
Caffeine	Yes, with a plan	3 to 6 mg per kg before, smaller doses late in a race.
Vitamin D	Only if a test shows you're low	Low levels are common in winter. Extra doses don't help if your level is fine.
Creatine	Optional	Helps strength work. No endurance benefit in trained athletes.
Laxatives	No, it's a medicine	Talk to a doctor. Never try one before a race.

Iron: Test Before You Take It

Iron helps your blood carry oxygen. Low iron can leave you tired and slow. It is common in runners, and much more common in women. In a study of 170 recreational marathon runners, 28 percent of the women had low iron stores. Fewer than 2 percent of the men did.² A review of athlete studies found low iron in about 15 to 35 percent of female athletes and about 5 to 11 percent of male athletes.³ Even among elite runners whose iron was being watched and treated, more than half of the women and almost a third of the men had low iron at least once.⁴

Why runners lose iron

Hard training makes iron harder to absorb. After a hard one-hour run, a hormone called hepcidin rose to 1.7 to 3.1 times its normal level for up to a day.⁵ Hepcidin blocks your gut from taking in iron. Running also breaks down some red blood cells. In a study of 10 triathletes, signs of red blood cell breakdown rose about four times more after running than after cycling, mostly from the pounding of each foot strike.⁶ Some iron is also lost through the gut, and a little in sweat. One guideline says the gut matters more than sweat.⁷

Get the right test

Ask your doctor to check ferritin (your iron stores) and hemoglobin. Other tests, like transferrin saturation, can help if the results are unclear.⁷ Ferritin drops first, before you become anemic.⁸ Experts use different cutoffs for "low." One Swiss guideline calls ferritin under 15 ng/mL empty and 15 to 30 ng/mL low.⁷ Another review calls anything under 35 ng/mL the first stage of iron loss.⁸ Your doctor can tell you what your number means for you.

Timing matters. Ferritin rises after hard efforts. After an ultra, it can double, and it takes about 6 days to come back down.⁷ Don't get tested in the week after a big race, or your iron may look better than it is.

Does iron make you faster?

Only if you're low. A review that pooled 17 studies of endurance athletes with low iron, but not anemia, found that iron treatment gave a moderate boost to VO2 max.⁹ Another review of 12 studies had mixed results. Every study that found a benefit treated athletes with ferritin of 20 ng/mL or less.¹⁰ These studies only looked at athletes who were low, so there's no evidence that extra iron helps runners whose iron is already normal.

Why not take it just in case?

Your body has no way to get rid of extra iron.⁷ In the marathon study, 15 percent of the men already had too much stored iron. The authors said runners should take iron only if a test shows they need it.² Iron pills can also upset your stomach. In a review of trials, people taking ferrous sulfate had about twice the odds of stomach side effects as people taking a placebo.¹¹ The IOC says high-dose iron should not be taken unless you are low.¹

If your doctor says you're low

• Eat iron-rich food. One guideline suggests meat, poultry, or fish at least 5 times a week, plus beans and green vegetables.⁷
• Pair it right. Vitamin C helps you absorb iron. Coffee, black tea, and calcium make it harder.⁷ Have orange juice with an iron-rich meal, and save your coffee for later.
• Follow your doctor's dose. The same guideline suggests 40 to 60 mg of iron once a day, with a retest after 6 to 8 weeks.⁷ In a study of women with low iron who were not athletes, taking iron every other day led to better absorption than taking it every day.¹² Ask your doctor which plan fits you.
• Morning may be better. In a study of 16 runners with ferritin under 50 ng/mL, they absorbed more iron from a breakfast eaten 30 minutes after a morning run than from breakfast on a rest day or dinner later that day.¹³

Protein: How Much Do Ultrarunners Need?

The International Society of Sports Nutrition (ISSN) has a position stand written just for ultrarunners. It says about 1.6 to 2.1 grams of protein per kilogram of body weight per day is enough to build and repair muscle.¹⁴ In your hardest training weeks, or if you aren't eating enough, you may need up to 2.5 grams.¹⁴ Below 1.6, runners in heavy training may break down more protein than they build.¹⁴

A lab study backs this up. In six trained men tested on the day of a 20 km run, the average need was about 1.65 grams per kilogram, and the safe target was 1.83 grams.¹⁵ Both are about twice the 0.8 grams per kilogram that the standard guideline suggests for most adults.¹⁵

Spread it out

Aim for about 20 to 40 grams of protein every 3 to 4 hours instead of one huge meal.¹⁶ The ultra position stand suggests about 20 grams every 3 waking hours, or up to 30 grams for runners over about 85 kg (187 pounds).¹⁴ In a study of 48 endurance-trained young men, 30 grams after a hard session was enough. More protein than that didn't add more muscle building.¹⁷

Don't stress about a short "window" after your run. When researchers accounted for total daily protein, the timing of protein around workouts made no real difference. That research was on weight training.¹⁸ Getting enough protein every day matters more.¹⁶

Food or powder?

Active people can get all the protein they need from food.¹⁹ Powder is a handy option, not a must. If you use one, the ultra position stand points to whey, because your body absorbs it well and it has all the amino acids you need.¹⁴ During a race, the ISSN suggests 5 to 10 grams of protein per hour along with your carbs, but the evidence for protein during ultras is weak and mixed.¹⁴

Is it safe? In a review of 28 trials with more than 1,300 healthy adults, high-protein diets did not harm kidney function.²⁰ If you have kidney disease, talk to your doctor before you eat more protein.²⁰

Caffeine: The Best-Proven Boost

Caffeine has some of the strongest evidence of any supplement. It improves performance at 3 to 6 mg per kilogram of body weight, and endurance exercise shows the most steady benefit.²¹ The most common timing is about 60 minutes before you start.²¹ Lower doses can work too.^1,21

Save some for late in the race

For ultras, the ultra position stand says to aim your caffeine at the later part of the race. That matters most in races over 24 hours, when lack of sleep starts to hurt your running and your safety.¹⁴ It suggests smaller repeat doses of 1 to 2 mg/kg (about 70 to 140 mg for a 154-pound runner), or about 50 mg per hour, mostly for running through the night.¹⁴ Caffeine may help some people stay sharp when they are short on sleep.^21,22 Very little research has looked at caffeine during ultras, so these numbers come mostly from expert advice.¹⁴

Coffee drinkers still get a boost. Research has not found a steady difference between people who have caffeine every day and people who don't.²¹

More isn't better. Very high doses, around 9 mg/kg, cause side effects like nausea, anxiety, trouble sleeping, and restlessness, with no added benefit.^1,21 Caffeine can also upset your stomach or give you a headache during a race.¹⁴ It stays in your body for about 4 to 6 hours on average, so a late dose can keep you up after you finish.²¹ Test your plan in training, and never try a new caffeine product on race day.¹

Vitamin D: Test If You Train Indoors or in Winter

Low vitamin D is common in athletes. A review of 23 studies with more than 2,300 athletes found that 56 percent had low levels.²³ Low levels were more common in winter and spring, in indoor sports, and farther north.²³ Athletes with darker skin also often test low, though experts say that may not always cause problems.²⁴

Does it help? Only if you're low. Raising vitamin D far above a healthy level does not boost performance.²⁴ In a study of more than 5,000 female Navy recruits, taking calcium and vitamin D together during 8 weeks of basic training led to about 20 percent fewer stress fractures.²⁵ That study tested the two together, in recruits rather than runners.

Get tested. The blood test is called 25(OH)D. A U.S. expert panel says a level of at least 20 ng/mL meets the needs of most people, and levels above 50 ng/mL raise concerns about harm.²⁶ Experts don't agree on the best level for athletes.¹ The panel set the safe upper limit at 4,000 IU a day for adults.²⁶ For most people, the IOC says 1,000 to 2,000 IU a day keeps levels healthy. There is no set dose for athletes yet.¹ More is not better, so let your doctor set the dose.

Creatine: Great in the Gym, Not for Race Day

Creatine is one of the best-studied supplements. It helps with strength, power, and short, repeated hard efforts, and it can lead to bigger gains in muscle and strength from training.^1,27 Endurance is a different story. A review of 13 studies in trained athletes found that creatine had no effect on endurance performance.²⁸

There's also a weight problem. Loading up on creatine can add 1 to 2 kg (2 to 4 pounds), mostly water. The IOC says that extra weight may hurt endurance performance.¹ Most of the extra water shows up in the first few days.²⁹

Where it might fit: creatine can support your strength training, and it may help with recovery.²⁷ In a study of marathoners, taking creatine for 5 days before a 30 km race lowered blood markers of muscle damage and inflammation. The study didn't test whether they ran faster or recovered sooner.³¹

If you take it: use plain creatine monohydrate. No other form has been shown to work better.²⁹ The usual dose is 3 to 5 grams a day.¹ You don't need to load. Taking 3 grams a day raised muscle creatine as much as a loading phase did, just more slowly, over about 4 weeks.³⁰ In healthy people, creatine has not been shown to harm the kidneys or cause cramps or dehydration.²⁹

What About Laxatives Like MiraLAX?

MiraLAX is an over-the-counter medicine for constipation, not a performance supplement. Gut trouble in ultras is common, and some of it runs the other way. In a study of 272 runners at Western States, 96 percent had stomach or gut problems during the race, and an urge to use the bathroom and diarrhea were among them.³² A review found that 30 to 50 percent of athletes have gut problems during exercise, and runners have them more often than cyclists.³³ Taking a laxative before a long run or race could make those problems worse.

If you get backed up during heavy training or travel, start simple. A review from the American Gastroenterological Association says to add fiber slowly, and to wait 7 to 10 days before changing the amount.³⁴ Drinking extra water only helps if you are dehydrated, and being more active is linked to less constipation.³⁴ In the days before a race, cut back on high-fiber foods.³³ Our gut training guide covers race week.

How to Buy Supplements Safely

Supplements aren't always what the label says. In a study of 634 supplements bought in 13 countries, about 15 percent contained steroids that weren't listed on the label. Those products were bought in 2000 and 2001.³⁵ A review of 23 studies found contamination rates from 12 to 58 percent.³⁶ Few of these studies tested a random mix of products, so no one knows the true rate.¹

Look for products tested by NSF Certified for Sport or Informed Sport. These programs lower your risk, but nothing can promise a product is 100 percent clean.^1,36 This matters most if you race in events with drug testing. Try any new supplement in training first, never on race day.¹

The Bottom Line

• Iron: get a ferritin test, especially if you're a woman or always feel tired. Take iron only if your doctor says you're low.
• Protein: eat 1.6 to 2.1 g per kg a day, spread across your meals. Food first, powder if it helps.
• Caffeine: worth it. Plan your doses, and save some for late in the race.
• Vitamin D: get tested, especially in winter. Take it only if you're low.
• Creatine: optional. It can help your gym work, but the research doesn't show it helps endurance racing.
• Laxatives: talk to a doctor. Keep them out of your race plan.