Feeling drained? Find out which iron supplements can help recharge your energy.
Reviewed by Dietitian Katey Davidson, M.Sc.FN, RD, CPT
Key Points
- Ferrous sulfate, gluconate, fumarate and bisglycinate are top iron options for better energy.
- Ferrous bisglycinate may be easier on the stomach than traditional iron salts.
- Get your iron levels tested before starting a supplement.
Running low on energy can make even the simplest everyday tasks feel more challenging. While fatigue can have many possible causes, low iron is one common culprit. Iron plays an important role in the body’s energy production, so not getting enough can leave you feeling more tired than usual.
If your health care provider has recently recommended an iron supplement, you may be wondering which type to buy for the best results. With so many forms and doses lining supplement shelves, knowing which to choose can feel overwhelming. Fortunately, we spoke with dietitians to break down the best types of iron for improving low energy and what to look for when choosing the ideal iron supplement.
Iron & Energy
Your body needs oxygen to turn food into energy, and iron helps make that possible. “Iron is essentially the delivery system for oxygen in your body. It’s a core component of hemoglobin, the protein in red blood cells that carries oxygen from your lungs to every tissue,” says Vanessa Rissetto, M.S., RD, CDN. She explains that it’s also required by enzymes inside your cells’ mitochondria—the tiny powerhouses responsible for converting nutrients into usable energy.
“So, when iron is low, you’re getting hit twice: less oxygen delivery and less efficient energy production, which is why fatigue is usually the first thing people notice.” Other common symptoms of iron deficiency include cold hands and feet, shortness of breath, exercise intolerance, brain fog, restless legs and unusual cravings for nonfood items like ice. Physical signs may also include pale skin, hair loss, brittle nails and a sore or swollen tongue. Rissetto notes that these symptoms are more common than people realize, with an estimated 14% of U.S. adults experiencing absolute iron deficiency, particularly young women.
The Best Types of Iron
Not all forms of iron are created equally. While iron in food is found as heme or non-heme iron, supplements contain specific forms such as ferrous and ferric iron. The best-absorbed iron supplements are generally the ferrous salts—ferrous sulfate, gluconate and fumarate—as well as ferrous bisglycinate, says Hannah Van Ark, M.S., RD.
Ferrous Sulfate
Ferrous sulfate is often considered the classic first-line treatment for low iron levels because it’s inexpensive, well absorbed and supported by decades of research, says Rissetto. In fact, studies show that supplementing with ferrous sulfate can effectively improve hemoglobin levels and replenish iron stores in people with iron deficiency.
However, that effectiveness doesn’t come without uncomfortable caveats. “The tradeoff is tolerability: nausea, constipation and stomach upset are common enough that many people quit taking it, and a supplement you don’t take doesn’t work,” warns Rissetto. Van Ark explains that these side effects tend to be more pronounced at higher doses because a lot of iron hits your gut at once, and any that isn’t absorbed can irritate the digestive tract.
Ferrous Gluconate
Ferrous gluconate is another ferrous form of iron that is well absorbed and commonly used to treat low energy associated with iron deficiency. However, like ferrous sulfate, it can cause digestive discomfort, nausea and constipation at high doses. Still, it can be an effective option for those who can tolerate it well.
Ferrous Fumarate
Ferrous fumarate is also an easily absorbed ferrous salt, providing the highest amount of elemental iron per dose. But it shares many of the same digestive side effects as the other traditional iron salts when taken in large doses. Choosing between ferrous sulfate, gluconate and fumarate often depends on the amount of elemental iron needed and how well a person tolerates the specific form.
Ferrous Bisglycinate
For those who experience digestive side effects from traditional iron salts, ferrous bisglycinate may be a gentler option. Rissetto highlights that this chelated form binds iron to an amino acid, often glycine, which makes it easier on the gut while still allowing the body to absorb the iron effectively.
“Ferrous bisglycinate has demonstrated roughly two-fold higher bioavailability compared to conventional iron salts like ferrous sulfate and ferrous fumarate,” she explains. “In a 2022 randomized controlled trial, it was associated with significantly fewer reports of nausea, abdominal pain, bloating, constipation and metallic taste.” This combination of improved absorption and tolerability makes it the most appealing choice for those who have trouble taking standard iron supplements.
Dosing
Along with choosing the right type of iron, the dose also matters. “For confirmed iron deficiency, a typical starting range is about 30 to 65 milligrams of elemental iron—and ‘elemental’ is the key word, because it’s the actual amount of usable iron, not the total compound weight on the front of the bottle,” says Rissetto.
When it comes to how often to take it, “every-other-day dosing is becoming more popular, since high daily doses increase side effects and raise hepcidin, a hormone that actually blocks iron absorption and makes supplementation less effective,” Van Ark explains. Rissetto points to research that shows meaningfully higher iron absorption with alternate-day dosing (21.8%) compared to daily dosing (16.3%). Other studies have found similar results, with fewer gastrointestinal side effects and improvements in iron deficiency.
However, both dietitians recommend getting your iron levels tested before starting a supplement. This is because low energy and fatigue can stem from many causes, and unnecessary iron supplementation can be harmful. “A simple ferritin and CBC panel tells you whether iron is actually the problem and how aggressive the dosing needs to be, and repeat labs at 8 to 12 weeks tell you when to stop,” Rissetto advises.
Safety & Precautions
“Before taking an iron supplement, the most important thing to know is that iron isn’t easily excreted once it’s absorbed, so too much can build up and become toxic to your organs,” Van Ark cautions. Symptoms of iron toxicity start with digestive disturbances and can progress to more serious problems, including seizures, abnormal heart rhythms, dangerous acid buildup and low oxygen levels. Therefore, you should always consult a health care provider to determine the right dose for your individualized needs.
Iron supplements can also interact with certain medications. “Interactions are where people get tripped up: iron blocks absorption of thyroid medication (levothyroxine) and certain antibiotics, so those need to be separated by several hours,” says Rissetto. She also points out that acid-reducing medications, such as proton pump inhibitors (PPIs), can lower iron absorption over time. Similarly, calcium—whether from supplements, dairy or fortified foods—competes with iron for absorption, so she recommends separating them by at least two hours.
Tips for Choosing an Iron Supplement
When choosing the right iron supplement to support your energy levels, keep these dietitian-approved tips in mind:
- Check the Dose. “Flip the bottle over and find the elemental iron per serving in the Supplement Facts panel—that’s the number that matters,” says Rissetto. Your individual needs will vary, so follow the dose recommended by your health care provider.
- Look for Independent Testing. Rissetto recommends keeping an eye out for independent verification symbols like USP or NSF. This is because supplements aren’t required to undergo FDA approval for safety and effectiveness before they’re marketed, and independent testing can help verify that a product contains what its label says.
- Choose the Form Intentionally. For those with sensitive stomachs, both dietitians recommend opting for ferrous bisglycinate. If cost is a priority, ferrous sulfate is a more affordable option, provided you can tolerate it.
- Consider the Supplement Formula. Rissetto suggests skipping heavily enteric-coated and “slow-release” formulas, as these can carry the iron past the part of the small intestine where it’s best absorbed. She also recommends steering clear of proprietary “energy blends” that bundle iron with other ingredients at undisclosed doses.
Our Expert Take
When low iron is the cause of your fatigue, correcting the deficiency with a supplement can help you regain the energy you’ve been missing. But choosing the right supplement involves more than grabbing the first one you see on the shelf. The best forms of iron for low energy include ferrous sulfate, gluconate, fumarate and bisglycinate, with the best choice depending on factors such as dose, tolerability and individual needs. However, always consult a health care professional before beginning supplementation to determine the right type and dose for your unique needs.