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Being iron-deficient: It’s more common than you’d think. Recent studies have found that 1 in 3 adults in the U.S.1 have an iron deficiency and are not getting enough of this key mineral in their daily diet. This is concerning because iron is not only essential in keeping your energy levels up but also in producing hemoglobin—the protein in your blood that carries oxygen from your lungs to the rest of your body. Meaning if your levels dip too low, you might feel sluggish, weak, and just plain tired.
Experts in This Article
endocrinologist and advisor for Hey Freya
Registered dietitian and culinary nutritionist specializing in polycystic ovary syndrome (PCOS).
While you can somewhat replenish your iron levels by eating iron-rich foods (e.g., spinach, red meat, or whole wheat bread), most people who have a true deficiency will need to take iron supplements, most often in the form of iron pills. But choosing the right type of iron—whether it’s ferrous gluconate vs. ferrous sulfate—can be confusing.
For starters, not all iron supplements are created equal: Choosing a pill that’s not right for your specific needs could result in unpleasant side effects (like an upset stomach or constipation) or simply not getting the boost in iron you need. Plus, while both ferrous gluconate and ferrous sulfate are the most popular iron options on the market, there are key differences between the two to note.
Here, we tapped a primary-care physician and a registered dietitian to create the ultimate guide on ferrous gluconate vs. ferrous sulfate, including the pros and cons of each and which is best to take for a deficiency.
5 differences between ferrous gluconate vs. ferrous sulfate
From how much elemental iron each has to how effective they are at treating deficiencies, here are five differences between ferrous gluconate vs. ferrous sulfate to know before trying them out.
1. Ferrous sulfate has more elemental iron
Iron supplements with ferrous sulfate have more elemental iron than ferrous gluconate, which basically refers to the pure form of iron. “Ferrous sulfate contains about 20 percent elemental iron, so a 325-milligram (mg) tablet has around 65 mg of elemental iron,” Amber Wheeler, MD, an endocrinologist and advisor for Hey Freya, tells Well+Good. In contrast, “ferrous gluconate contains about 12 percent elemental iron, meaning that a 325-mg tablet offers about 38 mg of elemental iron.”
Dr. Wheeler also notes that because iron supplements can cause digestive issues, a smaller dose of elemental iron is sometimes recommended to lower the risk of symptoms like nausea and constipation.
2. Ferrous gluconate may be better absorbed by your body
While there’s not much current research on this, older hallmark studies (like this 1996 clinical trial from Minerva Ginecologica2) show that ferrous gluconate might be better absorbed in the body than ferrous sulfate, especially when taken in liquid form.
That said, “it does contain less elemental iron than ferrous sulfate, so more of the supplement may be needed,” says Meggie Connelly, MS, RD, LDN, a registered dietitian and owner of Be Balanced Nutrition Services.
3. Ferrous gluconate may be better for deficiencies
While there’s not much research showing the benefit for adults, “research3 into the absorption of ferrous gluconate vs. ferrous sulfate in toddlers shows that both are effective at preventing iron deficiency anemia,” says Connelly. More specifically, “ferrous gluconate increased hemoglobin and ferritin (a blood protein that contains iron) more than the individuals who supplemented with ferrous sulfate.”
Why is this important? As we’ve learned, iron is a key component of hemoglobin—the protein that helps carry oxygen throughout the body. The iron in hemoglobin binds to oxygen, allowing it to be transported to all your major organ systems. The ferritin is also important, as it helps store iron in the body and acts as a reserve to make sure there’s enough available when needed.
This balance is crucial because, without enough stored iron (ferritin), the body might not be able to make enough hemoglobin, which can lead to iron deficiency anemia. Because ferrous gluconate has been found to increase hemoglobin and ferritin levels more than ferrous sulfate in small clinical trials in children, it’s possible that it might be better at preventing and treating iron deficiencies.
4. Ferrous gluconate may be a better choice during pregnancy
The American Society for Nutrition notes that when you are pregnant, your iron needs increase tenfold. This is why it’s important to keep your iron levels strong throughout pregnancy with iron-rich foods and supplements.
While both types of iron supplements can increase your levels and decrease the risk of anemia during pregnancy, Connelly says ferrous gluconate may be a better choice when it comes to absorption and tolerance. But “it does have less elemental iron, so more (milligrams) of it may be needed,” she adds. It’s also often more expensive than ferrous sulfate, which can be a barrier for some pregnant people.
For those who are expecting, Connelly also recommends ferrous bisglycinate, which has been shown to increase iron levels better4 than other forms of iron. She says it tends to cause fewer side effects in pregnant people—like constipation and nausea.
TIP
If you’re pregnant and need more iron, talk to your healthcare provider to decide which form and dose is best for your needs.
5. Ferrous gluconate is easier on the stomach
“Ferrous gluconate is a bit better for the stomach due to having less elemental iron,” says Connelly. That said, if you’re not tolerating your current supplement, she recommends trying one with ferrous bisglycinate, which studies have found is absorbed well and gentler on the stomach.
That said, some research (like this 2013 review in Current Medical Research and Opinion5), has found that extended-release ferrous sulfate with mucoproteins (a protein that binds to iron and helps protect the stomach lining and reduce the risk of GI effects) is better tolerated than other forms. However, it’s typically poorly absorbed and not recommended for treating an iron deficiency.
How to take iron supplements so they’re most effective
For best absorption, Connelly suggests taking iron pills about 30 minutes before a meal. “Taking your iron with a good source of vitamin C6, like a supplement or orange juice, can also help with absorption,” she adds.
Even though it’s best to take iron on an empty stomach, Connelly knows that can exacerbate digestive woes. If this is the case for you “it’s okay to take your iron supplement with a small amount of food,” she says. Just be sure to avoid high-calcium foods (like dairy), high-fiber foods, and caffeine around when you take the supplement, as they can all impair iron absorption.
As far as how often to take iron supplements? A 2019 study found that for people with iron-deficiency anemia, pills were better absorbed when taken every other day7 instead of daily. (For context, this particular study, which used ferrous sulfate, found that taking iron supplements every other day led to significantly higher iron absorption than those taking it daily.)
And what about dosage? Dr. Wheeler says the U.S. Recommended Dietary Allowance (RDA) of elemental iron for healthy adults ranges from 7 to 27 milligrams per day, depending on your age and factors like pregnancy and lactation.
Some good supplements to try include Walgreen’s High-Potency Iron Ferrous Gluconate ($9) or NatureMade Iron Ferrous Sulfate ($13). And if your doctor thinks iron bisglycinate is best for you, look for Thorne Iron Bisgylcinate tablets ($13).
Side effects of taking iron supplements
Just like there are different types of iron supplements, there are two main types of iron8, too: heme and non-heme. Heme iron comes from animal sources and is typically better absorbed, while non-heme iron comes mostly from plant sources.
“First-line iron supplements are usually non-heme sourced and can cause several side effects, particularly if the dose is above the RDA,” says Dr. Wheeler. Some of the most common side effects of taking iron supplements include the following:
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