Thorne · 60 capsules · $16.99
Iron is essential to the formation and function of healthy red blood cells.* An adequate amount of dietary iron is required for blood cells to efficiently transport oxygen throughout the body. According to the U.S. Centers for Disease Control (CDC), up to 16 percent of individuals in some populations is deficient in iron.
Individuals who can be prone to iron deficiency include:
Signs of iron deficiency:
Iron is difficult for the body to absorb, and supplementing with conventional forms of iron, including ferrous sulfate, can cause gastrointestinal side effects – such as nausea, vomiting, constipation, diarrhea, and dark-colored stools. Thorne's Iron Bisglycinate contains iron reacted with glycine to optimize its absorption in the gastrointestinal tract, thus minimizing the typical side effects of iron supplementation.*
Features of Thorne’s Iron Bisglycinate:
Because an athlete needs to know that his or her supplements are trustworthy and compliant, every batch of an NSF Certified for Sport® product is tested for compliance with label claims and to ensure the absence of more than 200 substances banned by many major athletic organizations, including stimulants, narcotics, steroids, diuretics, beta-2 agonists, and masking agents.
Iron (as Ferrochel® Ferrous Bisglycinate Chelate)† ... 25 mg
Iron bisglycinate is iron bound to two molecules of the amino acid glycine, a chelated form sold as a gentler oral iron. Research suggests that oral iron formulations differ in how well they are tolerated, with ferrous salts carrying a notable burden of gut complaints 1. Whether you need iron at all is a question for a blood test and a clinician, not for symptoms alone.
This page explains the ingredient. Amounts, timing and how to take any iron product belong on the product label and in a conversation with your clinician or pharmacist, not in an article.
Iron bisglycinate is a chelate, meaning the iron atom is held inside a ring formed by two glycine molecules rather than sitting as a loose salt. That structure is the whole point of the ingredient. A chelated mineral is shielded from some of the chemistry of the stomach and small intestine, which is why chelates are marketed as gentler and as less dependent on the conditions in the gut.
To understand why the form might matter, it helps to know how iron normally crosses the gut wall. Non-heme iron, the kind in plants, fortified foods and most supplements, is generally reduced to the ferrous state at the brush border and carried into the enterocyte by a transporter called DMT1, then either stored in the cell or exported into the blood 2. Reviews of absorption pathways describe this route as tightly controlled and as sensitive to everything else in the meal around it 3. Older reviews of iron absorption and transport describe the same bottleneck at the intestinal cell 4.
The claim made for bisglycinate is that a chelated molecule is less exposed to the inhibitors in food and less likely to leave reactive free iron sitting in the intestine. That is a plausible mechanism rather than a settled one.
Ferrous sulfate is the long-standing, inexpensive reference product for oral iron, while bisglycinate may be better tolerated, though how the two compare is less settled.
| Feature | Iron bisglycinate | Ferrous sulfate |
|---|---|---|
| Chemical form | Ferrous iron chelated to two glycine molecules | Simple ferrous salt |
| How it is thought to be handled | Partly protected by the chelate, with iron released for uptake at the intestinal cell | Dissociates in the gut lumen, releasing ferrous iron for uptake |
| Tolerability in published reviews | Chelated and modified formulations are described as better tolerated in some comparisons | Reviews report a significant excess of gastrointestinal side effects compared with placebo |
| Cost and availability | Usually costs more per serving | Inexpensive and very widely available |
| Amount per serving | Varies by product, read the label | Varies by product, read the label |
A systematic review of tolerability across oral iron supplements found that gastrointestinal complaints differ by preparation and that ferrous salts sit at the less comfortable end of that spectrum 5. A separate systematic review and meta-analysis concluded that ferrous sulfate is associated with significantly more gastrointestinal side effects than placebo in adults 6. Those two findings are the honest basis for the "gentle iron" positioning. They do not establish that bisglycinate works better, only that the comfort profile of oral iron is not uniform.
Iron absorption is governed by a liver hormone called hepcidin, which acts as a gate rather than a tap. When stores are full or inflammation is present, hepcidin rises, the export protein ferroportin is withdrawn from the intestinal cell surface, and less iron enters the blood regardless of what you swallowed. Reviews of iron metabolism describe hepcidin as the master regulator of systemic iron balance 7, and work on hepcidin in the diagnosis of iron disorders describes how it distinguishes true deficiency from iron that is present but locked away during inflammation 8.
Two practical consequences follow. First, taking more iron does not reliably mean absorbing more iron, because the gate closes. Second, the fraction that is not absorbed stays in the intestine, which is where most of the unpleasant effects come from. This is why the useful question is never "how much iron should I take" but "do I actually need iron, and does my body currently have a reason not to absorb it".
What is established here is narrow and specific, and nothing broader.
What it supports. Oral iron formulations differ in tolerability, and the reviews above found more gastrointestinal complaints with ferrous salts than with placebo 6. Absorption of non-heme iron is strongly modified by what it is eaten alongside, and in vitro and food-science reviews of fortified cereal products show how large those differences in bioavailability can be between iron compounds and food matrices 9. Studies suggest that certain prebiotic fibres may influence iron absorption, though that review describes the human findings as mixed and not yet actionable 10.
What it does not show. There is no basis here for saying that iron bisglycinate treats tiredness, improves athletic performance, supports hair growth or helps mood in someone whose iron status is already adequate. Iron corrects a shortfall. If there is no shortfall, the published absorption physiology predicts very little benefit and the same gut exposure. The reviews of newer oral iron formulations are explicit that old questions about optimal formulation, absorption and comparative outcomes remain unresolved 1. Supplements are not a substitute for diagnosis or for a prescribed treatment, and a licensed provider is the one who evaluates whether any iron therapy is appropriate for a given person.
Iron is one of the few nutrients where more is not neutral, because the body has no efficient route to excrete a surplus. That makes it a measure-first nutrient. The usual starting marker is ferritin, which reflects stored iron, supported by transferrin saturation, a full blood count and a marker of inflammation, because inflammation raises ferritin and can make depleted stores look adequate.
| Marker | What it reflects | Typical lab reference range |
|---|---|---|
| Ferritin | Stored iron, also rises with inflammation | Roughly 30 to 300 ng/mL in men, 15 to 200 ng/mL in women |
| Transferrin saturation | How much of the iron transport protein is carrying iron | Roughly 20 to 50 percent |
| Hemoglobin | Oxygen-carrying capacity of the blood | Roughly 13.5 to 17.5 g/dL in men, 12.0 to 15.5 g/dL in women |
| C-reactive protein | Inflammation, which can push ferritin upward | Typically under 3 mg/L |
These are typical lab reference ranges. They vary by laboratory, assay, age, sex and pregnancy status, and they are reference intervals rather than personal targets.
Read a result in three steps. The observation is the number itself, for example a ferritin sitting below the lower reference limit. The meaning is hedged, because a low ferritin may reflect genuinely depleted stores, while a ferritin in the middle of the range alongside a raised C-reactive protein may reflect inflammation masking a shortfall. The next step is a conversation. A low or unexpectedly high iron result is worth discussing with a clinician, and a markedly abnormal result is worth discussing promptly, because the cause matters more than the number. FitScript's membership includes 150+ biomarkers tested per year, with interpretation by Atlas, FitScript's AI health advisor.
Quite a lot, which is one reason iron supplementation is less predictable than it looks on the label.
Food components come first. Polyphenols in tea and coffee, calcium, and phytates in whole grains and legumes all reduce non-heme iron uptake, while vitamin C in the same meal increases it. Food-science reviews of iron fortification describe these interactions as large enough to change how much iron a product actually delivers 9. The chelate structure of bisglycinate is often said to blunt some of these effects, which remains a mechanistic argument rather than a proven advantage.
Medicines are the second category, and the rule here is simple. Tell your prescriber or pharmacist that you take iron, and let them decide whether anything needs to change. Several medicine classes are known to interact with minerals in either direction, including acid-reducing drugs, which change the stomach environment iron depends on. Do not stop, change or rearrange a prescribed medicine on your own. It is also worth noting that not every suspected interaction turns out to be real. A clinical study of mycophenolate mofetil found no interaction between iron and mycophenolate absorption 11, which is a useful reminder that interaction lists are hypotheses until tested.
Other supplements are the third. Minerals can compete for shared transport routes, so a pharmacist is the right person to review a full supplement list rather than a general rule applied blindly.
Most oral iron side effects are gastrointestinal, and they come from the iron that is not absorbed. Reviews of the mechanism describe unabsorbed iron interacting with the intestinal lining and the gut microbial community, generating oxidative stress and local irritation 12.
Can iron supplements cause constipation? Yes, constipation is one of the most commonly reported effects of oral iron, and the systematic review of tolerability across preparations found that the frequency of complaints differs between formulations 5. If constipation appears after starting iron, that is worth raising with the clinician who recommended it rather than quietly stopping.
Can iron pills cause black stool? Yes. Dark or black stools are an expected, harmless consequence of unabsorbed iron passing through the bowel and are not in themselves a sign of harm. The important caveat is that black, tarry, foul-smelling stool can also indicate bleeding in the upper digestive tract, and iron can mask that signal. Stool that is black and tarry, especially with abdominal pain, dizziness or vomiting, is worth discussing with a clinician promptly rather than assuming it is the supplement.
Can iron supplements cause diarrhea? They can. Although constipation is the more familiar complaint, diarrhea, nausea, cramping and a metallic taste all appear in the side-effect literature, and the meta-analysis of ferrous sulfate found a clear excess of gastrointestinal effects overall compared with placebo 6. Which direction a person's gut goes is individual.
Can you take iron and magnesium together? Many people take both, and there is no general prohibition, but minerals can compete for absorption and both can affect bowel habit, magnesium often loosening stools while iron often firms them. Whether to combine them, and how, is a question for the product labels and for a pharmacist who can see everything else you take.
One safety point sits above all of these. Iron products are a leading cause of poisoning in young children, so iron should be stored out of reach and out of sight.
Some people should not start iron on their own judgement at all.
Anyone with hereditary hemochromatosis or any diagnosed iron overload condition needs clinician direction, because the regulatory gate described above does not work normally and absorbed iron accumulates. Anyone with unexplained anemia needs the cause investigated before it is treated, because anemia is a finding rather than a diagnosis and supplementing can delay the search for what is driving it. The same applies to anyone with a family history of iron overload, anyone who has had repeated blood transfusions, anyone with chronic kidney, liver or inflammatory bowel disease, anyone who is pregnant or planning pregnancy, and anyone giving iron to a child.
For everyone else, the sensible sequence is to test, interpret the result with a clinician, decide whether iron is warranted, and then retest at an interval the clinician chooses. Iron stores rebuild slowly, so judging progress by how you feel is less reliable than judging it by a repeat measurement.
This article is educational and is not medical advice. Discuss your lab results and any supplement or treatment with a clinician or licensed provider before acting on them.
To see what iron stores look like on a lab report and how the marker is interpreted, read the Ferritin guide.
It is iron chelated to two molecules of the amino acid glycine. The chelate structure is intended to shield the iron from some of the chemistry of the stomach and intestine, which is why this form is commonly described as a gentler oral iron.
Published reviews report that oral iron preparations differ in tolerability and that ferrous sulfate is associated with significantly more gastrointestinal side effects than placebo in adults. That supports the general idea that formulations are not interchangeable for comfort, but whether bisglycinate specifically is gentler is less settled.
Yes. Constipation is among the most commonly reported effects of oral iron, and reviews of tolerability show the frequency varies between preparations. Unabsorbed iron in the bowel is thought to drive much of it. If it happens, raise it with the clinician who recommended the supplement rather than stopping quietly.
Yes, and on its own it is harmless. Unabsorbed iron darkens stool as it passes through. The caveat is that black, tarry, strong-smelling stool can also signal bleeding in the upper digestive tract, so that pattern, particularly with pain, dizziness or vomiting, is worth discussing with a clinician promptly.
They can. Constipation is more familiar, but diarrhea, nausea, cramping and a metallic taste are all recognised side effects, and ferrous sulfate is associated with more gastrointestinal effects than placebo. Which direction the gut goes is individual.
Many people take both and there is no blanket prohibition, but minerals can compete for absorption and both affect bowel habit in opposite directions. Whether and how to combine them is a question for the product labels and a pharmacist who can review everything else you take.
Polyphenols in tea and coffee reduce absorption of non-heme iron, while calcium and phytates in grains and legumes also lower it and vitamin C in the same meal raises it. Food-science reviews show these interactions are large enough to change how much iron a product actually delivers.
Through blood work rather than symptoms. Ferritin reflects stored iron, supported by transferrin saturation, a full blood count and an inflammation marker, because inflammation raises ferritin and can make depleted stores look adequate. A clinician interprets the pattern, not any single value.
Anyone with hemochromatosis or diagnosed iron overload, a family history of it, unexplained anemia, repeated transfusions, chronic kidney, liver or inflammatory bowel disease, anyone pregnant or planning pregnancy, and anyone considering iron for a child. Iron should also be stored out of reach of young children.
Iron corrects a shortfall in iron. There is no good basis for expecting a benefit to energy, performance, hair or mood in someone whose iron status is already adequate, and reviews of newer oral iron formulations describe many comparative questions as still unresolved.