Iron Supplement Forms: Ferrous Sulfate, Bisglycinate, and Heme — Elemental Iron Yield, Bioavailability, and Why the Label Number Means Three Different Things

Pick up three iron supplements off the same pharmacy shelf. The first is a 325 mg tablet of ferrous sulfate. The second is a 30 mg capsule of iron bisglycinate. The third is an 11 mg capsule of heme iron polypeptide. A consumer comparing the front panels reads three very different milligram numbers and concludes the 325 mg tablet must be the strongest. He buys it, takes it for a week, develops constipation and metallic mouth, and abandons the bottle in the cabinet next to the multivitamin he doesn't take either.

The three bottles are not three different doses of the same product. They are three different chemical forms of iron with three different elemental-iron yields, three different bioavailability profiles, three different GI tolerability profiles, and three different price-per-absorbed-milligram ratios. The 325 mg ferrous sulfate tablet yields about 65 mg of elemental iron with bioavailability typically 10-20%. The 30 mg iron bisglycinate capsule yields about 30 mg of elemental iron with bioavailability typically 25-35% and almost no GI side effects. The 11 mg heme iron polypeptide capsule delivers heme-form iron that absorbs at roughly 15-35% regardless of meal context. The label number is the salt weight or the molecule weight, not the iron the consumer's blood actually sees, and the front-of-bottle marketing does little to distinguish the three.

This post is the iron-form math: the elemental iron yield by form, the bioavailability picture, the dose ranges the trial literature actually supports, the constipation-and-tolerability differential that decides whether the supplement gets taken at all, and the label patterns that distinguish a usable iron product from one engineered to look cheap on the shelf.

The forms on the shelf

Ferrous sulfate. Iron(II) bound to sulfate as a salt. The cheapest form, the most common in mass-market and generic iron supplements, and the form most older clinical-iron-deficiency trials used. Ferrous sulfate is about 20% elemental iron by weight — a 325 mg tablet of ferrous sulfate delivers about 65 mg of elemental iron. Absorption depends heavily on stomach pH, meal context, and the inhibitor matrix in the same meal (calcium, polyphenols in tea and coffee, phytates in whole grains, calcium phosphate in dairy). Typical absorption: 10-20% on an empty stomach in iron-replete adults, climbing to 20-30% in iron-deficient adults whose hepcidin is suppressed.

Ferrous gluconate and ferrous fumarate. Two other ferrous-iron salts with different counter-ions. Ferrous gluconate is about 12% elemental iron by weight (a 325 mg tablet yields about 38 mg elemental iron). Ferrous fumarate is about 33% elemental iron by weight (a 325 mg tablet yields about 108 mg elemental iron). Both absorb similarly to ferrous sulfate per milligram of elemental iron. GI tolerability is marginally better than ferrous sulfate for some users, marginally worse for others, with no consistent population-level winner.

Iron bisglycinate (ferrous bisglycinate chelate). Iron(II) chelated to two glycine molecules. The chelation protects the iron from the inhibitor matrix in the gut and changes the absorption mechanism — iron bisglycinate is absorbed partly through a peptide-transporter mechanism rather than the classic DMT1 ferrous-iron transporter that ferrous sulfate uses. Iron bisglycinate is about 20% elemental iron by weight in the Ferrochel-branded form (the most clinically studied). A 30 mg capsule labeled "30 mg elemental iron from iron bisglycinate" delivers the 30 mg of elemental iron the label claims; a 150 mg capsule labeled "150 mg iron bisglycinate" delivers about 30 mg of elemental iron. The label discipline of "elemental iron from [form]" matters here more than for any other supplement form.

Bioavailability of iron bisglycinate is roughly 25-35% across meal contexts in healthy and iron-deficient adults — higher than ferrous sulfate, and importantly, less inhibited by calcium, polyphenols, or phytates. The chelation is also what produces the dramatic tolerability improvement: GI side effects (constipation, nausea, cramping, dark stool) are reduced 50-70% relative to ferrous sulfate at matched elemental-iron doses in head-to-head trials.

Heme iron polypeptide. Iron in its heme form (the iron-porphyrin complex from hemoglobin and myoglobin), typically extracted from bovine hemoglobin and capsulated. Heme iron is absorbed via a completely separate transporter (HCP1, the heme-carrier protein) and is functionally insensitive to the inhibitor matrix that limits non-heme iron absorption. Bioavailability runs about 15-35% depending on iron status, food context, and individual variability — the upper end of the range is similar to bisglycinate, but the variance is wider.

The label-relevant math, condensed:

| Form | Elemental iron yield | Tablet/capsule example | Elemental iron delivered | |---|---|---|---| | Ferrous sulfate | ~20% | 325 mg tablet | ~65 mg | | Ferrous gluconate | ~12% | 325 mg tablet | ~38 mg | | Ferrous fumarate | ~33% | 325 mg tablet | ~108 mg | | Iron bisglycinate | varies (Ferrochel ~20%) | "30 mg elemental" capsule | 30 mg | | Heme iron polypeptide | varies (heme-Fe-Pep ~1-5%) | "11 mg elemental" capsule | 11 mg |

The label trick is that the front of the bottle often shows the salt weight or the proprietary-ingredient weight (e.g., "iron 150 mg from iron bisglycinate"), not the elemental iron the consumer actually absorbs. The supplement-facts panel is where the elemental iron number lives; reading only the front panel produces a 5-7x overestimate of the dose.

The bioavailability question

Iron absorption is regulated almost entirely by the iron regulatory hormone hepcidin. In an iron-deficient or iron-depleted individual, hepcidin drops, ferroportin opens, and absorption efficiency rises. In an iron-replete individual, hepcidin rises, absorption drops, and excess oral iron passes through the gut. The form determines the ceiling on absorption efficiency at any given iron status; the iron status determines where on the absorption curve the individual sits.

Ferrous sulfate on an empty stomach in an iron-deficient adult: 20-30% absorption, dose-dependent. At doses above ~80 mg elemental iron, fractional absorption falls off as the gut's transporter capacity saturates and the unabsorbed iron drives inflammation and the next-dose hepcidin spike. The clinical implication is that 325 mg/day of ferrous sulfate (65 mg elemental) is not 2x as good as 162 mg/day; it is roughly the same total absorbed iron, with more GI side effects, and more inflammation that suppresses the next dose.

Iron bisglycinate at 25-30 mg elemental per day delivers absorbed iron comparable to ferrous sulfate at 65 mg elemental, with substantially fewer GI side effects and less hepcidin-mediated suppression of next-dose absorption. The trial literature on bisglycinate at 25-30 mg/day in iron-deficient adults shows ferritin and hemoglobin restoration trajectories that match ferrous sulfate at higher elemental doses, with markedly better adherence (because the dose is tolerated).

Heme iron at 7-15 mg elemental per day delivers absorbed iron comparable to higher-dose non-heme iron because the absorption mechanism is insensitive to the inhibitor matrix. Heme iron is also less likely to produce the gut-flora dysbiosis pattern that high-dose ferrous sulfate produces. The cost-per-absorbed-milligram is higher than ferrous sulfate but the tolerability is better than any non-heme form.

Alternate-day dosing matters across forms. A 2017 line of trials showed that daily dosing of iron drives a hepcidin spike that suppresses absorption of the next day's dose, while alternate-day dosing allows hepcidin to fall between doses and absorbs more total iron per week than daily dosing of the same form. The protocol that absorbs the most iron is often 50-100 mg elemental iron every other day, not 50 mg every day. The clinical literature on alternate-day dosing is now strong enough that many sports-medicine and primary-care iron-repletion protocols have moved to it.

The dose ranges the trials actually support

A quick survey:

Iron-deficiency anemia treatment (clinical). 80-200 mg elemental iron per day in divided doses, or 100-200 mg every other day per the alternate-day-dosing protocol. The classic ferrous-sulfate prescription is 325 mg three times daily (195 mg elemental iron), which the alternate-day-dosing literature has largely retired in favor of lower-frequency dosing at the same per-dose elemental iron.

Iron-deficiency without anemia (low ferritin, normal hemoglobin). 50-100 mg elemental iron per day or 100 mg every other day for 8-12 weeks, with ferritin recheck at the end of the course. The bisglycinate form at 25-30 mg/day is often used here because the tolerability supports adherence and the bioavailability is sufficient.

Pregnancy supplementation (prophylactic). 30-60 mg elemental iron per day per WHO guidance. The form choice often matters more here than the dose because GI tolerability decides whether the supplement is actually taken.

Athletic iron repletion (low ferritin, training-load-induced losses). 50-100 mg elemental iron per day or alternate-day at the same per-dose level, with form chosen for tolerability since athletes are typically asymptomatic and adherence drops fast with side effects.

General-health supplementation in iron-replete adults. The honest answer is "you probably don't need it." Iron supplementation in an iron-replete adult drives no clinically meaningful benefit and accumulates risk over time (oxidative stress, possible cardiovascular implications, the gut-flora question). Iron is one of the few supplements where the dose-response curve has a real cliff on the right side; an iron-replete adult does not benefit from a daily iron pill.

If the bottle says "iron 65 mg, take three tablets daily" and the form is ferrous sulfate, the dose is 195 mg elemental iron per day — the classic clinical-anemia dose, appropriate for documented iron-deficiency anemia, inappropriate as a general-health prophylactic. If the form is iron bisglycinate at 30 mg elemental per day, the dose is squarely in the maintenance/repletion range with a tolerability profile that supports long-term use. Three "iron 30 mg" or "iron 65 mg" bottles can deliver three completely different products at three completely different clinical use cases, none of which the front-of-bottle marketing accurately distinguishes.

The constipation, the dark stool, and the gut-flora question

Non-heme iron supplementation produces predictable GI side effects in a dose-dependent and form-dependent pattern: constipation, nausea, cramping, metallic taste, dark stool, and (at higher doses) frank diarrhea in a minority of users. The mechanism is partly local — unabsorbed iron in the lumen drives osmotic and inflammatory effects on the gut wall — and partly systemic, with the iron-induced hepcidin spike contributing to the GI distress.

Ferrous sulfate at 195 mg elemental per day produces clinically meaningful GI side effects in roughly 40-60% of users. The constipation in particular is the dominant adherence-killer; the dark stool, while harmless, is also often a quit reason for users who weren't warned.

Iron bisglycinate at matched elemental doses produces GI side effects in roughly 10-20% of users — a 50-70% reduction in side-effect rate that translates directly into higher adherence in trials lasting 8-12 weeks.

Heme iron polypeptide produces GI side effects at rates similar to or below bisglycinate at matched absorbed-iron doses.

Carbonyl iron (a pure-iron metal powder) is another tolerability-favorable form, though it is less commonly available in retail supplements. Bioavailability is somewhat lower than ferrous sulfate per milligram of elemental iron but the tolerability is meaningfully better.

The gut-flora question is the under-discussed side of the iron picture. High-dose unabsorbed ferrous iron in the lumen shifts the colonic microbiome toward iron-tolerant pathogens (some Salmonella and Shigella species, some pathogenic E. coli) and away from iron-sensitive commensals. The effect is dose-dependent and form-dependent: higher absorption efficiency means less unabsorbed iron in the colon, which means less gut-flora disturbance. Bisglycinate and heme iron, with their higher absorption efficiency at lower elemental doses, produce less gut-flora disruption than high-dose ferrous sulfate.

The clinical implication: for iron-deficiency that requires supplementation, the form choice often decides whether the supplement gets taken long enough to repete the iron stores. The "cheaper" ferrous sulfate is functionally more expensive than the better-tolerated bisglycinate or heme form if the side effects produce abandonment at week three of a twelve-week course.

The label patterns to scan

Pattern 1: "Iron 65 mg (as ferrous sulfate) — take 1-3 tablets daily." The default clinical-anemia prescription. Elemental iron is honestly disclosed. Form is honestly disclosed. The dose range matches the clinical literature for documented iron-deficiency anemia. Tolerability is the constraint, not the formulation.

Pattern 2: "Iron 30 mg (as Ferrochel iron bisglycinate chelate) — take 1 capsule daily." Legitimately dosed for the maintenance and mild-deficiency case. Elemental iron is honestly disclosed. Form is the trademarked bisglycinate ingredient with the strongest trial literature. The bottle delivers the trial-supported dose at the trial-supported tolerability profile.

Pattern 3: "Iron 150 mg from iron bisglycinate." The label-math test. The elemental iron delivered is roughly 30 mg (20% of 150 mg). If the supplement-facts panel reads "elemental iron 30 mg," the label is honest. If it reads "iron 150 mg" without specifying elemental, the math is being hidden behind the proprietary-ingredient weight to make the dose look larger than it is. The proprietary-blend variant of this pattern is worse — see the proprietary-blends post — but the single-ingredient version still trips up consumers who don't run the yield math.

Pattern 4: "Iron blend — ferrous fumarate + ferrous gluconate + heme iron polypeptide" (proprietary blend, total mass disclosed). The hide-the-elemental-iron pattern. The total mass of the blend is stated, but the elemental iron from each form is hidden. This is where heme iron in particular is most likely to be sub-trial-dosed; the trace amount of heme iron in such a blend is a marketing prop rather than a contributor to the actual dose. The proprietary blends post covers the broader pattern.

Pattern 5: "Gentle iron" without a stated form on the front panel. Read the supplement-facts panel. "Gentle iron" almost always means bisglycinate or carbonyl iron, both of which have legitimate tolerability advantages — but the form should be named explicitly. A bottle that uses "gentle" as the marketing term but doesn't name the form is doing brand work, not transparency work.

Pattern 6: Iron in a multivitamin at 18 mg elemental per serving. A multivitamin-style dose, appropriate for prophylactic supplementation in populations at elevated risk for iron-deficiency (pregnancy, regular blood donors, vegetarians with low iron intake) but not appropriate for documented iron-deficiency or iron-deficiency anemia. The 18 mg dose is also not appropriate as a daily supplement for iron-replete adults — see the next section.

The pattern Scythene considers the floor for an iron product worth selling: a single named form (bisglycinate or heme iron preferred for tolerability; ferrous sulfate acceptable for clinical-anemia use with explicit dose disclosure), elemental iron stated on the supplement-facts panel, no proprietary-blend obfuscation, and dosing instructions that match the use case the bottle implies.

When iron should NOT be supplemented

This is the question most iron product marketing avoids. Iron supplementation in iron-replete adults — no documented deficiency, normal ferritin, normal hemoglobin — is not a neutral choice. Excess iron accumulates over decades, drives oxidative stress, and carries some evidence of cardiovascular and metabolic-syndrome risk in observational populations. The "iron multivitamin" taken daily by an iron-replete middle-aged man is not contributing to his health; it is loading iron into a system that has no use for it.

The iron supplement decision should be made on the basis of: documented serum ferritin (the workup floor is below 30 ng/mL for symptomatic iron-deficiency in most populations, with sport-specific and life-stage variations), CBC (hemoglobin and red-cell indices to discriminate iron-deficiency anemia from other anemias), and the clinical context (pregnancy, regular blood donation, vegetarian or vegan diet, menstrual losses, endurance training load with documented iron-deficiency pattern, GI blood loss workup if indicated).

The default for a healthy iron-replete adult with no risk factor is to not supplement iron and to derive dietary iron from food sources (heme iron from animal protein sources, non-heme iron from legumes and fortified grains, vitamin C with non-heme iron sources to enhance absorption). A multivitamin without iron, or a multivitamin specifically labeled "iron-free" or "for men over 50," is the appropriate choice for most non-deficient adults.

What about timing and stacking

Timing. Iron absorbs best on an empty stomach, but the tolerability cost of an empty-stomach iron dose is high enough that many users take it with food at the cost of some absorption efficiency. Bisglycinate and heme iron are less affected by meal context and tolerate the with-food approach better than ferrous sulfate.

With vitamin C. Vitamin C (ascorbate) reduces ferric iron to the ferrous form that the DMT1 transporter prefers, and modestly enhances non-heme iron absorption. A 100-250 mg dose of vitamin C with a ferrous sulfate dose produces a measurable absorption improvement. The effect is smaller for bisglycinate (which uses a different absorption mechanism) and minimal for heme iron (which has its own transporter).

Away from calcium, coffee, tea, and high-phytate meals. Calcium and polyphenols (in coffee and tea) inhibit non-heme iron absorption, with effect sizes large enough to matter in the daily dose-by-dose math. The standard recommendation is to take non-heme iron at least 2 hours away from dairy, coffee, tea, and high-calcium-supplement doses. Bisglycinate is less affected by this inhibitor matrix than ferrous sulfate.

Drug interactions. Iron interacts with the absorption of thyroid hormone (levothyroxine), fluoroquinolone and tetracycline antibiotics, bisphosphonates, and a few other classes. The standard separation is 4 hours between iron and these medications. Anyone on a chronic prescription should confirm the spacing with the prescribing physician.

The bottom line

Three bottles, all labeled "iron," can deliver three completely different products: a clinical-anemia treatment dose, a tolerability-optimized maintenance dose, or a hidden-low-elemental marketing-driven blend. The front-of-bottle "iron" claim does not distinguish them. The supplement-facts panel does, if the consumer knows which form to look for and how to run the elemental-iron yield math.

For documented iron-deficiency anemia under clinical supervision: ferrous sulfate or ferrous fumarate at 65-200 mg elemental iron per day, increasingly via alternate-day dosing to optimize absorption and tolerability, is the workhorse. The cost is the GI side-effect profile; the offset is the dosing flexibility and the clinical-trial depth.

For mild iron-deficiency (low ferritin, normal hemoglobin), prophylactic supplementation, or any case where tolerability decides adherence: iron bisglycinate at 25-30 mg elemental iron per day is the strongest-tolerability form with bioavailability competitive to higher-dose ferrous sulfate. The bottle that names the form (preferably the trademarked Ferrochel ingredient or an equivalent) and states elemental iron explicitly is the bottle doing what the label implies.

For users who prefer heme-form iron with the meal-context-insensitive absorption profile: heme iron polypeptide at 7-15 mg elemental iron per day is the alternate-form option. The cost-per-absorbed-milligram is higher; the tolerability is excellent.

For iron-replete adults with no documented deficiency: don't supplement iron. The default multivitamin should not contain iron unless there is a clinical reason for it.

Any bottle that lists "iron" without specifying the form on the supplement-facts panel, that hides the form inside a proprietary blend, or that overstates the dose by labeling the salt weight or the proprietary-ingredient weight as the iron number is doing label work, not pharmacology work. Read the panel, check the form, run the elemental-iron yield math, and the choice gets a lot less ambiguous.

Iron is the supplement category where the form choice has the largest practical effect on whether the supplement actually delivers on its purpose. The cheapest tablet on the shelf is often the most expensive option per absorbed milligram once the abandonment rate is accounted for. The bottle that costs more per pill but gets taken for the full course is the bottle that does the job.