Do supplements compete with each other, and does timing matter?

Minerals such as calcium, iron and zinc share absorption routes in the gut, and some compounds simply bind others. Calcium also lowered the absorption of heme iron, the form found in meat, which points to competition at the gut wall rather than only a chemical reaction³. Levothyroxine sticks to calcium carbonate in an acid environment, and iron forms a poorly soluble complex with thyroxine^1,2.

What the studies measured

• Calcium and iron. Calcium cut iron absorption by 50 to 60 percent at 300 to 600 mg, and even 165 mg from milk or cheese did the same³.
• Calcium and zinc. A 600 mg calcium carbonate supplement taken with a meal cut zinc absorption by 50 percent; adding zinc to the supplement offset it⁸.
• Iron and zinc. High iron doses lowered zinc absorption when both were given in water, but not when given in a meal; iron experts still advise taking iron supplements between meals⁴.
• Coffee and tea. A cup of coffee cut iron absorption from a meal by 39 percent and tea by 64 percent. Coffee an hour before the meal did no harm, but coffee an hour after it still blocked absorption⁹.
• Vitamin C. It boosts iron absorption in single meals, yet across a full day's diet the effect was far smaller¹⁰.
• Dietary fat. Fat in the meal raised vitamin D3 absorption by about a third⁵, though blood vitamin D levels at one and three months were no better in another trial¹¹.

A simple schedule

1. On waking, with water: thyroid medicine, alone. In one study coffee taken 60 minutes after the tablet did not reduce absorption⁶.
2. With your biggest meal: vitamin D and other fat-soluble vitamins, plus calcium carbonate, which needs food; calcium citrate works at any time. Keep each calcium dose to 500 mg of elemental calcium or less¹².
3. Between meals: iron, away from calcium, tea, coffee and dairy^4,3. If a clinician has prescribed iron, taking it every other day absorbed more than daily dosing⁷.
4. At a different meal from calcium: zinc⁸.

Keep it in proportion. Single-meal tests overstated vitamin C's effect on iron absorption¹⁰, and the same caution applies to the other pairs. Spacing matters most if you are deficient, pregnant, anemic, or take thyroid medicine. See our pages on iron, vitamin D supplementation and supporting thyroid health.

Minerals competing with each other

Hallberg and colleagues gave 126 volunteers wheat rolls with calcium added in doses of 40 to 600 mg and measured iron absorption. Inhibition was dose related up to 300 mg, and iron absorption fell 50 to 60 percent at 300 to 600 mg of calcium. The same effect appeared when 165 mg of calcium was given as milk, cheese or calcium chloride, and heme iron absorption fell too³. Wood and Zheng fed 18 postmenopausal women a controlled diet for 36 days: an extra 468 mg of calcium a day, as milk or calcium phosphate, reduced net zinc absorption and zinc balance by about 2 mg a day. In a second group, 600 mg of calcium as carbonate halved zinc absorption from a single meal, and an extra 7.8 mg of zinc offset the loss⁸. Whittaker's review found that high iron doses lowered zinc absorption when given in solution but not in a meal, and that iron fortification at current levels did not impair zinc absorption⁴.

Tea, coffee and vitamin C

Morck and colleagues used dual-isotope studies in iron-replete volunteers. A cup of coffee reduced iron absorption from a hamburger meal by 39 percent versus 64 percent for tea, and stronger coffee inhibited more. Coffee an hour before the meal had no effect, while coffee an hour after the meal inhibited as much as coffee taken together with it⁹. For vitamin C, Cook and Reddy measured iron absorption in 12 people across three 5-day periods in which diet vitamin C ranged from 51 to 247 mg a day. Mean absorption did not differ significantly between periods, although vitamin C was positively associated with absorption after adjustment. The authors concluded that vitamin C's effect on a complete diet is far weaker than on single meals, which may explain why long-term vitamin C supplements have not clearly improved iron status¹⁰.

Iron dosing schedule

In two randomized trials in iron-depleted young women, Stoffel and colleagues used isotope-labeled iron. In 40 women, 60 mg of iron on alternate days gave higher cumulative fractional absorption than the same dose on consecutive days (21.8 percent versus 16.3 percent, p=0.0013), with lower hepcidin, a hormone that reduces iron absorption. In 20 women, one 120 mg dose and two 60 mg doses a day absorbed similar amounts, but the split dose raised hepcidin more⁷. The trials were small, short and did not test anemic patients or clinical outcomes.

Thyroid medicine

Singh and colleagues followed 20 hypothyroid patients on stable levothyroxine who added 1,200 mg of elemental calcium as carbonate for three months: mean TSH rose from 1.6 to 2.7 mIU/L, free T4 fell, and 20 percent had TSH above the normal range¹. Campbell and colleagues gave 14 patients 300 mg ferrous sulfate with their thyroxine for 12 weeks in an uncontrolled trial: mean TSH rose from 1.6 to 5.4 mU/L and nine patients had more hypothyroid symptoms². Benvenga and colleagues found that coffee swallowed with the tablets lowered the area under the absorption curve by 36 percent in patients and 27 percent in volunteers, whereas coffee 60 minutes after the tablets did not reduce absorption relative to water alone⁶. All three studies were small, and two were before-and-after designs without a parallel control group.

Fat-soluble vitamins and meals

Dawson-Hughes and colleagues randomly assigned 50 older adults to a fat-free meal or a meal with 30 percent of calories from fat; after a single 50,000 IU dose of vitamin D3, peak plasma D3 was 32 percent higher (95% CI 11 to 52 percent) with fat. The type of fat made no difference⁵. A second randomized trial in 62 older adults gave monthly 50,000 IU doses with no meal, a high-fat meal or a low-fat meal. Absorption was highest with the low-fat meal, yet 25-hydroxyvitamin D did not differ between groups at 30 or 90 days¹¹. Absorption is not the same as status, and these two trials do not agree on the best meal. Trials comparing meal conditions for vitamins A, E and K in supplement users are scarce.

Calcium forms and doses

A clinical review recommends calcium carbonate with a meal, calcium citrate with or without food (the better choice with low stomach acid or acid-blocking drugs), and no more than 500 mg of elemental calcium at one time¹². This is expert guidance rather than trial evidence.

What is not shown

Single-meal studies overstated vitamin C's effect in a whole-diet comparison¹⁰, and none of the mineral studies measured disease outcomes. The thyroid findings are the most clinically relevant but come from small studies.

Bottom line (confidence 3 of 5). Calcium, iron, zinc, coffee and tea reduce each other's or thyroid medicine's absorption in careful experiments. Spacing doses costs nothing, but it matters mainly for people who are deficient, pregnant or on thyroid medicine.