Nutrient Absorption & Cofactors
Taking the right nutrient in the wrong context is barely better than not taking it. Absorption depends on what else you eat, when you take it, and which cofactors are present or missing.
Vitamin D + K2 + Magnesium
Vitamin D increases calcium absorption from the gut. Without K2, that calcium deposits in arteries instead of bones. K2 (specifically MK-7) activates osteocalcin (directs calcium to bone) and matrix GLA protein (prevents arterial calcification).
Magnesium is the often-missed third piece. It's required to convert vitamin D from its storage form (25-OH) to its active form (1,25-dihydroxy). Without sufficient magnesium, supplementing D can remain ineffective regardless of dose.
Practical ratio: for every 1,000 IU D3, ensure 100 mcg K2 (MK-7) and at least 200 mg magnesium daily from all sources.
Iron + Vitamin C
Vitamin C converts non-heme iron (plant sources, supplements) from ferric (Fe3+) to ferrous (Fe2+) form, which the intestinal lining can actually absorb. This can increase absorption by 3-6x. As little as 25 mg of vitamin C (a few strawberries) with an iron supplement makes a measurable difference.
This is specifically important for vegetarians and vegans whose iron comes entirely from non-heme sources. Meat-eaters get heme iron which absorbs independently of vitamin C.
Zinc & Copper Balance
Zinc and copper compete for the same absorption pathway (metallothionein in intestinal cells). Supplementing zinc at 30+ mg/day without copper creates copper deficiency over time, causing anemia, neutropenia, and neurological symptoms that mimic B12 deficiency.
The optimal ratio is roughly 8-15:1 zinc to copper. If supplementing 30 mg zinc, include 2 mg copper. Most quality zinc supplements include copper for this reason.
Key Antagonists
Calcium blocks iron
Calcium inhibits both heme and non-heme iron absorption by up to 60%. Never take iron and calcium supplements at the same meal. Separate by 2+ hours.
Caffeine blocks iron
Coffee and tea (tannins and polyphenols) reduce non-heme iron absorption by 39-90% depending on strength and timing. Wait 1 hour after meals before coffee if iron status is a concern.
Phytates block zinc and iron
Found in whole grains, legumes, nuts, and seeds. Phytic acid chelates minerals in the gut, reducing absorption by 50-65%. Soaking, sprouting, or fermenting reduces phytate content significantly.
High-dose zinc blocks copper
As noted above. This is actually used therapeutically in Wilson's disease (copper overload), but it's an unintended side effect for most supplement users.
Excess calcium blocks magnesium
Both compete for absorption. High calcium intake without matching magnesium worsens the existing magnesium deficit most adults already have.
Timing Strategy
Morning, with food: Vitamin D3 + K2 (fat-soluble, need dietary fat), B vitamins (energizing, can disrupt sleep if taken late), iron with vitamin C (empty stomach if tolerated, otherwise with light meal).
Evening: Magnesium glycinate (calming, supports sleep), calcium (if supplementing, separate from iron by 2+ hours), zinc (away from high-phytate foods).
Key rule: fat-soluble vitamins (A, D, E, K) need dietary fat to absorb. Taking them on an empty stomach wastes money. Water-soluble vitamins (B, C) absorb fine without food but excess is excreted within hours.