Lemon.

Ascorbate and citrate from lemon reduce ferric iron to the ferrous form and hold it soluble at intestinal pH, which is what DMT1 takes up. This is one of the most fully characterised food-level absorption effects there is.

Vitamin C keeps supplemental iron in the reduced, soluble ferrous state through the upper gut and counters the binding effect of phytate and polyphenols in the same meal. Lemon juice supplies both the ascorbate and the organic acid.

Lemon's main measurable nutrient is ascorbic acid. A formula carrying both is loading the same compound from two sources.

Green tea catechins degrade quickly at the near-neutral pH of the small intestine, and ascorbate plus citric acid keep the medium acidic and reducing so more catechin survives to be absorbed. This is why citrus is added to tea beverages.

The ascorbate and citrate in lemon slow the oxidative breakdown of catechins in the intestinal lumen. More of the intact catechin reaches the absorptive surface.

Lemon is among the highest natural sources of citrate, and citrate raises urinary citrate while binding calcium in solution. The two deliver the same anion by different routes.

Citrate keeps calcium soluble across a range of gastric pH, which is why the citrate salt does not depend on a strongly acidic stomach. Lemon's own citric acid works the same way on calcium in a meal.

Ascorbate reduces the tocopheroxyl radical back to alpha-tocopherol at the membrane surface. Lemon's vitamin C content puts it on the aqueous side of that recycling step.

Ascorbate regenerates flavonoid phenoxyl radicals back to the parent flavonol, and lemon also carries its own flavanones such as hesperidin and eriocitrin. Vitamin C and flavonoids occur together in citrus for this reason.

Citric acid forms soluble low molecular weight complexes with zinc and keeps it available for uptake where phytate would otherwise bind it. The effect is smaller than for iron but runs the same direction.

Ascorbate from lemon reduces ferric iron to the ferrous form and holds it soluble at intestinal pH, which is the standard mechanism behind citrus raising non-heme iron uptake. Bisglycinate is already chelated and enters partly by a different route, so the ascorbate effect on it is smaller than on a simple iron salt. The pairing is common and the size of the gain depends on which iron form is used.

Prolyl and lysyl hydroxylase need ascorbate to keep their iron centres reduced during collagen synthesis, and lemon is a practical dietary ascorbate source. Collagen peptides supply the glycine, proline and hydroxyproline building blocks. The cofactor relationship is settled biochemistry and needs no trial to state.

Proline residues in the procollagen chain are hydroxylated to hydroxyproline by an ascorbate-dependent enzyme, and that hydroxylation is what stabilises the triple helix. Proline supplies the substrate, ascorbate supports the enzyme. Two halves of one reaction.

Lysyl hydroxylase is the second ascorbate-dependent step in collagen maturation and it acts on lysine residues, which then form the cross-links that give collagen its tensile strength. Lysine is the substrate for that step. The pairing with a vitamin C source is textbook.

Calcium carbonate needs gastric acid to dissolve before the calcium ion is available for absorption, which is why it is taken with food. An acidic vehicle such as lemon juice lowers gastric pH and helps that dissolution. The same is not true of calcium citrate, which dissolves independently of stomach acid.

Betaine hydrochloride is used to lower gastric pH in people with low stomach acid, and lemon's citric acid contributes acidity in the same direction. The two act on the same variable, so combining them is additive rather than complementary. Anyone stacking acidifiers should account for that overlap.

Sustained high ascorbate intakes have been described as reducing copper status, and ascorbate also reduces cupric to cuprous copper, changing how it is handled. Lemon at dietary amounts is nowhere near that range. The interaction matters for high-dose vitamin C, not for a squeeze of juice.

Rutin belongs to the same citrus flavonoid family as lemon's eriocitrin and hesperidin, and formulas often combine a flavonoid concentrate with the ascorbate fraction that naturally accompanies it in the fruit. Flavonoids and ascorbate can regenerate one another in vitro. Whether that translates to a measurable human effect is unresolved.

The reduction of nitrite to nitric oxide is favoured in acidic conditions and ascorbate accelerates it, which is the basis for pairing a vitamin C source with dietary nitrate. Lemon supplies both acidity and ascorbate. The mechanism is well described; the practical size of the effect from a culinary amount is not established.

Lemon juice carries potassium alongside citrate, and citrate is metabolised to bicarbonate, so a potassium-citrate load acts on both the electrolyte and the acid-base side. Formulas combining lemon with potassium salts are stacking the same two effects. Worth accounting for in the total potassium figure.

Magnesium oxide and carbonate are poorly soluble and depend on gastric acid for dissolution, while citrate-bound magnesium is not. An acidic lemon vehicle helps the oxide and carbonate forms into solution. The effect is form-dependent, which is the whole point of stating it.

Bicarbonate and citric acid react directly in the glass, releasing carbon dioxide and neutralising both before either is swallowed. Anyone taking bicarbonate for an alkalinising purpose loses part of the dose by mixing it with lemon. Separate the two rather than combining them.

Catechins including EGCG degrade quickly at intestinal pH, and an acidic ascorbate-containing vehicle such as lemon juice slows that breakdown, which raises the fraction surviving to absorption. The effect has been described for citrus juice added to tea. It is a stability effect rather than a change in the catechin itself.

Citrate binds calcium in solution, which is what keeps calcium citrate soluble and what raises urinary citrate's capacity to hold calcium in urine. In the gut the same binding can shift how much free calcium ion is present at a given moment. The direction depends on the calcium salt involved.