Place · Level 3
Vitamin C
水溶 · 容易流失 · 抗氧化 · 帮铁进门 · 让胶原蛋白成形
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Chapter 1
Fresh food
Fresh food
Ascorbic acid is destroyed by three things: heat, oxygen, water.
Cut fruit left in air for 24 hours loses 30–50%Vegetables (especially leafy greens) boiled 10 minutes lose ~50%Shredded and soaked in water — you're literally washing the water-soluble C away
Highest food sources (mg / 100g):
Bell peppers 130–190 mgKiwi 90 mgBroccoli, Chinese kale 80–90 mg (raw)Strawberries 60 mgCitrus 50 mg
Humans, primates, and guinea pigs are among the few mammals unable to synthesize vitamin C. Other mammals have L-gulono-lactone oxidase in liver and kidney, which makes C directly from glucose; in humans this gene (GULO) has degraded into a pseudogene, so vitamin C must come from food daily. This is why ancient sailors began dying of scurvy around week 8 of food rationing — once the body pool burned out, there was no buffer left.
Cut fruit left in air for 24 hours loses 30–50%Vegetables (especially leafy greens) boiled 10 minutes lose ~50%Shredded and soaked in water — you're literally washing the water-soluble C away
Highest food sources (mg / 100g):
Bell peppers 130–190 mgKiwi 90 mgBroccoli, Chinese kale 80–90 mg (raw)Strawberries 60 mgCitrus 50 mg
Humans, primates, and guinea pigs are among the few mammals unable to synthesize vitamin C. Other mammals have L-gulono-lactone oxidase in liver and kidney, which makes C directly from glucose; in humans this gene (GULO) has degraded into a pseudogene, so vitamin C must come from food daily. This is why ancient sailors began dying of scurvy around week 8 of food rationing — once the body pool burned out, there was no buffer left.
Cooking to preserve C
Vitamin C is the most cooking-loss-prone vitamin — remember three variables and you keep most of it.Approximate loss by cooking method:
Boiling 10 min ~50% (C leaches into the water)Steaming 5–7 min ~20%Microwave (small water) ~15%Roasting, dry-frying ~25% (high heat but no water dissolution)Raw 0%
Highest-retention methods: stir-frying (oil + 60–90 s high heat) actually keeps 70–80%; steaming or quick-boiling and drinking the broth saves the water-dissolved C; refrigerated produce degrades 3–5× slower than at room temperature.
Easily missed loss scenarios:
Shredded then soaked (some restaurants do this) — 50% gone in 30 minutesReheating overnight leftovers repeatedly accelerates lossCanned vegetables: low C after processing + long storage (minerals + fiber still present)Fresh juice opens, sits at room temp 24 h, loses ~50%; industrial UHT juice labeled '100% vitamin C' is usually re-fortified
The steadiest practice: 2 daily fresh fruits (kiwi + citrus ≈ 200 mg C), 1–2 daily raw vegetable portions (salad, peppers, tomato), pair a little raw with cooked vegetables — don't cook everything. Hitting the 75–90 mg/day RDA becomes nearly impossible to miss.
Chapter 2
Gut · saturating uptake
Gut · saturating uptake
Vitamin C is absorbed in the small intestine via sodium-dependent vitamin C transporter 1 (SVCT1). This channel has a ceiling, and absorption rate falls steeply with dose:
The excess is almost entirely excreted in urine — this is why mega-dose C makes urine darker and occasionally causes osmotic diarrhea. Small frequent doses are 3–5× more efficient than one big hit.
SVCT2 is the sister transporter, distributed in leukocytes, adrenals, brain, retina — these tissues actively concentrate C to 10–100× plasma, suggesting C is most needed where oxidative stress is highest.
| Single dose | Absorption | Actually absorbed |
|---|---|---|
| 200 mg | ~100% | 200 mg |
| 500 mg | ~70% | 350 mg |
| 1000 mg | ~50% | 500 mg |
| 5000 mg | <20% | <1000 mg |
The excess is almost entirely excreted in urine — this is why mega-dose C makes urine darker and occasionally causes osmotic diarrhea. Small frequent doses are 3–5× more efficient than one big hit.
SVCT2 is the sister transporter, distributed in leukocytes, adrenals, brain, retina — these tissues actively concentrate C to 10–100× plasma, suggesting C is most needed where oxidative stress is highest.
Liposomal C myths
The marketing pitch for liposomal vitamin C goes: regular C has limited absorption, liposomal encapsulation bypasses SVCT1, achieves 'near-100% absorption + direct cell entry', 'equivalent to IV dosing' — and the price is 5–10× higher.Walking through the data:
A few small studies (Davis 2016, Hickey 2008) show oral liposomal C has 1.5–2× peak plasma vs regular CNo large RCT has compared clinical endpoints (colds, wound healing, exercise)'Equivalent to IV C' is overstated: IV C can push plasma >10 mmol/L, unreachable by any oral form (oral physiologic ceiling ~220 µmol/L)True pharmacologic-dose C (e.g., cancer adjunct) must be given IV — there's no oral substitute
Practical judgments:
For daily 200–500 mg/day supplementation, regular tablets, chewables, or powder are equivalent — pick the cheapestTo reach higher intakes (1–2 g), split doses are more efficient than a single doseLiposomal C is unnecessary for normal people — 5–10× the price with unclear clinical benefit'Natural C (acerola / rose hip) is better than synthetic C' has no evidence — the ascorbate molecule is identical
Useful timing tricks:
Adding 25–75 mg C to an iron-containing meal doubles non-heme iron absorption — timing matters more than total C doseDon't take large doses of antioxidants before exercise; it may blunt training adaptation (Paulsen 2014 RCT) — avoid C/E 1–2 hours before resistance trainingSmokers add 35 mg/day to RDA — one orange covers it
Chapter 3
Plasma & tissues
Plasma & tissues
At healthy intakes, plasma vitamin C steady-state is 50–80 µmol/L. But intracellular leukocyte concentration is ~1500 µmol/L (20× plasma), adrenal medulla ~4000 µmol/L, vitreous humor >2000 µmol/L — all sites of highest oxidative stress.
The body's total C pool is ~1.5 g, scurvy onset threshold ~300 mg (pool burned down to 1/5). From cessation of intake to scurvy onset takes ~2–3 months — the conclusion from James Lind's 1747 first controlled trial in the British navy (lemon + orange vs cider + seawater).
Plasma C half-life ranges from 8–40 days depending on steady state. Severe oxidative stress (bad colds, post-surgery, burns) acutely depletes the C pool — this is the chemical basis for 'the body really does need more C when sick'. But it doesn't mean '10 g vitamin C daily prevents everything' works — see the antioxidant section downstream.
The body's total C pool is ~1.5 g, scurvy onset threshold ~300 mg (pool burned down to 1/5). From cessation of intake to scurvy onset takes ~2–3 months — the conclusion from James Lind's 1747 first controlled trial in the British navy (lemon + orange vs cider + seawater).
Plasma C half-life ranges from 8–40 days depending on steady state. Severe oxidative stress (bad colds, post-surgery, burns) acutely depletes the C pool — this is the chemical basis for 'the body really does need more C when sick'. But it doesn't mean '10 g vitamin C daily prevents everything' works — see the antioxidant section downstream.
Who needs more C
RDA is 90 mg (men) / 75 mg (women) per day, but several populations have significantly higher real needs:Smokers: official recommendation is +35 mg/day on top of RDA. Oxidants in smoke deplete the C pool — smokers' plasma C is typically ~35% lower than non-smokersSecond-hand smoke exposure also accelerates depletion, though IOM hasn't issued a separate recommendationHeavy alcohol use interferes with C absorption and metabolism — C is commonly deficient in chronic alcohol-related malnutritionPost-surgery / burn / severe trauma: trauma rapidly depletes C pool (dual demand from collagen hydroxylation + antioxidant), ICU burn patients commonly receive 1–3 g/dayChronic inflammation (RA, IBD, COPD) releases continuous oxidative stressSepsis: ICU's Marik protocol trialed high-dose IV vitamin C (1.5 g every 6 hours) + thiamine + hydrocortisone — RCT results inconsistent, but mechanism plausibleType 2 diabetes: hyperglycemia raises oxidative stress, plasma C tends lowerSVCT1 polymorphisms (~3% of population) have low absorption efficiency — even normal intake leaves them low
Worth remembering: heavy smoking + heavy alcohol + chronic inflammation stacked — actual C status is usually far below routine indicators. For these people, 200 mg daily (one citrus + one bell pepper) is worth far more than for average people.
But 'severely ill so supplement 5 g vitamin C' folk practice has no RCT support. Clinical evidence strength is limited to 'adequate dietary RDA' plus 'acute high-dose IV trials in ICU settings' — at-home oral mega-dose mostly causes diarrhea and kidney stones.
Chapter 4
Antioxidant
Antioxidant
C's antioxidant mechanism is direct electron donation — it neutralizes free radicals (•OH hydroxyl radical, •O₂⁻ superoxide, •nitric oxide: A small signal molecule from the vessel lining that relaxes the vessel-wall muscle so the vessel widens.₂, peroxyl radicals) into stable molecules. Each electron donation oxidizes C into dehydroascorbate (DHA, not that DHA), its oxidized form.
A key mechanism is that C reduces oxidized vitamin E back to its active form: vitamin E in the membrane neutralizes oxidation and becomes tocopheryl radical, then aqueous-phase C reduces it back to tocopherol, putting E back into service. C + E + glutathione (GSH) are the body's antioxidant partners — supplementing any one alone has limited meaning; the cycle running together is normal physiology.
But 'supplement C to prevent everything' is overreach. Epidemiologically, high dietary C correlates strongly with reduced cardiovascular and certain cancer incidence; but interventional trials of high-dose C supplements are generally null. One explanation: at normal doses in food form, C is antioxidant; at very high doses, C may become pro-oxidant (Fenton reaction releases Fe²⁺ generating •OH) — this is the chemical basis for 'less is more.'
A key mechanism is that C reduces oxidized vitamin E back to its active form: vitamin E in the membrane neutralizes oxidation and becomes tocopheryl radical, then aqueous-phase C reduces it back to tocopherol, putting E back into service. C + E + glutathione (GSH) are the body's antioxidant partners — supplementing any one alone has limited meaning; the cycle running together is normal physiology.
But 'supplement C to prevent everything' is overreach. Epidemiologically, high dietary C correlates strongly with reduced cardiovascular and certain cancer incidence; but interventional trials of high-dose C supplements are generally null. One explanation: at normal doses in food form, C is antioxidant; at very high doses, C may become pro-oxidant (Fenton reaction releases Fe²⁺ generating •OH) — this is the chemical basis for 'less is more.'
Cold prevention: what trials show
'Vitamin C prevents colds' is the famous 20th-century hypothesis from Linus Pauling (he himself took 18 g per day). 50 years on, the evidence is now clear.Per the Cochrane review (Hemilä & Chalker 2013, 29 RCTs, n=11,306):
For prevention (general population, >200 mg/day):
No significant reduction in overall cold incidenceBut in extreme-physical-stress groups (marathoners, soldiers in winter training, Arctic military): incidence drops ~50% — in people with extremely high oxidative stress, C is a genuine preventive tool
For shortening duration (started within symptom onset, ≥200 mg/day):
Adult duration shortens ~8% (from 7 days to 6.5 days) — subjectively minorChildren shorten ~14% — slightly more notableMild improvement in symptom intensity, subjective experience inconsistent
Very high doses (~8 g) single hit on day 1 of cold: small RCTs show slightly stronger effect, but diarrhea and GI upset are nearly guaranteed.
Practical takeaways: 'daily 1 g vitamin C prevents colds' has no evidence for normal people; for extreme exercise / extreme environments there is evidence — a genuine prevention tool; once cold has started, larger doses can mildly shorten duration but don't cure; overall, food sources + dietary variety beat mega-dose supplements, which mostly produce expensive urine.
Where Pauling was wrong: he conflated 'dose response when correcting severe deficiency' with 'marginal effect when adding more after sufficient' — a classic cognitive trap in nutrition science.
Chapter 5
Collagen
Collagen
Collagen accounts for ~30% of body protein — the structural foundation of skin, blood vessels, bone matrix, tendons, cartilage. Its strength comes from three intertwined strands; the key is that proline (Pro) and lysine (Lys) residues must be hydroxylated before they form the stable hydrogen-bonded crosslinks.
The enzymes for this step are prolyl hydroxylase (P4H) + lysyl hydroxylase (LH), whose active sites carry an Fe²⁺. During the reaction, Fe²⁺ gets oxidized to Fe³⁺ → enzyme inactivates; at this point vitamin C reduces Fe³⁺ back to Fe²⁺, letting the enzyme work again. So C is the cofactor for these two enzymes.
Scurvy = no C → hydroxylation stops → collagen triple helix slackens → vessel wall leakage + gum bleeding + old wounds re-opening + bone pain. Modern people rarely get scurvy, but post-surgery healing, sports injury recovery, chronic wounds (diabetic foot, pressure ulcers) do raise C demand — this is the chemical basis for surgeons telling post-op patients to take C.
The enzymes for this step are prolyl hydroxylase (P4H) + lysyl hydroxylase (LH), whose active sites carry an Fe²⁺. During the reaction, Fe²⁺ gets oxidized to Fe³⁺ → enzyme inactivates; at this point vitamin C reduces Fe³⁺ back to Fe²⁺, letting the enzyme work again. So C is the cofactor for these two enzymes.
Scurvy = no C → hydroxylation stops → collagen triple helix slackens → vessel wall leakage + gum bleeding + old wounds re-opening + bone pain. Modern people rarely get scurvy, but post-surgery healing, sports injury recovery, chronic wounds (diabetic foot, pressure ulcers) do raise C demand — this is the chemical basis for surgeons telling post-op patients to take C.
Collagen supplements
'Drink collagen' / collagen peptides have been one of the hottest beauty and joint supplements over the past 5 years — mixed truth, claim by claim:The evidence-backed part: oral collagen peptides (2.5–15 g/day) do improve certain endpoints in RCTs:
Skin elasticity + hydration (Choi 2019 meta, 19 RCTs): visible improvement at 6–12 weeksNail growth rate + reduced brittleness (Hexsel 2017)Joint pain (Bello 2006, Lugo 2016 RCTs, moderate evidence)Athlete joint comfort (UC-II type significant)
Mechanistic hypothesis: peptide fragments enter blood as signaling peptides activating fibroblasts, while also supplying Gly/Pro/Hyp amino acids.
The overstated part:
'Directly puts collagen back into your skin' doesn't hold — oral collagen is fully hydrolyzed by gut into free amino acids / dipeptides / tripeptides; whole molecules can't be absorbed into skin'Better than regular protein' — most studies lack a protein-control arm, so the effect is likely mostly the total protein increase'Collagen in drinks' — most collagen drinks contain <2.5 g, far below RCT doses
Cheaper equivalents: adequate protein (1.2–1.6 g/kg) + vitamin C lets your body synthesize collagen; gelatin is partially hydrolyzed collagen at 1/10 the price; whey and egg protein supply similar Gly/Pro amounts.
Real clinical use cases:
Major surgery / burn / severe wounds: protein + C + zinc upregulated simultaneously — no need to specifically add collagenAthletes + tendon injury: oral collagen peptides 15 g + C 50 mg 1 hour before training, Shaw 2017's small study showed it boosts tendon collagen synthesis — interesting but small sampleSkin beauty: sunscreen + moisturizer + retinol + quit smoking + adequate sleep stack far outperforms any oral supplement
Chapter 6
Iron co-absorption
Iron co-absorption
Plant-source non-heme iron (Fe³⁺) has poor solubility at neutral pH, and is often locked by phytate, tannin (tea / coffee), oxalate — absorption is only 2–10%.
Vitamin C does two things here:
1. Reduction: Fe³⁺ → Fe²⁺ (donating one electron). Fe²⁺ is much more soluble in the mildly acidic small intestine environment and no longer gets grabbed by phytate.
2. Chelation: C forms a soluble complex with Fe²⁺, escorting iron to the intestinal cell membrane's DMT1 (divalent metal transporter 1) — DMT1 only transports Fe²⁺, not Fe³⁺.
The effect is dose-dependent, roughly: 25 mg of C with a meal doubles non-heme iron absorption; 75 mg (one kiwi or half a bell pepper) triples it; above 100 mg, marginal returns diminish.
Practically, vegetarians or iron-deficient people can pair iron-containing meals (spinach, legumes, fortified grains) with C-containing foods (citrus, peppers, tomatoes) in the same meal — cheaper and more natural than taking separate iron tablets. Push tea and coffee back at least an hour after meals — tannins will reverse-lock iron.
Vitamin C does two things here:
1. Reduction: Fe³⁺ → Fe²⁺ (donating one electron). Fe²⁺ is much more soluble in the mildly acidic small intestine environment and no longer gets grabbed by phytate.
2. Chelation: C forms a soluble complex with Fe²⁺, escorting iron to the intestinal cell membrane's DMT1 (divalent metal transporter 1) — DMT1 only transports Fe²⁺, not Fe³⁺.
The effect is dose-dependent, roughly: 25 mg of C with a meal doubles non-heme iron absorption; 75 mg (one kiwi or half a bell pepper) triples it; above 100 mg, marginal returns diminish.
Practically, vegetarians or iron-deficient people can pair iron-containing meals (spinach, legumes, fortified grains) with C-containing foods (citrus, peppers, tomatoes) in the same meal — cheaper and more natural than taking separate iron tablets. Push tea and coffee back at least an hour after meals — tannins will reverse-lock iron.
Pairing C with iron in practice
For women with heavy menstruation, vegetarians, and iron-deficiency anemia patients, pairing C with iron meals is one of the cheapest and most effective non-pharmacologic interventions.An executable one-day combo:
Morning: oats + almonds (non-heme Fe) + 1 kiwi (~90 mg C), iron absorption ~3×Noon: lentil soup / red bean rice + bell pepper / tomato, Fe and C in the same mealEvening: spinach (oxalate locks some iron but non-heme Fe still partly available) + a squeeze of lemonPush tea and coffee outside the 1-hour window before/after meals to avoid tannin locking iron
Common traditional pairing anti-patterns:
Spinach + milk: oxalate + calcium double-block ironRice + tea (common Chinese / Indian diet): tannin locks ironIron supplement + calcium supplement together: compete for absorption channel
When diet isn't enough and ferritin <30, typically oral 60–200 mg Fe + 250 mg C, on empty stomach. Moretti 2015 showed alternate-day dosing beats daily, related to hepcidin rhythm; retest at 8–12 weeks.
C augments rather than replaces: severe anemia (Hb <90) or uncontrollable heavy menstruation still requires medical evaluation + oral / IV iron + menstrual evaluation; postmenopausal or male iron deficiency must rule out GI bleeding (colorectal cancer, peptic ulcer) — 'add C + iron' won't solve it.