Vitamin K1 (phylloquinone) is the plant-derived form of vitamin K, essential for blood clotting. This serum assay measures circulating K1 only. It does NOT reflect K2 (menaquinone) status, which requires a separate test (undercarboxylated osteocalcin is the preferred proxy).
Vitamin K2 (menaquinone) directs calcium into bones rather than arteries, making it increasingly recognized in cardiovascular and bone health. Low K2 status is common in modern diets lacking fermented foods and grass-fed animal products.
Optimal Vitamin K1 (Phylloquinone): 0.2-3.2 ng/mL.
Typically re-tested every 16 weeks.
Vitamin K1 is the fat-soluble form of vitamin K produced by plants, and it is the main dietary source of vitamin K for most people. It concentrates in green leafy vegetables such as kale, spinach, collards and broccoli, and in some vegetable oils. The other major family, the menaquinones (vitamin K2), comes from fermented foods, animal products and bacterial synthesis, and behaves differently in the body.
Mechanically, phylloquinone is absorbed in the small intestine along with dietary fat, which means it needs bile and pancreatic enzymes to get across the gut wall. Once absorbed, it travels in the blood on triglyceride-rich lipoproteins and is taken up largely by the liver. There it acts as a cofactor for the enzyme gamma-glutamyl carboxylase, which adds a carboxyl group to specific glutamate residues on a family of proteins known as vitamin K dependent proteins 8. That single chemical step is what switches those proteins on, allowing them to bind calcium and do their job.
The best known of these proteins are the clotting factors II, VII, IX and X, plus the anticoagulant proteins C, S and Z. Outside the liver, the same carboxylation step activates osteocalcin in bone and matrix Gla protein in blood vessel walls. During the reaction vitamin K is oxidised, then recycled back to its active form through the vitamin K epoxide cycle, which is why a relatively small intake can support a lot of enzymatic work 9. Because circulating phylloquinone clears quickly and tracks recent meals, laboratory guidance regards it as one piece of a wider assessment rather than a standalone verdict on vitamin K status 1.
It matters because the proteins it activates sit at the intersection of three systems that age badly when neglected, blood clotting, bone mineralisation and vascular calcium handling.
The clotting role is the most immediate. Without enough vitamin K, the liver still makes clotting factors, but they are undercarboxylated and functionally inactive, which lengthens clotting times and shows up clinically as bruising or bleeding. This is the pathway that vitamin K antagonist medications deliberately interrupt, and it is the reason clotting tests often move before anyone thinks to measure the vitamin itself 1.
The bone role is slower and more interesting for anyone thinking in decades. Osteocalcin, one of the most abundant non-collagen proteins in bone, requires vitamin K dependent carboxylation to bind calcium properly 10. Research suggests that a higher proportion of undercarboxylated osteocalcin in the blood is associated with lower bone mineral density and with fracture risk in older adults, although the relationship between circulating phylloquinone itself and bone outcomes is less consistent across studies 2. Reviews of vitamin K and osteoporosis describe a plausible mechanism and supportive observational data, while noting that intervention trials have produced mixed results and vary widely in design 3. A systematic review and meta-analysis of randomised trials in postmenopausal women reported benefit signals for vitamin K2 specifically on bone density measures, which does not automatically transfer to K1 because the two forms differ in half-life and tissue distribution 4.
The vascular role runs through matrix Gla protein, which when fully carboxylated appears to act as an inhibitor of calcium deposition in artery walls and soft tissue 11. Studies indicate that markers of poor vitamin K status, including high circulating dephosphorylated uncarboxylated matrix Gla protein, are common in people with chronic kidney disease and are associated with vascular calcification in that population 5. That is an association, not a demonstrated cause, and the clinical value of correcting it is still being studied.
Most laboratories report serum or plasma phylloquinone against a reference range of roughly 0.1 to 5 ng/mL, and FitScript uses a narrower optimal band inside it.
| Measure | Range | How to read it |
|---|---|---|
| Typical lab reference range | 0.1 to 5 ng/mL | The interval most laboratories use to flag a result as inside or outside expected limits |
| FitScript optimal range | 0.2 to 3.2 ng/mL | A tighter target band used for optimization, sitting comfortably within the standard range |
Reference ranges vary by laboratory, assay method, age and sex, so always read your result against the range printed on your own report. FitScript's optimal range is an optimization target, not a diagnostic cut-off, and a value outside it is not a diagnosis of anything.
Two practical caveats matter more here than for most nutrients. First, phylloquinone has a short circulating half-life, so a single value describes recent intake and absorption rather than tissue reserves. Second, because it rides on triglyceride-rich lipoproteins, the number moves with blood lipids, and some laboratories report it alongside triglycerides for that reason. Functional markers give a different angle, including prothrombin time or INR, PIVKA-II (also called undercarboxylated prothrombin) and undercarboxylated osteocalcin, and laboratory reviews regard these functional measures as a useful complement when vitamin K status is genuinely in question 1.
A high phylloquinone result most often means the sample was drawn soon after a vitamin K rich meal or a supplement, and it is not usually interpreted as a sign of toxicity.
What people report. Typically nothing.
Common causes. A non-fasting draw after a large salad, a green smoothie or a meal cooked in vegetable oil. Multivitamins and standalone vitamin K supplements. Very high triglycerides, which can raise the measured concentration because the vitamin travels with lipoproteins. Occasionally, sample handling or an assay quirk.
Longer view. A one-off high value in someone eating plenty of greens is generally unremarkable, and the usual response is to repeat the test under standard fasting conditions rather than to act on it. There is one situation that deserves a conversation. In people taking a vitamin K antagonist anticoagulant, swings in vitamin K intake can shift INR control in either direction, and any change to intake or medication is a decision for the prescribing clinician 12. Persistently high results combined with very high triglycerides are worth discussing with a clinician, because the lipid picture is usually the more informative finding.
A low result suggests limited recent intake, poor fat absorption or increased turnover, and it raises the question of whether vitamin K dependent proteins are being fully activated.
What people report. Many people with a mildly low value notice nothing at all, which is why this is sometimes described as subclinical insufficiency. When vitamin K status falls far enough to affect clotting, reported signs include easy bruising, bleeding gums, frequent nosebleeds, heavier than usual menstrual bleeding, blood in urine or stool, and cuts that take longer than expected to stop. Any of those, especially bleeding that is new or unexplained, is worth discussing with a clinician promptly rather than monitoring at home.
Common causes. Low intake of green vegetables over a sustained period. Fat malabsorption from conditions such as coeliac disease, inflammatory bowel disease, cystic fibrosis, pancreatic insufficiency, bile duct obstruction or cholestatic liver disease. Previous bariatric surgery or significant bowel resection. Chronic liver disease, which affects both storage and the proteins made downstream. Medications that reduce fat absorption or bind bile acids. Newborns are a distinct case, because placental transfer is limited, breast milk is low in vitamin K and the infant gut is not yet colonised, which is why prophylaxis at birth is standard practice in paediatric guidance 6. A multicentre study in term healthy infants reported measurable vitamin K insufficiency in that population and examined how prophylaxis strategy relates to it 7.
Longer view. Beyond clotting, the concern with chronically low status is the slow stuff, incompletely carboxylated osteocalcin in bone 2 and incompletely carboxylated matrix Gla protein in arteries, particularly in people with reduced kidney function where poor vitamin K status is reported more often 5.
More things move this number than move most vitamin results, and several of them have nothing to do with your long-term nutrition.
Testing conditions. Time since your last meal is the single biggest influence, since levels rise within hours of eating vitamin K rich food and fall again quickly. The fat content of that meal matters too, because absorption depends on it. Phylloquinone is light-sensitive, so sample protection, transport and processing time can all affect the reported value, and methods differ between laboratories, which makes comparing results across providers unreliable. High triglycerides can raise the measured concentration independently of tissue status.
Medications. Vitamin K antagonist anticoagulants act directly on the recycling enzyme in this pathway 9. Bile acid sequestrants, orlistat and mineral oil can reduce absorption of fat-soluble vitamins generally. Never start, stop or change a prescription medication based on a lab value, that is a decision a licensed provider makes after reviewing the full clinical picture.
Habits and physiology. Weeks of minimal vegetable intake, very low fat eating patterns, recent gastrointestinal illness, heavy alcohol use affecting liver function, and supplement use including multivitamins and combination bone formulas. Pregnancy, age and body composition can also shift how the vitamin is distributed. This page does not give amounts or schedules for any supplement or medication, because the right approach depends on your clinical context.
FitScript does not currently test vitamin K1 in a panel, so there is no result to generate from membership testing alone.
If you already have a phylloquinone result from another laboratory, you can upload your labs so Atlas, FitScript's AI health advisor, can read it alongside the rest of your markers. That context is usually where the value lies, since a single vitamin K1 number in isolation says very little, while the same number next to clotting times, liver enzymes, bone chemistry and iron studies starts to tell a coherent story. Atlas uses the markers available to write your FitScript, the personalised output built from your own biology, and membership testing covers 150+ biomarkers tested per year across metabolic, hormonal and nutritional categories.
Where a result points toward something that might need prescription input, a licensed provider evaluates whether any treatment is appropriate after reviewing you as a whole person. Nothing on this page replaces that evaluation.
Vitamin K1 is most informative when read with the other markers that share its absorption pathway or its downstream targets.
Research suggests that vitamin K status is best judged from a combination of intake, circulating vitamin and functional carboxylation markers rather than any single number 1.
Vitamin K1 is the plant-derived form of vitamin K that your liver uses to activate clotting factors, and that bone and blood vessels use to activate calcium-binding proteins. A blood result captures recent intake and absorption rather than stored reserves, which is why timing, meal composition and triglycerides all influence it. High results are usually dietary and unremarkable. Low results point toward intake, digestion or absorption, and matter most when read next to clotting, bone and iron markers. Trial results for vitamin K supplementation in postmenopausal bone health have been mixed 4.
This article is educational and is not medical advice. Discuss your results and any treatment decisions with a clinician or licensed provider who can review your full history.
For related reading on fat-soluble vitamin testing and what nutrient status can and cannot explain, see does vitamin d affect sleep and does vitamin d affect testosterone, and if you hold results from an outside laboratory you can bring them into one view when you upload your existing labs.
Vitamin K1 (phylloquinone) comes from plants, mainly green leafy vegetables and some vegetable oils, and is taken up largely by the liver for clotting factor activation. Vitamin K2 (menaquinones) comes from fermented foods, animal products and gut bacteria, and generally has a longer half-life with wider distribution to bone and blood vessels.
Not necessarily. Circulating phylloquinone clears quickly and mostly reflects what you ate in the hours before the draw, so a normal value is a snapshot rather than a measure of tissue reserves. Laboratory guidance regards functional markers such as prothrombin time, PIVKA-II and undercarboxylated osteocalcin as useful complements when status is genuinely in question. Your clinician decides which of those is worth adding.
Follow the instructions from the laboratory running the test, because methods differ. In general, timing relative to meals has a large effect on this marker, since levels rise within hours of eating vitamin K rich food and fall again quickly. High triglycerides can also raise the measured concentration, because phylloquinone travels in the blood on triglyceride-rich lipoproteins.
Low vitamin K status is associated with impaired activation of clotting factors, and reported signs include easy bruising, bleeding gums, nosebleeds and cuts that take longer to stop. That said, bruising has many possible explanations, including platelet issues, medications and iron status, so a single low value does not identify a cause. New or unexplained bleeding is worth discussing with a clinician promptly.
Low results turn up most often in people with fat malabsorption, including coeliac disease, inflammatory bowel disease, cystic fibrosis, pancreatic insufficiency and bile duct or cholestatic liver conditions, and after bariatric surgery or bowel resection. Newborns are a separate situation addressed by standard paediatric prophylaxis guidance.
The usual next step is repeating the test under standard conditions.
FitScript does not currently test vitamin K1 in a panel. If you already have a result from another laboratory, you can upload your labs so Atlas, FitScript's AI health advisor, reads it alongside your other markers such as vitamin D, calcium, alkaline phosphatase and iron studies. Where prescription options come into the conversation, a licensed provider evaluates whether anything is appropriate.
There is no universal interval, and the sensible timing depends on why it was measured in the first place. Because the value swings with recent meals, repeat testing is usually more informative when the underlying question is clear, for example tracking an absorption problem or following up after a change in diet or digestive health. Ask the clinician who ordered the test what interval fits your situation.