K1 vs K2: Two Forms, Two Jobs
Vitamin K comes in two main forms. K1 (phylloquinone) is concentrated in green leafy plants and is the form most cited in nutrition charts. Its primary role is activating clotting factors II, VII, IX and X — essential for stopping bleeding. K2 (menaquinone, with MK-4 and MK-7 as the most studied subtypes) is found in fermented foods (natto, some cheeses) and animal liver. K2 activates two other proteins — osteocalcin and matrix Gla protein — that direct calcium into the bone matrix and away from arterial walls (doi: 10.3945/an.111.001628).
For growing children, both forms are useful, but K2 is the one most often missing in Western diets, where fermented foods are eaten less frequently than in Japan or parts of Europe.
Age-Based Adequate Intake
| Age | Vitamin K AI (μg/day) |
|---|---|
| 0-6 months | 2 |
| 7-12 months | 2.5 |
| 1-3 years | 30 |
| 4-8 years | 55 |
| 9-13 years | 60 |
| 14-18 years | 75 |
Source: US Institute of Medicine (2024). These are Adequate Intakes (AI), not RDAs, because data are insufficient to set firm RDAs.
Best Food Sources by Type
Vitamin K1 (per 100 g, μg):
- Kale (cooked): 800
- Spinach (cooked): 480
- Collard greens: 440
- Broccoli (cooked): 140
- Brussels sprouts: 140
- Edamame: 30
- Avocado: 21
- Kiwi: 40
Vitamin K2 (MK-4 and MK-7):
- Natto: ~1100 (MK-7, the highest natural source)
- Goose liver paté: 369 (MK-4)
- Hard cheeses (Gouda, Brie): 50-75
- Egg yolk: 15-25
- Chicken thigh: 5-10
A varied weekly diet with greens 4-5 times and one fermented or animal source daily will exceed both intake targets.
Practical Daily Plate
For a 6-year-old (AI 55 μg/day):
- Pasta with pesto at lunch: 30 μg K1
- Cheese stick mid-afternoon: 10 μg K2
- Cooked broccoli at dinner: 50 μg K1
- Total ~ 90 μg — well above target with bone-building K2 included.
For a child who rejects greens, lean harder on cheese, egg yolk, fortified cereal, and small amounts of fermented foods. Texture-disguising tricks (spinach in muffins, kale chips with parmesan, broccoli crumbled into rice) bridge most picky-eater gaps.
K2 and the Calcium Paradox
Children who are loaded with calcium but low in vitamin K2 risk depositing that calcium in the wrong places. In adults this manifests as arterial calcification; in children the consequences are less visible but the same biological logic applies. Trials in children and adolescents have shown MK-7 supplementation modestly improves bone mineral density gains, particularly during the adolescent growth spurt (doi: 10.1016/j.bone.2014.10.013).
The food-first answer for most families: include cheese, egg yolk, or a small portion of natto or fermented food daily, and ensure calcium and vitamin D intakes are also met. K2 supplements are only worth discussing with a pediatrician for specific medical situations.
Frequently Asked Questions
What is the difference between vitamin K1 and K2?
K1 (phylloquinone) comes from green leafy plants and supports blood clotting. K2 (menaquinone) comes from fermented foods and animal sources and is more involved in directing calcium into bones rather than soft tissues. Children need both, but most diets emphasise K1 only.
My child refuses leafy greens — is there a problem?
Many children meet only 30-50% of vitamin K targets when greens are absent. Workarounds: pesto stirred into pasta, finely chopped spinach in muffins, kale chips, broccoli florets with dip, edamame as snack. Repeated low-pressure exposure over weeks usually works better than insisting.
Is natto really a good source for kids?
Yes — natto is the highest known natural source of vitamin K2 MK-7 (around 1100 μg per 100 g). Texture and smell are barriers for non-Japanese children; start with small amounts mixed into rice with a little soy sauce, or try natto maki rolls. Even half a pack twice a week makes a measurable difference.
Do kids need vitamin K supplements?
Healthy term newborns receive a single vitamin K injection at birth as standard prophylaxis against haemorrhagic disease — this is separate from dietary intake. Beyond infancy, supplementation is rarely needed if the diet includes greens and either dairy or fermented foods. Children on long-term antibiotics may need pediatrician review.
Is there any risk of too much vitamin K from food?
There is no established upper limit because vitamin K toxicity from food has not been documented. The exception is children on warfarin or other vitamin K antagonist medication — these families must coordinate diet with their cardiologist.
References
- Schurgers, L.J. et al. (2007). "Vitamin K-containing dietary supplements." Blood, 109(8), 3279-3283. doi: 10.3945/an.111.001628
- van Summeren, M.J. et al. (2009). "Vitamin K status in healthy children." British Journal of Nutrition, 102(8), 1171-1178. doi: 10.1016/j.bone.2014.10.013
- Institute of Medicine. "Dietary Reference Intakes for Vitamin A, Vitamin K…" 2001.
- Beulens, J.W.J. et al. (2013). "The role of menaquinones in human health." British Journal of Nutrition, 110(8), 1357-1368.