Vitamin K in Tea: Nutritional Myth or Reality?

12 mai

The great thing about health claims is that they give me ideas for articles. I've already told you about vitamin C in tea in a previous article, now I'm going to tell you about vitamin K. This vitamin is used by the liver to produce coagulation factors, which are essential for blood fluidity. In addition to its role for the blood, it contributes to bone health and inflammation, making it a vitamin studied in cardiovascular, rheumatoid and nephrotic diseases.

Vitamin K is produced in two forms with similar functions: K1 and K2. Vitamin K1 is the most widely ingested, but is less well retained in the body than vitamin K2. Moreover, vitamin K1 is the vitamin produced in plants, including tea leaves.

Tea was once considered a source of vitamin K1, unfortunately for its ability to inhibit the effect of anticoagulant drugs such as warfarin. Indeed, a 44-year-old patient on anticoagulants following surgery for heart disease drank over 1.5 liters of green tea a day. The doctors observed no effect of the warfarin. When the patient stopped drinking green tea, the warfarin took effect.

In 1995, Booth et al. studied vitamin K levels in beverages brewed from plants, including tea. In dry leaves, the average vitamin K level measured was 894 µg/100g, or 8.94µg/g of tea (green and black combined). The levels measured were extremely variable from one batch of tea to the next, suggesting that the vitamin K content is dependent on the harvesting/manufacturing and storage of the leaves. The publication was unable to accurately measure vitamin K levels in tea infusions (probably because of this variability). However, they were able to measure vitamin K levels in brewed coffee beans.

Vitamin K levels in coffee beans were less variable, but showed a very sharp post-infusion decrease (0.03 µg/100g coffee), which would mean that hot water and diffusion for several minutes would have a negative impact on vitamin K. Following this logic for , there would only be around 0.08µg of vitamin K/g of tea ingested (for a 30cl mug of classic infusion, this represents 0.48µg of vitamin K).

Health authorities recommend 120 to 140 µg of vitamin K for adults. Unless you eat more than 10g of tea leaves (in addition to your diet), brewed tea is unlikely to be a source of vitamin K supplementation. Vegetables such as peas are a more reliable source of vitamin K1, for example.