Ask Dr Miriam

Q I’ve just been put on warfarin for a heart condition and the diet sheet I’ve been given says to avoid sprouts and spinach. Why is that?

A This is because green leafy vegetables such as sprouts and spinach are high in vitamin K, which the body uses to make clotting factors in the blood. This is the opposite of what warfarin does, which is to thin the blood. So vitamin K theoretically damps down the effect of warfarin.

You might wonder if another green leafy vegetable, cabbage, can affect you and the answer is yes. Cabbage, cauliflower and kale contain substances that can affect thyroid function by blocking the production of the hormone, thyroxine. However, the effects are small so cabbage is unlikely to cause problems in healthy people and those who are taking the drug levothyroxine for an underactive thyroid.

But these vegetables may be a problem for people who have undiagnosed low thyroid function giving symptoms of tiredness and weight gain.