Taking vitamin D3 and K2 for your arteries? What the big trials actually found

Capsules combining vitamin D3 and K2 promise to steer calcium out of your arteries and into your bones. The biology behind K2 is interesting, but the human trials are far less convincing. Here is what vitamin D3 and K2 do, what the large trials found, how much vitamin D is recommended, which foods supply K2 and who needs to be careful. (Based on the insights of Dr. Alex)

Why Calcium in the Artery Wall Matters

Calcium belongs in bones and teeth, but as we age, a surprising amount ends up in the walls of our arteries. If you scanned a large group of people in their 70s, most would have at least some calcium in the blood vessels around the heart, alongside the fatty plaque that narrows those arteries. Calcium also stiffens the wall, turning a soft, stretchy tube into something more like a rigid pipe.

A stiff artery cannot cushion the pressure wave from each heartbeat. The top number of your blood pressure, the systolic pressure, creeps up, the heart has to push harder, and the harder pulse reaches the small, delicate vessels of the brain and kidneys. Over decades, this is linked to strokes, heart attacks, kidney decline and even memory problems. That is why supplements that claim to steer calcium out of the arteries have become popular, and why it is worth asking what vitamin D3 and K2 can and cannot do.

Vitamin D3: Good for Bones, Unproven for the Heart

Vitamin D comes as D3, which your skin makes in sunlight and which is found in oily fish and eggs, and D2, which comes from plants and fungi and is used to fortify foods. Compared head to head, D3 tends to raise blood levels more and hold them longer, so it is usually the form chosen for supplements.

Its main job is helping you absorb calcium from the gut. Without enough, bones become softer and weaker, with aching bones, weak muscles around the hips and thighs, and more falls and fractures. People with low vitamin D also tend to have more heart disease in observational studies, so the obvious question was whether giving everyone vitamin D prevents it. The largest trial, called VITAL, gave nearly 26,000 healthy American adults either 2,000 units of vitamin D a day or a placebo and followed them for about five years. There was no difference in heart attacks, strokes or deaths from heart disease, and other large trials have found much the same. A low vitamin D level may be mostly a marker of being unwell, less active or indoors more. So vitamin D is worth getting enough of for your bones and muscles, but not because it protects your heart.

How Much Vitamin D, and When

• UK: 10 micrograms (400 units) a day is recommended, and everyone is advised to consider it from October to March, when the sun is too low for the skin to make much. Some groups should take it all year: people who rarely go outdoors, people in care homes, people who cover most of their skin and people with darker skin, which needs more sunlight to make the same amount.
• US: 600 units a day for most adults and 800 units over age 70.
• Upper limit: 100 micrograms (4,000 units) a day for adults. More than that without a reason can push blood calcium too high, causing thirst, confusion and kidney stones, and, ironically, calcium deposits in places you do not want them.
• Food: oily fish such as salmon, mackerel and sardines are good sources, along with egg yolks and fortified foods, although food rarely gets you there in winter.

A blood test called 25-hydroxy vitamin D measures your stored level. It is not needed by everyone, but your doctor may check it if you have bone pain, muscle weakness, frequent falls or a condition that affects absorption, such as celiac disease.

Vitamin K2: Real Biology, Mixed Human Results

Vitamin K comes in two main forms. K1, in leafy greens such as spinach, kale and broccoli, is used mostly by the liver to make clotting factors. K2 is made by bacteria, so it is found in fermented foods and some animal foods, and it seems to act elsewhere in the body. One thing it does is switch on matrix Gla protein, made by cells in the artery wall, which works a bit like a security guard stopping calcium crystals from forming, but can only start work once vitamin K has handed it its keys. People on warfarin, which blocks vitamin K, tend to have more calcium in their arteries. In mice lacking this protein, arteries calcify. So the biology is real.

The question is what happens in people:

• The Rotterdam Study found that older adults who ate the most K2, mostly from cheese, had lower rates of death from heart disease. That is an observational finding.
• One randomized trial in postmenopausal women found that three years of a vitamin K2 (menaquinone-7) supplement slightly improved a measure of arterial stiffness.
• A Danish trial in men who already had calcium building up in a heart valve found that K2 plus vitamin D for nearly two years did not do much.
• No trial has shown that K2 supplements reduce heart attacks or strokes.

So no one can say that a K2 supplement protects your arteries. Food is different.

Do You Need D3 and K2 Together?

The logic behind combination capsules is that vitamin D makes you absorb more calcium, which will end up in your arteries unless K2 directs it away. It sounds sensible, but there is no convincing evidence that normal doses of vitamin D cause calcium to build up in the arteries. In VITAL, people took 2,000 units a day for five years with no K2 at all and had no more heart disease than the placebo group. At sensible doses, there is no evidence that you need K2 alongside your vitamin D.

Where to Get Vitamin K2

• Natto, fermented soybeans eaten in Japan, is by far the richest source, although it is sticky and an acquired taste.
• Aged cheeses such as Gouda, Edam and Jarlsberg.
• Egg yolks and chicken.
• Leafy greens provide K1.

Who Needs to Be Careful

• Warfarin: do not start any vitamin K supplement without speaking to your anticoagulation clinic or doctor, and avoid a large sudden jump in leafy greens, which could undo the effect of the medicine and raise the risk of a clot.
• Kidney disease, past kidney stones, high blood calcium or sarcoidosis: speak to your doctor before taking vitamin D, because even normal doses could push calcium too high.

If you plan to take these vitamins anyway, check with your own doctor first.

What Actually Protects Your Arteries

• Eat oily fish two or three times a week, eggs, plenty of leafy greens and, if you enjoy them, aged cheeses or fermented foods.
• Get outside in daylight in the warmer months without burning.
• Take vitamin D3 in the dark months if advised, at the recommended dose, unless your doctor says otherwise.
• Keep blood pressure down, do not smoke, keep blood sugar in check and move every day.

These habits do far more for artery stiffness than anything in a capsule. Skip high-dose vitamin D without a reason, and skip combination capsules marketed for artery or heart health. See a doctor for bone pain, weakness or any persistent or worsening symptom.

Frequently Asked Questions

This article is for general educational purposes and is not a substitute for personalized medical advice. Do not start vitamin K supplements if you take warfarin without speaking to your doctor, and talk to your doctor before taking vitamin D if you have kidney disease, kidney stones, high calcium or sarcoidosis.