Calcium supplements, taken by millions of elderly people and post-menopausal women to prevent bone thinning, may double the risk of having a heart attack, a study has found.
Researchers warned that the pills should be "taken with caution", and experts commenting on the findings questioned their safety.
Previous studies linked higher calcium intake with a reduction of heart disease risk factors such as high blood pressure, obesity and Type 2 diabetes.
But the new research from Germany points to a vital difference between dietary calcium from sources such as milk, cheese, greens and kale, and supplements.
Taken in supplement form, the mineral floods the bloodstream, causing changes that may produce hard deposits on the walls of arteries, scientists believe.
Researchers analysed data on 23,980 German men and women aged 35 to 64 taking part in a study called the European Prospective Investigation into Cancer and Nutrition.
Over a period of 11 years, a total of 354 heart attacks, 260 strokes and 267 associated deaths were recorded.
Participants whose diets included a moderate intake of calcium - around 820 milligrams (mg) daily - from all sources had a 31% lower heart attack risk than those with the lowest intake.
But no significant benefit was seen when calcium intakes rose to more than 1,100 mg per day.
The picture changed for the worse when the scientists focused on supplements. People taking supplements that included calcium were 86% more likely to suffer a heart attack than those taking no supplements.
For participants who only used calcium supplements, heart attack risk more than doubled.
The researchers led by Professor Sabine Rohrmann, from the University of Zurich, Switzerland, wrote in the online edition of the journal Heart: "In conclusion, this study suggests that increasing dietary calcium intake might not confer significant cardiovascular benefits, while calcium supplements, which might raise MI (myocardial infarction, or heart attack) risk, should be taken with caution."
In a comment article accompanying the research paper, two experts from New Zealand examined the safety of calcium supplements.
Professors Ian Reid and Mark Bolland, from the University of Auckland, pointed out that previous research had linked the supplements to kidney stones and gut and abdominal symptoms.
For many healthy middle-aged women, taking calcium supplements to ward off brittle bones, the overall protective effect was only about 10%.
Profs Reid and Bolland stressed that dietary calcium, taken in small amounts spread throughout the day, was absorbed slowly.
But supplements caused calcium levels in the blood to soar above the normal range, possibly increasing the risk of artery calcification.
"It is now becoming clear that taking this micronutrient in one or two daily boluses is not natural, in that it does not reproduce the same metabolic effects as calcium in food," wrote the experts, from the University of Auckland's faculty of medical and health sciences.
"The evidence is also becoming steadily stronger that it is not safe, nor is it particularly effective. Therefore, the bolus administration of this micronutrient should not be encouraged, rather, people should be advised to obtain their calcium intake from an appropriately balanced diet."
Find out which foods help keep your heart healthy...
Foods That Ward Off Heart Disease
Eat yourself to a healthy heart with these cardiovascular-friendly foods.
Oats contain beta glucan, a soluble fibre that helps reduce cholesterol levels, especially LDL (bad cholesterol), which damage the heart.
Green Leafy Vegetables
Green leafy vegetables like spinach, fenugreek, pak choy, radish leaves, lettuce are known to reduce the risk of heart disease as they are rich sources of folic acid, magnesium, calcium and potassium - the essential minerals for keeping the heart functioning properly. Studies have shown that one daily serving of green leafy vegetables can lower the risk of heart disease by 11%.
Soy is a healthy protein alternative to red meat, as it has a low saturated fat content, no cholesterol and even increases your HDL 'good' cholesterol, which is good news for your heart.
Regular consumption of tomatoes is known to reduce the risk of heart disease, as they contain a rich source of vitamin K, which help prevent hemorrhages.
Wholegrains contain high levels of vitamin E, iron, magnesium and a host of anti-oxidants, which are all beneficial to the heart as they help reduce blood pressure.
Apples contain guercetin, a photochemical containing anti-inflammatory properties, vital for keeping blood clots at bay, which can lead to heart attacks.
Almonds, when eaten in moderation, are known to lower cholesterol levels as they contain monosaturate fats (the 'good' fats), as well as vitamin B17, vitamin E and minerals like magnesium, iron and zinc.
Red wine (when drank in moderation) can be good for the heart as it contains a powerful antioxidant called resveratrol, which helps prevent damage to blood vessels, reduces "bad" cholesterol and prevents blood clots.
The British Heart Foundation urged people not to stop taking calcium supplements prescribed by their doctors.
Natasha Stewart, the charity's senior cardiac nurse, said: "This research indicates that there may be an increased risk of having a heart attack for people who take calcium supplements. However, this does not mean that these supplements cause heart attacks.
"Further research is needed to shed light on the relationship between calcium supplements and heart health. We need to determine whether potential risks of the supplements outweigh the benefits calcium can give sufferers of conditions such as osteoporosis.
"If you have been prescribed calcium supplements, you should still keep taking your medication, but speak to your doctor if you have any concerns."
Dr Carrie Ruxton, from the Health Supplements Information Service, which is funded by an association representing supplement manufacturers, criticised the research.
She pointed out that calcium intakes were only measured once, and no attempt was made to observe the effect of modifying them or make comparisons with a "dummy" placebo supplement.
She said: "Vitamin and mineral supplements were never intended to treat chronic health conditions. Instead, their role is to help people meet recommended nutrient intakes.
"According to the UK National Diet and Nutrition Survey, women consume only 740mg of calcium per day from food sources on average.
"This is 43mg lower than in 2000/01. Osteoporosis (brittle bone disease) is a real issue for women and it is irresponsible for scientists to advise that women cut out calcium supplements on the basis of one flawed survey, particularly when the link between calcium, vitamin D and bone health is endorsed by the European Food Safety Authority."
Natural Ways To Strengthen Your Bones
Try Soy-Based Foods
Calcium-enriched soy foods, like tofu, contain as much as 861mg of calcium, which is a large proportion of your recommended daily allowance. Soy has a similar effect as oestrogen on the body, helping strengthen bones.
Milk is one of the richest sources of calcium. One glass of milk contains 300mg of calcium.
Go In The Sun
Sunshine is a natural vitamin D stimulant. Without vitamin D, the body cannot absorb calcium, a crucial mineral which helps build healthy bones.
Feast On Sardines
Another rich source of calcium. Just 3 ounces of canned sardines has the same calcium equivalent as a cup of milk.
Eight ounces of milk contains the same amount of calcium than milk. Lactose and dairy-free products contain the same amount of calcium than its dairy counterpart.
Cut Back On Salt
A high intake of salt deprives the body of calcium - the vital mineral for maintaining healthy bones. The more salt that is consumed, the more calcium is flushed away through the urine. Sticking to a low-salt diet helps the body retain its natural calcium levels for longer.
Peanuts (unsalted) and almonds are great sources of potassium, which helps protect against the loss of calcium in the body. Nuts are also excellent sources of protein and other vital nutrients that play a supportive role in building strong and healthy bones.
Top Up On Your Greens
Dark,leafy greens are packed full of calcium. Greens such as bok choy, Chinese cabbage and kale have the highest levels of the mineral.