A large UK study has found further evidence that people with more calcium in their diet - equivalent to a glass of milk a day - can help reduce their risk of bowel cancer.

The researchers analysed the diets of more than half a million women over 16 years and found dark leafy greens, bread and non-dairy milks containing calcium also had a protective effect.

They also found more evidence that consuming too much alcohol and processed meat has the opposite effect, increasing the risk of the disease.

Cancer charities say having a healthy, balanced diet, being a healthy weight and stopping smoking were the best ways to lower your risk of bowel cancer.

A recent review found dairy products "probably" decreased the risk of colorectal (bowel) cancer.

This study, from Oxford University and Cancer Research UK, suggests that is down to calcium, from dairy or non-dairy foods.

• an extra 300mg of calcium a day in the diet, or a large glass of milk, lowers your risk by 17%

"It highlights the potential protective role of dairy, largely due to calcium, in the development of bowel cancer," said lead researcher Dr Keren Papier, from Oxford.

Breakfast cereal, fruit, wholegrains, carbohydrates, fibre and vitamin C also showed they lowered the risk of the cancer, but only slightly.

It is already well-known that eating too much processed meat and red meat probably increases your risk of bowel cancer, as does alcohol.

This study provides more evidence of that link:

• drinking an additional large glass of wine a day, or 0.7oz (20g) of alcohol, increases your risk by 15%
• eating 1oz more red and processed meat a day, such as a slice of ham, increases your risk by 8%

Quantifying exactly what those percentages mean is tricky, because everyone's risk of bowel cancer is different depending on their lifestyle, diet, habits and genetics.

Calcium is an important mineral for strengthening bones and keeping your teeth healthy, but there is growing evidence it also protects against some cancers.

There is lots of calcium in milk, yoghurt and cheese. Dairy products are one of the main sources of it in the UK diet (we love our cereals in the morning).

It is also present in other foods like soya and rice drinks, white bread, nuts, seeds and fruits like dried figs, curly kale and canned sardines, and it is also in lactose-free milk.

The study says calcium may protect against bowel cancer "because it is able to bind to bile acids and free fatty acids in the colon, lowering their potentially carcinogenic effects".

There are about 44,000 cases of bowel cancer every year in the UK, making it the fourth most common cancer.

Although most cases are in older people, rates of the cancer are rising among younger adults under 50 - but there is no clear reason why.

Experts say poor diet and obesity may be among the factors involved.

Symptoms of bowel cancer include:

• a change in your bowel habits, such as looser poo, pooing more often or constipation
• bleeding from your bottom or finding blood in your poo
• losing weight when you have not been trying to
• unexplained tiredness or breathlessness

Advice is to talk to your doctor if you notice any one of them.

This was an observational study, not a trial, so it cannot prove categorically that calcium or any other food product protects against cancer or makes it more likely.

However, the researchers say the study is "the largest on diet and bowel cancer to date", which gives them confidence they are on the right track. The findings are also in line with previous studies' conclusions.

More than 12,000 women in the study developed bowel cancer, and nearly 100 food products and nutrients in their diets were investigated to assess potential links.

Nutrition expert Prof Janet Cade, from the University of Leeds, said the paper "provides important evidence showing that overall diet can influence risk of colorectal cancer".

Prof Andrew Prentice, from the London School of Hygiene & Tropical Medicine, wonders whether, in the light of the study results, calcium supplements might be protective although he says "the jury is out on this".

The take-home message for Prof Tom Sanders, from King's College London, is that "drinking above the safe limits of alcohol intake (more than 14 units per week) increases [the] risk of colorectal cancer in women, but that drinking about half a pint of cows' milk a day is probably protective".

Dr Lisa Wilde, from charity Bowel Cancer UK, says someone is diagnosed with bowel cancer "every 12 minutes" and half of all bowel cancers could be prevented with healthier lifestyles.

"If you don't drink dairy milk there are other ways you can get calcium, for example from broccoli or tofu, and still reduce your bowel cancer risk," she says.