It’s the time of year many of us are tempted to swap our usual coffee for a special festive hot drink.

However, a new study by the charity Action Against Sugar shows just how high in sugar those drinks can be – and how high street coffee chains are failing to reduce these levels.

The charity, based at Queen Mary University in London, analysed the sugar and calorie content of the largest available sizes of 124 hot chocolates and 79 seasonal lattes made with milk and milk alternatives. It found that almost all of the large drinks received a red traffic light for total sugars, with some containing as much sugar as three cans of Coca-Cola.

One particularly surprising thing to come out of the report is how people who choose the vegan option often don’t realise how high in sugar it is, because plant-based milks are seen as more ‘virtuous’ and have a health halo around them. In actual fact, many coffee shops use pre-sweetened dairy-free milks, and nutritional information about them is often hard to find.

While 27% of products directly comparable to a similar study done in 2016 have seen an increase in sugar, others have fared better – like some drinks from Costa, which have reduced in sugar content by more than 50% since 2016.

"The UK guidelines for an adult’s daily intake of sugar state a maximum of seven teaspoons," says Dr. Diana Gall of Doctor4U. "With some festive drinks containing over three times this daily limit, it’s important to know what happens to your body when you consume so much sugar in such a short space of time,"

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The first place sugar hits is your mouth. "Within minutes of consuming something sugary, the bacteria in your mouth combines with the sugar and forms an acid that can wear away tooth enamel," Gall explains.

"Over time, this can cause dental problems such as tooth decay and cavities, potentially leading to tooth loss."

Read more: How to protect children's teeth this Christmas

The sugar is then absorbed into your bloodstream.  "Blood glucose rises, causing a spike in insulin levels," Gall says. "Insulin helps to regulate blood glucose levels, but with some festive drinks having excessive sugar content, some of it will need to be stored as fat.

"Increased blood glucose levels caused by sugar consumption can also raise blood pressure and increase the heart rate, due to the effect it can have on the arteries."

More immediately, Gall says you might experience hunger and sugar cravings when your blood glucose levels drop back to normal, known as the dreaded ‘sugar crash’.

Gall says: "Some sugar in your diet is good, as are most things when eaten in moderation. However, long-term excessive sugar consumption can lead to chronic and serious conditions such as diabetes, obesity, heart disease, and even put you at risk of some cancers.

"The immediate effects of sugar are bad enough without needing to worry about long-term consequences. "