The Starbucks Trenta question: How much coffee is OK?

The Trenta will be offered only for three drinks

Starbucks is launching a huge new measure for its coffee shops. The Trenta will contain 916ml of iced coffee but is it bad for you to drink that much in a sitting?

In a nation fighting obesity, the arrival of a giant new cup in the US coffee shops of Starbucks might leave some people scratching their heads.

The 31-US fluid ounce (916ml) Trenta dwarfs their previous largest measure - the Venti cup at a paltry 20 US fluid ounces (591ml).

Scaling up a 591ml coffee to a 916ml coffee, you soon see there might be health implications.

Taking a Starbucks Venti whole milk caffe latte and making it a Trenta, you would be left with a drink that contained 447 calories, including 23.2g of fat with 13.3g of that saturated fat.

You also end up with an alarming-sounding proposition if you scale up an iced hazelnut mocha made with semi-skimmed milk and whipped cream (from Starbucks' UK offering). As a Trenta this would add up to 929.2 calories, 24.3g of saturated fat, and 122g of sugar.

But these drinks remain purely fictional, because Starbucks says it is only using the new measure for a very limited range of drinks - two types of iced tea and plain iced coffee. There will also be a significant proportion of ice in the drinks.

For its Trenta iced coffee, Starbucks advertises total fat of 4.5g, of which 2.5g is saturated. The calorie count is 230. The sugar clocks in at 42g and the caffeine content is 195mg, equivalent to 2.6 of the firm's espresso measures.

Only the sugar content might cause concern, with Emma Williams, a nutrition scientist at the British Nutrition Foundation, suggesting an RDA of 90g a day for women and 120g a day for men. The Starbucks Trenta iced tea lemonade contains considerably more sugar with 59g.

As far as the caffeine goes, two Trenta iced coffee cups a day would be fine as 400mg of caffeine a day is widely considered safe.

"If somebody only drinks one or two cups of coffee throughout the day and spaces them out with water they should be fine," says Dr Sarah Schenker, a registered dietician.

"But as adults we don't get enough calcium so if coffee is the way we can up our calcium intake then that is the way to do it."

She admits the new 916ml Starbucks Trenta would go through a drinker very quickly. But the 195mg of caffeine within the drink would not have a huge effect on a person who drinks coffee regularly.

"The safest intake of caffeine through the course of the day is about 400mg. The average mug of instant coffee is around 80-90mg."

She admits that coffee can have its positives as it is a rich source of antioxidants, but pregnant women should not consume more than 200mg of caffeine a day.

And of course for most people, the problems with a 916ml iced coffee will be purely logistical - where do you put it and how quickly can you drink it?