A single unit of alcohol, as defined by the Government, is 125ml (4fl oz) of a wine that is 8 per cent alcohol by volume (ABV). This measure was devised in the late 1980s when people were knocking back glasses of Liebfraumilch and lambrusco, both low in alcohol. But times have changed and so has our taste in wine. Nowadays, it’s difficult to find wine with an ABV under 12 per cent and it is not uncommon for it to go up to 15 per cent, almost twice the strength of 1980s wine.

New World wines are partly responsible. Their grapes are grown in a hot climate and can be left on the vine for a long time, which causes alcohol levels to soar. They also use techniques, such as adding sugar, which can increase the amount of alcohol in the brew.

And it’s not just alcohol levels that have risen. Glass sizes in bars and pubs have also increased. Most high street chain bars, such as All Bar One and Pitcher and Piano sell wine in 175ml glasses, with a large glass containing 250ml — that’s a third of a bottle in a glass. This means a glass of wine, traditionally thought of as one unit, could be as much as 3.7 units. Suddenly the hangover mystery — “Why do feel this way when I had only two glasses?” — is easier to understand.

The combination of large glasses and high alcohol levels makes it difficult to drink moderately when out, particularly at Christmas, when a certain amount of excess seems to be expected. And some bars will make it even more difficult. Often there is no mention of alcohol levels at point of sale and it is rare that a wine with an ABV of less than 12 per cent is available. Some pubs insist on selling only 250ml glasses of wine, hardly an encouragement to drink sensibly.

Buying wine from an off-licence or supermarket and drinking at home makes it easier to keep tabs on units. Bottles packaged for Marks & Spencer, Sainsbury’s and Tesco have how many units they contain written on the label but bottles imported by wholesalers don’t.

“Most European countries have their own unit of measurement,” says David Tromans, a spokesman for the Wine and Spirits Trade Association. “Importers could ask for units to be put on labels of wine headed for Britain but they don’t, maybe because of the extra cost.”

So, quite often, it’s up to us to figure out how many units we are consuming. The Drink Aware Trust reports that its website gets more than 56,000 hits a day from people wanting to find out how many units they are consuming. You can do this with an equation: units drunk equals ABV multiplied by the volume of wine in ml divided by 1,000 (see box below for our easy cut-out-and-keep keep guide). However, given the vogue for outsized wine glasses, some holding an entire bottle of wine, it’s important to monitor exactly how much wine is being poured.

The chances are that you might be an accidental binge drinker. The official figure for binge drinking is only six units for women, or two large glasses of wine, and eight units for men. The average UK adult binge drinks 28 times a year, compared to an Italian, where the figure is just eight times a year, or 14 times a year for a Spanish adult. And it’s not just about how much we drink, it’s the way we drink. In Italy, 51 per cent of people only drink alcohol when they are eating. In the UK, only 7 per cent of people only drink when they are eating. Drinking with food slows down the rate at which alcohol is absorbed, which lessens its effect and reduces drunkeness.

HOW WOMEN AND MEN DIGEST ALCOHOL

As soon as we have a drink our bodies get to work to break down the alcohol and get it out of our bodies as quickly as possible. About 90 to 98 per cent of it is broken down by a group of six enzymes, known collectively as alcohol dehydrogenase. These enzymes are present in small amounts in our stomachs and in larger amounts in our livers. Very small amounts of alcohol are lost in our urine about 40 minutes after we start drinking and tiny amounts exit our bodies in sweat and tears.

Women have less alcohol dehydrogenase in their stomachs than men, which is part of the reason why they feel the affects of alcohol more rapidly. But this is only part of the explanation. Women also have more fat and less water in their bodies and so alcohol is less diluted in their systems. All in all, this means that it is physiologically impossible for girls to keep up with the boys.

It is thought that, on average, a healthy person can metabolise just under a unit of alcohol an hour, although this varies a lot depending on sex and size, ethnic group and how regularly and how much you drink, not to mention whether you are taking certain medications. A big bloke who drinks every day may make more alcohol dehydrogenase since the production of these enzymes can be induced the more you drink. He, therefore, may be capable of clearing alcohol at a rate of almost three times this, three units an hour, although this won’t help with the pressure his liver is put under. Small women who drink infrequently could take two hours to metabolise just one unit.

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