As the festive season approaches many people will be enjoying their Christmas spirit in a liquid form. How much does this contribute to their seasonal bulge?

And how much do our everyday drinking habits affect our weight? This briefing paper aims to provide a basic scientific background to these questions.

The calories in alcohol
The body can use four main foodstuffs as fuel: fat, carbohydrate, protein and alcohol. Pure alcohol contains 7 kcal/gram compared to 9 kcal for fat, and only 4 kcal for carbohydrates and protein. Thus, on a weight-for-weight basis alcohol is the second most calorific energy source. However, we never drink pure alcohol: it is always diluted by water even in the strongest of spirits. Table 1 indicates the approximate energy content of a variety of drinks. The energy content is not always directly related to the alcohol content since many drinks contain sugars, carbohydrates, and even fat.

Beer(per pint)170 kcal
Strong ale(per bottle = 250 ml)200 kcal
Wine(red, rosé per 125 ml glass)90 kcal
Wine(dessert white per small glass = 50 ml)50 kcal
Champagne(or dry white wine per 125 ml glass)80 kcal
Port(per 60 ml glass)95 kcal
Spirits(per measure = 25 ml 70% proof)55 kcal
Liqueurs (per measure = 25 ml) 60 kcal

The average energy content of a variety of drinks.
(Source: McCance and Widdowson's Composition of Foods, HMSO)


Are all these calories absorbed?
Alcohol is absorbed very efficiently in the stomach. In spite of its diuretic effect (which might be thought to rid the body of some alcohol), and the powerful smell of alcohol on drinkers' breath, less than 2% of any alcohol consumed escapes from the body. The only way in which the body can rid itself of alcohol is by burning (oxidising) it in the liver and other tissues. This is a crucial process since alcohol is essentially a toxin, and since the body has nowhere to store alcohol.



Is alcohol converted to fat?
There are biochemical pathways in the liver which can partially break down alcohol and reform it into fat. Thus, it is certainly possible for alcohol to be converted directly to fat, but this is probably a rare event which is only stimulated when a great deal of alcohol is consumed with other foods. The more usual way for alcohol to lead to fat storage is through a `sparing' effect. When alcohol is consumed, it forces the body to choose it as its primary fuel (because it is a toxin which must be eliminated), and hence it suppresses the amount of fat which is being burnt. This unburnt fat is then stored. It should be noted that this `fat sparing' effect is usually exaggerated by the fact that we often drink alcohol at the very time we are also overeating fat - especially at Christmas.



Is alcohol metabolised differently by men and women?
Alcohol does have quantitatively different effects in men and women. Although it's not universally true, most men can tolerate more alcohol than most women. Firstly, men are usually bigger than women. Secondly, men generally carry less body fat than women when expressed as a proportion of body weight. (the UK averages are about 15-20% fat for men and 30-35% for women). Since alcohol does not enter fat this means that men have a greater `alcohol space' in their bloodstream and cells, and hence, an equivalent amount of alcohol will be less concentrated in men than in women. There has also been a long-standing controversy as to whether men actually have a different biochemical pathway than women. It was proposed that men's stomach and digestive tract burn up a proportion of alcohol before it ever enters the bloodstream, and that women lack this process (known as `first pass metabolism'). Further work has shown that both men and women possess this process, and it only works better in men because they are larger. Finally, it is well known that the liver enzymes responsible for breaking down alcohol are inducible (i.e. they get better with practice). The induction occurs over 1 to 3 days, and diminishes over a similar timescale when we stop drinking. This training effect also explains why men on average are more alcohol-tolerant than women - because men generally practice harder!



How much alcohol does the average Briton consume?
To a great extent this is a superfluous question when thinking about alcohol and obesity, since peoples' intakes vary widely and since what matters is individual intakes. People are notorious at underestimating how much they drink. If you apply a questionnaire at the end of a party, and then compare it to the empty wine bottles and beer cans, the self-reported consumption will often be about half the amount that has disappeared! Exact sales figures should be obtained from the industry. However, in nutritional terms it can be stated that, on average, women obtain about 2-4% of their daily calorie intake from alcohol, and men about 4-8%. These rather small figures help to put some of the apparent paradoxes in perspective.

Does this mean that alcohol aids slimming?
Certainly not! Much as it would be tempting to publish a `gin and tonic' diet, the plain facts are that alcoholic drinks contain energy that will be fattening if consumed in addition to a person's daily energy needs. So, how can we explain the paradox (especially in women) that drinkers seem to be leaner than non-drinkers, and that in men alcohol intake seems to be neutral with respect to body fatness?

Thirty years ago, there was a popular theory that alcohol calories `didn't count' because they were simply burnt away as heat. Several complicated pathways were proposed which might explain this. However, experiments in which men are given alcohol while living in metabolic chambers (to measure heat production) show that the heat liberated from alcohol is very similar to that liberated from carbohydrate - thus, disproving the `wasted calories' theory. Remarkably, these experiments have also been done in alcoholics, who were given 22 units of alcohol intravenously per day and were still able to be tested while riding a cycle ergometer! The result was the same.

If the wasted calories theory is untrue, then we must find another explanation. It appears that this has little to do with alcohol itself, but can be found in the other behaviours which correlate with drinking. For instance, drinkers are more likely to smoke, and smoking suppresses weight gain. Drinkers are also more likely to participate in sport and active leisure and, perhaps surprisingly, are also more likely to be well-educated and of high socio-economic status, both of which are associated with leanness.



What about the archetypal beer-bellied couch potato?
Of course, there are individuals who don't abide by the population averages described above. The big-bellied beer-swilling man certainly exists, and beer is certainly a contributor to his body weight and shape, but he's actually quite a rare individual who makes little impact on the nationwide statistics. We have a tendency to overplay the significance of such people because they have become such a caricature. The Whitehall II Study of men in different professional grades picks up some of these men in the unskilled category and shows that their fatness tends to be related more to beer consumption than to wines and spirits, but again they make little impact on the overall statistics. What does appear to be true is that excess beer consumption does tend to encourage intra-abdominal (belly) fat, which is more dangerous to health. The reasons for this are not fully understood, but may relate to the release of fat from the liver. It is often suggested that it is the carbohydrate in beer which actually causes the weight gain, but it should be noted that in a typical draft beer the calories from alcohol are 2.5 times those from carbohydrate (Source: McCance and Widdowson's Composition of Foods, HMSO).



Alcohol and dieting
There are several views about alcohol and dieting. One is that since alcohol only contributes a small proportion to most people's daily energy intake, there is little point in draconian recommendations. Indeed, since diets are already hard enough to stick to, many would argue that the odd drink is actually useful in helping people not to feel too deprived and, hence, to abandon their diet or to bounce back with a vengeance as soon as their `diet' is over.

However, one disadvantage of alcohol is its `disinhibition' effect. When people have had a drink or two they're much more likely to say "to hell with my diet" and to over-indulge. Dieters are warned of this.

Many men, particularly business men, find that cutting down on booze alone is a very effective way of losing weight. This often makes them realise that their consumption levels had been creeping up to undesirable levels.



Can pills and supplements block the fattening effect of alcohol?
In the lead up to Christmas, advertisements are already beginning to appear for a variety of supplements which are claimed to block the carbohydrates in beer from being absorbed and laid down as fat. These are highly misleading for several reasons. First, there is no proof that they work. Second, it should be remembered that a pint of beer typically contains only 12 grams of carbohydrate - a fraction of the amount in a slice of bread or in a chocolate bar. Third, if the carbohydrate really is blocked from being digested in the upper intestine, it will give the micro-organisms in the lower gut a field day and cause massive flatulence. This is a Catch 22: if the supplement works you'll have flatulence; if you've no flatulence, then the supplement isn't working as claimed. You have been warned!