Wine contains a lot of calories: there are about 550 calories in a typical bottle of dry red or white wine with an alcoholic strength of 13% abv. That’s about 90 calories per small (125ml) glass, or 130 in a large (175ml) glass. What’s worse, they are so-called ’empty’ calories, because there is no other nutritional value in wine. The calories are contained in the alcohol, so low-alcohol wines are generally less calorific. There are additional calories in any residual sugar in the wine, so you might think that sweet wines are more fattening.
In fact, that is not always the case. A German Auslese Riesling may be 5% sugar, which is enough to lend noticeable sweetness, but be as low as 7% alcohol, so there are not so many calories in total: about 450 per bottle. Near the top of the calorie scale would be Recioto della Valpolicella, a sweet red wine from Veneto in Italy. It is made from grapes that have first been dried for several months until raisined, to concentrate the sugars, and then the fermentation is stopped before all the sugar has been converted into alcohol. Recioto may contain 9% sugar as well as 14.5% alcohol so it is certainly very calorific, around 650 calories per bottle.
The most calorific wines are fortified wines, to which extra alcohol has been added after the fermentation is complete (if dry), or to interrupt the fermentation (if sweet). Dry or off-dry examples include fino, amontillado and oloroso sherry, but most fortified wines are sweet to varying extents, including cream and PX sherries; most Port and Madeira; ‘vin doux naturel’ (VDN) from France; and wines in similar styles to VDN such as Australia’s Rutherglen muscat. Fortified wines may come in at around 1000 calories per bottle.
Leaving fortified wine aside, let’s follow through what happens to the calories in the grape sugar when the juice is made into wine. The juice is fermented by yeast – either naturally present or added for the job. The yeast consumes the sugar and produces several other compounds, but mostly alcohol (ethanol) and carbon dioxide gas, which is released to the atmosphere. In the process, heat is produced, so clearly some calories are lost this way. This tells us that the alcohol that’s produced contains fewer calories than the sugar that we started out with. In round numbers, each gram of sugar contains four calories, but converts into 0.5g of alcohol containing three calories, so one calorie is lost as heat.
This means that if the fermentation is cut short to produce a sweet wine, this will be more calorific than if fermentation is allowed to finish to make a dry wine. But, as we saw, that doesn’t mean that sweet wines are always more calorific than dry wines because there are other factors involved.
A major source of variation is the amount of sugar in the grapes. The reason that an Auslese Riesling may be low in calories even though it is somewhat sweet is that the grapes don’t contain much sugar: by law the must has to be between 83-100 Oechsle (the German scale for sugar content), or approximately 8-10% sugar, but it’s usually near the bottom of this scale. This is largely due to the cool, Northern climate and high river valleys in which they are grown. By contrast, a typical dry red wine around 13-14% abv will require grapes with around 100 Oechlse, while the dried grapes used for Recioto would be around 125 Oechsle.
So to summarise: if you like sweet wine but not the calories, it’s best to avoid fortified wines, as well as wines made from dried grapes – these include ‘botrytised’ or rotten grapes, as used for Sauternes and sweet Chenin Blanc. Your best bet is a cool climate wine like that Riesling, whether from Germany, Alsace, Australia (not forgetting Tasmania) and New Zealand. For example, Giesen in Marlborough – a region more famous for its Sauvignon Blanc – produces an off-dry Riesling with 10% abv. The Muscat family of grapes also makes some waistline-friendly sweet wines, the best known surely being the sparkling Asti, which is not more than 8% abv and about 7-12% sugar. Its equally sweet but less fizzy cousin Moscato d’Asti may not exceed 6.5% abv – that’s at most 300 calories per bottle. Enjoy!
 To combat this, German wine law allows up to 15% Süssreserve, unfermented grape juice, which if added to the wine will increase the sugar while diluting the alcohol. Although this increases the calorific content slightly, the final level is still low.