German wine is sweet. Sweet wine will make you fat whereas dry wine won't. Therefore German wine will ruin your diet. Actually, both statements and the conclusion are wrong.
First of all let me say that the majority of wine made in Germany is dry; it just so happens the we export more of the sweet stuff. Now the more interesting question in the context of this posting: what about residual sugar and calories? After all, some of the fruity Rieslings have dozens of grams of sugar - isn't that bad for my waistline? Yes, but no. Actually, it is the alcohol level you should be concerned about too: While 1 gram of alcohol has about 7 kcal, 1 gram of sugar has only about 4 kcal.
The key process in making wine is fermentation during which sugar is transformed into alcohol. To oversimplify the process: stop the fermentation earlier and you get more sugar and less alcohol, but without dramatically changing the calorie count in the process. This means we have to look at the individual wine to find out how bad it really is. An example:
I just tasted the WeightWatchers 2008 Fruity White that is said to have 64kcal per 100 ml. Sounds pretty good, doesn't it? After all, this is a WeightWatchers wine, so it must be as low as the calorie count can get - and that would mean dramatically less than a sugar heavy wine.
Compare the 2006 Zeltinger Sonnenuhr Spätlese, a wine that, as winemaker Molitor informs us, has 7.5% alcohol and the shocking amount of 106.7 grams of residual sugar. One would assume that this one is a calories bomb, as the Germans call it. But let's do some maths.
The Molitor has 7.5% alcohol, which means that 7.5% of the wine's volume (750ml per bottle), are alcohol. Therefore the wine contains 56,25 ml or about 45 g of ethanol (there are 0.789 g ethanol per ml). As 1 g ethanol has about 7 kcal, we are looking at 315 kcal because of the alcohol level, to which we add 4.1*106.7=437,47kcal because of the sugar level to get about 750 kcal for the sweet wine from the Mosel. Seems shocking, but we have to divide that by 7.5 as this is per bottle compared to the 64kcl per 100 ml WeightWatchers wine. And we get: almost exactly 100 kcal.
In plain English, this means that the super sugar heavy late harvest has only 50% more calories than the diet wine - despite having over 100 g residual sugar! Compare that to the standard version of Coca-Cola whose calorie count is about 100 times higher than that of the diet version!
Obviously, if you were on a diet the difference between the Molitor and the WeightWatchers would still make a, hm, difference. However, wouldn't it be better to compare the diet wine to a 'normal' wine, let's say a typical fruity Riesling Kabinett from the Mosel, at 10% alcohol (415 kcl) and 16 g (66 kcal) residual sugar? This wine would have about 64 kcal per 100 ml - which is pretty much identical to the WeightWatchers but almost certainly much more tasty.
And as a final comparison take a wine made by winemakers Trockene Schmitts ('Dry Schmitts'), the Silvaner Konsequent. With 13% alcohol, this - as you could think - diet friendly wine with only 0.5 g residual sugar does actually have about 72 kcal per 100 ml - so despite having 200 times less sugar than the Molitor sugar bomb it still has more than 2/3 of its calorie count. So remember to keep an eye on the alcohol level!
This actually means that if your are on a diet, go for a fruity German Riesling, as long as you stay away from the sugar heavy Spätlese or Auslese. The category you will be looking for on the label is 'halbtrocken' or half-dry, meaning that the wine should have 9-18 g of residual sugar.