I discovered that when I loaf all day, eating huge amounts of junk food and cakes and drinking beer, wine etc I gain weight. When I eat less, reduce my drinking and walk around a lot I lose weight. Should I write a diet book?

sure! The only problem is expanding "Eat more get fatter. Eat less get slimmer" into min. 200 pages. And of course you can also sell some diet food which is made from sawdust and cheapest artificial flavor at similar price as caviar.

Of course you also need some photoshopped before/after picture. Sure someone here can sell you the photo from his belly and sure at some bodybuilding forum you can get the "after" picture so you only need to mount the same head.

That is the way this business runs.

I also believe that going lower / low carb helps... (remember sugars are bad)

I discovered that when I loaf all day, eating huge amounts of junk food and cakes and drinking beer, wine etc I gain weight. When I eat less, reduce my drinking and walk around a lot I lose weight. Should I write a diet book?

How about using your newly discovered knowledge to encourage/guide/influence someone in doing the same, and by that help a single person in your network (family/friends) being healthier?

And if it works, yeah, why not share your story on paper, as it could positivelly impact more people. But don't call it "diet book" ... it's actually a lifestyle change!

You think my sarcasm will help others lose weight?

With the right dosage, I actually do believe it can be a great motivator ... provided you genuinely want to help (http://www.thefreedictionary.com/help)