Proper nutrition + adequate exercise in sports.
As for the lumbar region, also known as the "lifebuoy", there is an interesting diet. It is called intermittent fasting. Link to read: http://fitnessguides.ru/periodicheskoe-golodanie-kak-dieta-dlya-sushki-tela/#gref
Honestly, I tried it. Helps. The main thing is to strictly adhere to the established rules.
The most important thing is not to poison yourself with all sorts of diets, a la kefir, chocolate, etc.
Local fat loss is unfortunately impossible. With any method of losing weight (except for a knife or a saw), the body will get rid of fat more or less evenly, the only thing is that fat disappears faster from where it appeared last. But since a man is characterized by the accumulation of fat in the lower back, do not expect to be able to quickly get rid of it