Great Danes, like most giant dogs, have a fairly slow metabolism. This results in less energy and less food consumption per pound of dog than in small breeds.Great Danes have some health problems that are common to large breeds. Bloat (a painful distending and twisting of the stomach) is a critical condition that affects Great Danes and results rapidly in death if not quickly addressed. It is a commonly recommended practice for Great Danes to have their stomachs tacked (Gaxtroplexy) to the interior rib lining during routine surgery such asspaying and neutering if the dog or its relatives have a history of bloat. Another problem common to the breed is in the hips (Hip displasia). Typically an x-ray of the parents can certify whether their hips are healthy and can serve as a guideline for whether the animals should be bred and are likely to have healthy pups.

Dilated cardiomyopathy (DCM) and many congential heart diseases are also commonly found in the Great Dane.
Also, some Danes may develop yeast infections, when not fed all needed nutritional requirements. The yeast infection may also lead to minor recurring staph infection(s).
Great Danes also suffer from several genetic disorders that are specific to the breed. For example, if a Great Dane lacks color (not white) near its eyes or ears then that organ does not develop and usually, the dog will be either blind or deaf.

Wikipeda: Verifiability, 2007