The red colour compound betanin is not broken down in the body, and in higher concentrations may temporarily cause urine or stools to assume a reddish colour, in the case of urine a condition called beeturia.[21] Although harmless, this effect may cause initial concern due to the visual similarity to what appears to be blood in the stool, haematochezia (blood passing through the anus, usually in or with stool) or haematuria (blood in the urine). Beets contain oxalic acid, which when combined with calcium and/or vitamin C can form oxalates. Excessive amounts of oxalates in the body can lead to urinary tract infections, kidney stones, and even kidney failure.
If you have kidney disease or are on a low-oxalate diet for another reason, your doctor may recommend limiting your consumption of beets (especially their greens).
Nitrosamine formation in beetroot juice can reliably be prevented by adding ascorbic acid
Source: Beets Nutrition Facts and Health Benefits.