Beets have been grown for food since ancient times. Both the leaves and the root are edible. Beets' most distinguishing characteristic is their bright red pigment, sometimes used as a dye. They are a good source of folate and a moderate source of iron, potassium, vitamin C and fiber. Consuming beets is generally safe, without major side effects. For people with certain conditions or genetic dispositions, however, eating beets can result in some mild to moderate side effects.
For about 10 to 14 percent of the population, eating beets results in passing pink or red urine. This tinted urine looks alarmingly like the blood-filled urine that can accompany a urinary tract infection. The condition is common enough to have earned a medical term: beeturia. Beeturia is sometimes theorized to be caused by a certain recessive gene or set of genes. It is also linked to iron deficiencies in the body. Beeturia can come and go depending on, among other things, the amount of pigment in the beets eaten and in what form they were consumed as well as the amount of iron needed by the body.
Beeturia also sometimes affects your bowel movements. The red pigment found in beets can turn your stool dark. Sometimes you'll even witness suspicious reddish streaks when you have a bowel movement, visually similar to those left by hemorrhoids or fissures.
Beets are quite high in oxalic acid. Oxalic acid, or oxalate, can interfere with absorption of certain nutrients such as calcium. In part because of this, some doctors believe there is a link between consumption of high-oxalate foods and the formation of calcium oxalate kidney stones. However, the "New York Times" states that kidney-stone sufferers should not eliminate high-oxalate foods from their diet without a specific recommendation from a doctor.
Although unrelated to kidney stones, gallstones are also formed of oxalic acid crystals. Therefore your doctor may advise you to avoid high-oxalate foods such as beets if you're prone to gallstones. Again, without a specific recommendation from your doctor, you should not avoid beets or other high-oxalate foods.