I missed that one - but I do not think it is really cautioning against using beetroot juice in significant amounts, just advising to stagger out the doses and to take it with other juices:
World Research into Beets, Broccoli, Tomato, Carrots, Garlic
"Our previous studies identified the extract of Beta vulgaris (beetroot), commercially also known as betanin, as a potent cancer chemopreventive agent. An in vivo anti-tumor promoting activity evaluation against the mice skin and lung bioassays revealed a significant tumor inhibitory effect. The combined findings suggest that beetroot ingestion can be one of the useful means to prevent cancer. The most interesting observation is that the cancer chemopreventive effect was exhibited at a very low dose used in the study and thus indicating that beetroot warrants more attention for possible human applications in the control of malignancy." [Department of Pharmaceutical Sciences, Howard University, Washington, DC]
A team of researchers at the University of Wisconsin-Madison has shown that red beetroot pigments boost levels of proteins, called phase II enzymes, that help detoxify potential cancer-causing substances and purge them from the body. [Journal of Agricultural and Food Chemistry]
CAUTION: TOO MUCH BEET JUICE CAN OVER-TOXIFY THE LIVER & CAUSE HARM OR EVEN DEATH
"Beets clean up cancer faster than the liver is capable of processing all the wastes dumped into it at any one time. Consequently, the internal administration of beetroot needs to be staggered out somewhat, and closer attention given to detoxifying the liver and colon at the same time the beetroot therapy is commenced. Never drink beet juice by itself. Pure beet juice can temporarily paralyze your vocal chords."