When we take a medication, we trust there is evidence that it will work. When we apply a cosmetic, we trust there is evidence that it is safe. When we put on a sunscreen, we trust there is evidence that it filters ultraviolet light. But evidence is not white or black; it runs the gamut from anecdotal to incontrovertible.
Some people claim that placing a bar of soap under the sheet when they sleep solves the problem of restless leg syndrome. That鈥檚 what we call anecdotal evidence, and it remains so until it is confirmed or dismissed by proper randomized double-blind controlled trials. On the other hand, evidence that gold conducts electricity is ironclad. There are no ifs or buts about it. Often, though, the use of the term 鈥渆vidence鈥� is open to interpretation. An interesting example is a recent paper published in Nature, one of the world鈥檚 foremost scientific journals, intriguingly titled: 鈥淓vidence for human transmission of amyloid-beta pathology and cerebral amyloid angiopathy.鈥�
Let鈥檚 dissect this title. Amyloid-beta proteins are one of the hallmarks of Alzheimer鈥檚 disease, so the title implies that evidence has been found that the disease can be transmitted from person to person. Little wonder that the paper generated headlines in the lay press ranging from 鈥淎lzheimer鈥檚 may be a transmissible infection鈥� and 鈥淵ou can catch Alzheimer鈥檚鈥� to 鈥淎lzheimer鈥檚 bombshell.鈥� All of these are highly misleading because the paper, in spite of its provocative title, does not provide evidence for the transmission of Alzheimer鈥檚 disease between humans.
So what did the researchers, led by Dr. John Collinge of University College London, actually find? They investigated the brains of eight people who had been injected with human growth hormone as children due to stunted growth back when this hormone was extracted from the pituitary glands of dead donors. Unfortunately, the donors from whom the hormone was extracted for these children had been harbouring proteins known as 鈥減rions鈥� that cause Creutzfeldt-Jakob disease, a terminal neurological affliction. The recipients ended up dying from the disease they had contracted via the hormone.
Collinge found that six of the eight people also had amyloid plaques typical of Alzheimer鈥檚 disease. But none of 116 people who had died of Creutzfeldt-Jakob disease who had not received contaminated growth hormone showed any sign of amyloid protein deposits. Dr. Collinge therefore suggests that molecules that lead to amyloid plaque formation were passed to the recipients along with the growth hormone.
A very interesting hypothesis to be sure. But the study did not show that the patients would actually have developed Alzheimer鈥檚 had they lived longer. A more appropriate title for the paper would have been 鈥淧ossibility for human transmission of amyloid-beta pathology via contaminated growth hormone.鈥� The word 鈥渆vidence鈥� should not have appeared in the title. The authors point out clearly that 鈥渢here is no suggestion that Alzheimer鈥檚 disease is a contagious disease and no supportive evidence from epidemiological studies that Alzheimer鈥檚 disease is transmissible.鈥� Nevertheless it was the term 鈥渆vidence鈥� that caught journalists鈥� eye and created undue public alarm with the suggestion that Alzheimer鈥檚 disease can be 鈥渃aught.鈥�
