Originally, I got my information from the nutritional cookbook Nourishing Traditions by Sally Fallon and Mary D. Enig, PhD. In their section on Sprouted Grains, Nuts, & Seeds, on page 113, it says this:
"There is only one seed we do not recommend in sprouted form (or in any form) and that is--surprisingly--alfalfa! After mung beans, alfalfa is the variety of sprout that has caught on in the health food world. Unfortunately, it seems that all the praise heaped on the alfalfa sprout is ill advised. Tests have shown that alfalfa sprouts inhibit the immune system and can contribute to inflammatory arthritis and lupus. Alfalfa seeds contain an amino acid called canavanine that can be toxic to man and animals when taken in quantity. (Canavanine is not found in mature alfalfa plants; it is apparently metabolized during growth.)"
On the site Nutrition and Metabolism, I found this page about Canavanine. From the page:
"L-canavanine is a common non-protein amino acid found naturally in alfalfa sprouts, broad beans, jack beans, and a number of other legume foods and animal feed ingredients  at up to 2.4% of food dry matter. This analog of arginine (Figure 1.) can also block NO synthesis [2-5], interfere with normal ammonia disposal [6,7], charge tRNAarg, cause the synthesis of canavanyl proteins , as well as prevent normal reproduction in arthropods  and rodents .
"Canavanine has also been reported to induce a condition that mimics systemic lupus erythematosus (SLE) in primates [11,12], to increase antibodies to nuclear components and promote SLE-like lesions in auto immune-susceptible (e.g., (NZB X NZW)F1) mice ."
My Google search also turned up this site. From the article:
"The jack bean, Canavalia ensiformis (L.) D.C., belongs to the agriculturally utilized legumes of the tropics. Both, the pods and seeds, as well as the other plant parts are often used as stockfeed. In the literature, several reports indicate that the seeds and other parts of this plant lead to the development of toxic effects in animals."Investigations about these manifestations of poisoning, primarily observed in agricultural practice, are to date only isolated. Not in all cases could a toxic effect of the plant be shown. Addison  observed no toxic effects after feeding young oxen with Canavalia meal that was fed together with silage and maize straw. The same result came from experiments with Jersey cows (Addison ). In contrast, Orru & Cesaris Demel  observed a toxic effect of Canavalia meal with rats."
"These findings are of special importance because Canavalia ensiformis is not only used as stockfeed but also for human consumption. They confirm the observations of other authors that Canavalia can have toxic effects. The scattered reports about poisoning by this plant probably stand in no relation to actual number of incidences that are caused by it in agricultural practice, because the cause is difficult to recognize.
"Only in a few cases do poisonings end fatal, because the lethal dose (30 g seed flour/ kg body weight) is apparently only reached in exceptional cases. However, even small doses allow the recognition of clear effects. Shone  observed that milk production was markedly reduced after feeding Canavalia flour to dairy cows. The composition of the feed seems also to have had something to do with it. When Canavalia flour is given together with protein-rich fodder, then no or little effect is noted. Feed containing up to 30% Canavalia can, according to Shone, be given without danger. It is, however, to be expected that even thereby some detrimental effect on the animals occurs."
I also found this here:"L-Canavanine is a potentially deleterious arginine antimetabolite whose toxicity is expressed in canavanine-sensitive organisms ranging from viruses to humans. Canavanine, a substrate for arginyl-tRNA synthetase, is incorporated into nascent polypeptide chains in place of arginine. This substitution results in the production of structurally aberrant, canavanyl proteins. Chemical, physical, and immunological studies of native and canavanine-containing vitellogenin obtained from female migratory locusts (Locusta migratoria migratorioides (Orthoptera] provide the first experimental evidence that canavanine can disrupt the tertiary and/or quaternary structure that yields the three-dimensional conformation unique to the protein. These findings enhance our understanding of the biochemical basis for canavanine's antimetabolic and potent insecticidal properties."
More can be found by Googling "Canavanine."
From this experiment, the conclusion was:
"Whether daily ingestion of canavanine exacerbated SLE in two patients consuming 15 and 8 alfalfa tablets (0.27 and 0.15 mg, respectively) is uncertain and warrants further investigation."
The same site posted results from this experiment. From the post:
"Bacillus cereus UW85 suppresses diseases of alfalfa seedlings, although alfalfa seed exudate inhibits the growth of UW85 in culture (J. L. Milner, S. J. Raffel, B. J. Lethbridge, and J. Handelsman, Appl. Microbiol. Biotechnol. 43:685-691, 1995). In this study, we determined the chemical basis for and biological role of the inhibitory activity. All of the alfalfa germ plasm tested included seeds that released inhibitory material....These results indicate that canavanine exuded from alfalfa seeds affects the population biology of B. cereus."
Probably the most relevant (and easy-to-read) article I found was here. Although I don't agree that natural foods contain more toxic substances than man-made ones, we all know that there are both beneficial and toxic plants in nature. Alfalfa seeds and sprouts should definitely be avoided as being on the toxic side.
From the article:
"Canavanine: a toxic arginine analogue
Small-seeded legumes like alfalfa and broom have developed a protective chemical called canavanine. Alfalfa seeds are about 0.5% canavanine, compared with 13% in seeds of the tropical legume Dioclea megacarpa. Just 0.02% canavanine can harm insect larvae. Any animal that ingests canavanine makes incorrect proteins that malfunction as enzymes. The damage is non-specific and widespread, affecting RNA and DNA metabolism, as well as a key enzyme for destroying alcohol. Because it messes up so many aspects of metabolism, canavanine is a highly toxic chemical to animals. Pigs refuse to eat feed containing too much canavanine.Although we humans are not immune to canavanine, we don't seem to taste it."Hopefully this will provide some answers to those of you who thought I was previously off my rocker.
Continue to eat sprouts of other seeds, however--these are extremely good for you! Perhaps in another post, I will provide instructions on how to sprout your own. Right now, I have to go make supper!