DL-methionine (the synthetic version) is so ubiquitous in chicken feed that organic chicken producers are having a hard time finding a natural replacement.
Adding synthetic methionine is cheaper than adding natural protein. This is why modern chicken feedstuffs rely on it. In other words, it's purely about industrial scale production. Yet the practice also seems to have percolated down to backyard chicken feeds.
Methionine is one of the crucial building blocks of life. According to the Evonik website (http://history.evonik.com/sites/geschichte/en/chemicals/inventions/degussa/methionine/pages/default.aspx), DL-methionine came into widespread use from 1974-7, but had first been synthesised decades earlier as a way to combat nutritional insufficiencies in soldiers.
The question is whether there is a difference in metabolic terms between synthetic methionine and natural forms.
As it happens, there is. You see, DL-methionine contains both the naturally occurring isomer of methionine (L) and its mirror image (D) in equal ratios. But only L-methionine can be utilised to make animal protein. To be used in the body, D-methionine (half the DL) must be converted to L first, and this conversion is not 100% but around 90% in chickens.
What happens to the extra 10%? Well if you read what happens in calves, it seems likely that it results in elevated plasma methionine: ‘Although the unnatural D-isomer of methionine can be used efficiently by growing calves for protein deposition[…], it leads to higher concentrations of plasma methionine than does the natural L-methoinine[…]’ (See J. P. Felix D'Mello, Amino acids in animal nutrition, CABI publishing, UK, page 341.)
If the use of DL-methionine has a tendency to raise methionine plasma levels in calves, then it stands to reason that carcasses of DL-fed broilers could easily contain traces of D-methionine. This substance is not well used in humans. (See:
As it happens, excess dietary methionine is also associated with artherosclerosis. The excess methionine artherosclerosis association seems to exist whether or not there is also elevated homocysteine (a subsequent product of methionine, normally described in association with artherosclerosis). While the source for this paragraph doesn't discuss different isomers of methionine, it's worth wondering to what extent methionine excess occurs after ingestion of D isomers? See
‘The atherogenic effect of excess methionine intake’ at http://www.ncbi.nlm.nih.gov/pmc/articles/PMC299913/
Needless to say, DL-methionine seems to have adverse effects on the chickens themselves. In 'Effect of herbal or synthetic methionine on performance, cost benefit ratio, meat and feather quality of broiler chicken' (see * below for the extended citation), Halder and Roy set out to compare performance between a control group fed no added methionine, a group fed DL-methionine (the synthetic form), and a final group fed a natural version (herbal-derived methionine).
In general, performance between both methionine-supplemented groups was similar (being above the unsupplemented group), but liver triglycerides were noticeably higher in the DL-methionine supplemented group than in the natural-methionine supplemented one, and the livers in synthetic supplemented birds were larger as well. In their words: '[...]supplementation of HerboMethione® in diets decreased liver triglyceride markedly when compared to bird fed DL-methionine (synthetic) supplemented diets.'
Of course, liver triglycerides are associated with heart disease, among other problems. The DL-methionine fed birds were also slightly lower in carcass muscle but higher in carcass fat than the natural methionine fed group.
Interestingly, the DL-methionine group also had a higher death rate than either the unsupplemented or natural methionine supplemented groups, though the study authors state that most deaths overall were accidental. As a side note, subtle neurological damage typically produces increased rates of accidents in humans. Chickens might not be intellectual superstars, but I'd like to see a study on the use of synthetic methionine in relation to neurological health.
When it comes to feeding the home flock, or for that matter feeding our children, I feel we backyarders are over a bit of a barrel. We either trust that science is on top of these questions, or, looking at the increasing prevalence of modern mystery syndromes, suspect that we've been playing a long-term guessing game.
All the more reason to go back to dual purpose utility breeds and aim for a natural diet. Remember, variety covers a host of sins, and will help keep ahead of deficiency. After all, wild chickens don't base their entire diet on a couple of staples like wheat and soy. And in 'olden days' (pre-1950s) chickens got by quite well on non-synthetic diets because they included milk and milk byproducts as well as other methionine sources. See http://attra.ncat.org/attra-pub/methionine.html.
* For the study quoted in my main text, see Halder, G. and B. Roy, 2007. Effect of herbal or synthetic methionine on performance, cost benefit ratio, meat and feather quality of broiler chicken. Int. J. Agric. Res., 2: 987-996.