The short version: scant evidence for "leaky gut syndrome" at all; none for it being caused by vaccines; none for it being a defining or causal characteristic in autism.
Let's start with some definitions. "Leaky gut", formally, is "intestinal permeability", and increased intestinal permeability really does exist in some conditions.
The general "leaky gut syndrome" storyline runs like this: a typically-developing child is vaccinated; the child develops "leaky gut", which allows particles from food (especially gluten and casein) leak into the blood and affect the brain. Child's behavior changes.
In a previous post, we reviewed the evidence that vaccines do not affect the intestines.
Leaky gut syndrome is almost certainly a "fad diagnosis", without a medical basis.
..."leaky gut syndrome," is described by proponents as a condition in which the intestinal lining becomes irritated and porous so that unwanted food particles, "toxins," bacteria, parasites, and "Candida" enter the bloodstream and result in "a weakened immune system, digestive disorders, and eventually chronic and autoimmune disease." Treatment of this alleged condition can include dietary changes (such as not eating protein and starch at the same meal); "cleansing" with herbal products; "reestablishing good balance" of intestinal bacteria; and supplement concoctions claimed to strengthen and repair the intestinal lining. Note: Some medical scientists use the term "leaky gut" for problems associated with abnormal intestinal permeabilty, but "leaky gut syndrome" is not one of them.
The origins of the "leaky gut" myth start with Jaak Panksepp, who published "A neurochemical theory of autism" in 1979. In that paper, Panksepp proposed that autism arises from an upset in the opiate systems in the brain. Then in 1991, Karl Ernst (Kalle) Reichelt published a paper in Norwegian on leaky gut, and later published other papers in English. Since then, the idea that certain foodstuffs (and/or vaccination) cause a host of symptoms and problems have passed into "common knowledge", without the benefit having a base of evidence.
The UK's National Health System has this to say about "Leaky gut syndrome"
'Leaky gut syndrome' is a proposed condition some health practitioners claim is the cause of a wide range of long-term conditions, including chronic fatigue syndrome and multiple sclerosis.
Proponents of 'leaky gut syndrome' claim that many symptoms and diseases are caused by the immune system reacting to germs, toxins or other large molecules that have been absorbed into the bloodstream via a porous ('leaky') bowel.
There is little evidence to support this theory, and no evidence that so-called 'treatments' for 'leaky gut syndrome', such as nutritional supplements and a gluten-free diet, have any beneficial effect for most of the conditions they are claimed to help.
While it is true that certain factors can make the bowel more permeable, this probably does not lead to anything more than temporary mild inflammation of an area of the bowel.
Why we should be sceptical about 'leaky gut syndrome'
Exponents of 'leaky gut syndrome' – largely nutritionists and practitioners of complementary and alternative medicine – believe the bowel lining can become irritated and 'leaky' as the result of a much wider range of factors, including an overgrowth of yeast or bacteria in the bowel, a poor diet and the overuse of antibiotics.
They believe that undigested food particles, bacterial toxins and germs can pass through the 'leaky' gut wall and into the bloodstream, triggering the immune system and causing persistent inflammation throughout the body. This, they say, is linked to a much wider range of health problems and diseases, including:
The above theory is vague and currently largely unproven.
There has been one small pilot study on the leaky gut hypothesis in autism.
Robertson MA, Sigalet DL, Holst JJ, Meddings JB, Wood J, Sharkey KA. Intestinal permeability and glucagon-like peptide-2 in children with autism: a controlled pilot study. J Autism Dev Disord. 2008 Jul;38(6):1066-71. doi: 10.1007/s10803-007-0482-1. Epub 2008 Feb 29.
We measured small intestinal permeability using a lactulose:mannitol sugar permeability test in a group of children with autism, with current or previous gastrointestinal complaints. Secondly, we examined whether children with autism had an abnormal glucagon-like peptide-2 (GLP-2) response to feeding. Results were compared with sibling controls and children without developmental disabilities. We enrolled 14 children with autism, 7 developmentally normal siblings of these children and 8 healthy, developmentally normal, unrelated children. Our study did not detect differences in these measures of gastrointestinal function in a group of children with autism.
Edited to add:
Thjodleifsson B, Davídsdóttir K, Agnarsson U, Sigthórsson G, Kjeld M, Bjarnason I. Effect of Pentavac and measles-mumps-rubella (MMR) vaccination on the intestine. Gut. 2002 Dec;51(6):816-7. "The failure of the MMR vaccination to cause an intestinal inflammatory response provides evidence against the proposed gut-brain interaction that is central to the autistic "enterocolitis" hypothesis."
End of story. No such evidence-based thing as leaky gut; no evidence that vaccines damage the gastrointestinal tract.