Vegan diets cause brain shrinkage
The finding concerns B12 deficiency, not plant-based eating. Anyone deficient is exposed, including many omnivores, and supplementation removes the risk.
Studies have linked low vitamin B12 to faster brain atrophy and cognitive decline, and B12 is found reliably only in animal foods, so a diet that removes them strips out the brain's natural supply of a nutrient it cannot do without. The damage builds slowly and silently, often with no warning signs until it is hard to reverse, which makes it reckless to adopt a way of eating whose basic design leaves the brain exposed. A diet that people need to be rescued from by a pill has a built-in flaw, and that flaw falls on the organ that matters most.
The research behind this claim associates low B12 status with accelerated brain atrophy and cognitive decline in older adults. It applies to anyone who is deficient, which includes a substantial number of people eating meat whose absorption is impaired. It is not a finding about plant foods.
A vegan taking B12 is not in the exposed group. The claim takes a real risk and attaches it to the wrong cause: the causal factor is a specific nutrient, and that nutrient costs a few pence a week. B12 is the supplement most clearly required on a vegan diet, though vitamin D, iodine and algal omega-3 are also commonly worth considering depending on diet and location.
Inferring that one thing caused another from the fact that they occurred together or one after the other. Correlation and sequence are not causation.
How it applies here: Attributes the effect to the diet when the measured cause is a specific nutrient deficiency that occurs across all diets. The association holds for deficient omnivores too, which is the test the explanation fails.
Presenting only the evidence that supports a conclusion while leaving out evidence that tells against it.
How it applies here: Cites the deficiency research while omitting the part that settles the question: that supplemented individuals do not show the effect.