Fatty fish, such as salmon etc, are natural sources of vitamin D. (source - Wikipedia)
How often do we see a statistical risk reduction of some disease 8-times (relative risk reduction RR=0.12) or by a factor of 12 (RR=0.08)? One cannot be 100% sure based on the outcome of just two studies but I am tempted to entertain an idea of the TOTAL PROTECTION, with the residual incidents possibly explained by non-conformance or observational errors.
Intake of vitamin D and risk of type 1 diabetes: a birth-cohort study.
Vitamin D supplementation was associated with a decreased frequency of type 1 diabetes when adjusted for neonatal, anthropometric, and social characteristics (rate ratio [RR] for regular vs no supplementation 0.12, 95% CI 0.03-0.51, and irregular vs no supplementation 0.16, 0.04-0.74. Children who regularly took the recommended dose of vitamin D (2000 IU daily) had a RR of 0.22 (0.05-0.89) compared with those who regularly received less than the recommended amount. Children suspected of having rickets during the first year of life had a RR of 3.0 (1.0-9.0) compared with those without such a suspicion.
Vitamin D supplementation during the first year of life and risk of schizophrenia: a Finnish birth cohort study.
RESULTS: In males, the use of either irregular or regular vitamin D supplements was associated with a reduced risk of schizophrenia (Risk ratio (RR)=0.08, 95% CI 0.01-0.95; RR=0.12, 95% CI 0.02-0.90, respectively) compared with no supplementation. In males, the use of at least 2000 IU of vitamin D was associated with a reduced risk of schizophrenia (RR=0.23, 95% CI 0.06-0.95) compared to those on lower doses. There were no significant associations between either the frequency or dose of vitamin D supplements and (a) schizophrenia in females, nor with (b) nonpsychotic disorder or psychotic disorders other than schizophrenia in either males or females.
CONCLUSION: Vitamin D supplementation during the first year of life is associated with a reduced risk of schizophrenia in males. Preventing hypovitaminosis D during early life may reduce the incidence of schizophrenia.
Another paper on the subject (thanks Neonomide):
Relation of schizophrenia prevalence to latitude, climate, fish consumption, infant mortality, and skin color: a role for prenatal vitamin d deficiency and infections?
A review article by J.J. Cannell MD of "The Vitamin D Council":
The Vitamin D Newsletter August 2009, Vitamin D and Schizophrenia
More update (18/09/2010)
Paper: Schizophrenia, gluten, and low-carbohydrate, ketogenic diets: a case report and review of the literature
C.D. is a 70 year-old Caucasian female with a diagnosis of schizophrenia since the age of seventeen. Her diagnosis was based on paranoia, disorganized speech, and hallucinations. She reported both auditory and visual hallucinations,... she has had these hallucinations on almost a daily basis since the age of seven. ...has also been hospitalized at least five times over the last six years for suicide attempts and increased psychotic symptoms.
... Over the course of 12 months, C.D. has continued the low-carbohydrate, ketogenic diet and has had no recurrence of her auditory or visual hallucinations. She has also continued to lose weight (body weight 131.4 kilograms) and experience improvements in her energy level.