Type 1 diabetes (T1D) has both genetic and environmental contributors, and the early years of life are when several of the environmental candidates act. Study results are still inconsistent about the effects of pre-natal factors on T1D pathogenesis, but there is some information about the early-childhood factors that can influence it.
Microbial exposure and the hygiene hypothesis
Early exposure to microbial agents has fallen sharply with the urbanization of human life, and this has been proposed as an explanation for the recent rise in T1D. On this view, the hygiene hypothesis, early exposure to microbes trains and improves the immune system and helps it avoid autoimmunity.
The gut microbiome, the community of microorganisms living in the intestine, is mainly established during the first 3 years of life, and patients who develop T1D seem to have less diversity and less protection from it.
Nutritional exposures
Several dietary exposures have been examined, and the evidence ranges from suggestive to inconclusive; breastfeeding is taken first because it links directly to the gut microbiome described above.
The clearest candidate is breastfeeding. Several studies suggest a protective effect against T1D, while others show none. The hypothesis behind the protective effect is that breast milk improves the gut microbiome, which can play an important role in preventing T1D; breastfeeding is thought to increase both the diversity and the protection the microbiome provides.
Other dietary candidates are less clear. Cow’s milk was proposed to contain cow’s insulin that could accelerate the course of T1D, but there is still not enough informative data on this to draw a conclusion. Gluten introduction has been studied at different ages in the newborn diet, at 6 months and at 12 months, with no difference in T1D pathogenesis between the two times.
Vitamin D is harder to study than the others, because supplementation in newborns is already recommended, which makes a randomized controlled trial impractical. Supporting, though not conclusive, evidence comes from the observation that Northern European countries with a higher prevalence of vitamin D deficiency also have a higher incidence of T1D.
Viral infections
Infections form the last group of candidates, and here the evidence is again limited to a few associations.
Congenital rubella, rubella infection acquired before birth, is associated with T1D, but for other viral infections there is no conclusive information. One clinical pattern is well recognised: a viral infection very commonly precedes clinical T1D, so it is possible that an acute viral infection converts sub-clinical T1D into clinical symptomatic T1D.