A recent study was carried out in the USA, from Alaska to Florida, to determine whether the same phenomenon was observed in that country.The authors delved into results of two prospective cohorts of nurses, i.e. 175,912 women regularly followed up since 1976 and 1989, respectively.
Figures very close to those seen in Europe were observed. 257 women had developed Crohn's disease (CD) and 313 showed ulcerative colitis (UC), giving an annual incidence of 5.6 to 10.3 according to age for CD and 6.2 to 12.9 for UC.

When taking into account the place of residence at birth and up to the ages of 15 and 30, the place of residence at the age of 30 seemed to be most influential, with a 36% decreased incidence of UC and a 53% decreased incidence of CD for women residing in the southern US compared to those living in the north. Women living at mean latitudes had intermediate incidences.

The effect of the state of residence at birth and at age 15 was less clear-cut, at the limit of statistical significance.
The severity of the diseases did not differ from north to south.

So what happens in the north to explain all this?

While less sunshine would appear to play a role, other factors may be present: air pollution, variations in body temperature, a less relaxed way of life, a more optimistic attitude, and the quality of fruits and vegetables.
Several factors argue in favor of sunshine via deficiency in vitamin D, the anti-inflammatory role of which is well known (especially due to its suppressive effect upon the T cell immune response). Indeed, there exist models of experimental colitis in mice deficient in vitamin D and also in mice rendered invalid for the vitamin D receptor.
Using a prediction index for the serum vitamin D value (as predicted by exposure) the authors of this study showed that a high serum level was associated with a lower incidence of CD.
This work warrants further study before we can envisage using vitamin D as a preventive or curative treatment.

References

Khalili H, Huang ES, Ananthakrishnan AN, Higuchi L, Richter JM, Fuchs CS, Chan AT. Geographical variation and incidence of inflammatory bowel disease among US women. Gut. 2012; 61: 1686-92.
Division of Gastroenterology, Massachusetts General Hospital, Harvard Medical
School, Boston, Massachusetts 02114, USA.