While the lactose tolerance allele may have some positive health effects, notably because milk is one of the few good dietary sources of calcium, it seems to correlate also with some negative effects, namely obesity.
Ricardo Almon et al., Association of the European Lactase Persistence Variant (LCT-13910 C>T Polymorphism) with Obesity in the Canary Islands. PLoS ONE 2012. Open access ··> LINK [doi:10.1371/journal.pone.0043978]
Canary Islands, in spite of its subtropical geography, is one of the regions of the European Union where milk is most consumed, at levels comparable to Scandinavia.
Although there is a strong correlation between being lactose tolerant and milk consumption it is not fully clear yet if it is excess milk consumption what makes people obese or an unknown collateral effect of the European lactase persistence allele.
Interestingly the correlation, very strong, is only found for obesity and not for being overweight:
|Fig. 1 – BMI classification by LCT genotypes (LP: n = 330; LNP: n = 221)|