The data from the present study expand previous studies of association of FABP genes in the risk of having type 2 diabetes and insulin resistance. One single nucleotide polymorphism of the FABP-1 gene, rs2197076 and one haplotype were associated with an increased risk of type 2 diabetes even when adjusted for age, sex and BMI. The replication analysis, not only for the individual SNP but also for the haplotype in a population with a different metabolic profile supports our initial findings. In this last population, the SNP and the haplotype could also be related with HOMA-IR. Carriers of the A allele of the rs2197076 of FABP1 or those with the haplotype AT of the rs2197076 and rs2241883 had significantly higher levels of HOMA-IR than those with the major allele G of the rs2197076.
The evaluation of the total, saturated or poly-unsaturated fat intake, as potential environmental factors, which could influence the genetic association, was inconclusive. Although the level of association was more evident in the low fat intake group, the formal test for interaction was not significant.
The implication of the FABP
genes on the risk of DM2 or insulin resistance has been supported by experimental studies in animals as well as in humans 
. Liver fatty acid binding protein (FABP1
) is an abundant cytosolic lipid-binding protein that regulates lipid transport and metabolism. Deletion of the FABP1
gene shows no obvious phenotype in mice receiving a low fat chow diet, but leads to decreased hepatic triglyceride accumulation in the prolonged fasted state, which exposes mice to an increased fatty acid flux to the liver. The function of the FABP1
gene may be regulated by polymorphisms in coding regions, probably leading to modifications in hepatic triglycerides accumulation and hepatic insulin resistance. Impairment of the function of this gene can lead to several metabolic disturbances such as insulin resistance, non-alcoholic esteatohepatitis and several components of the so-called metabolic syndrome 
Among all the polymorphisms which have been tested for their relationship with obesity and type 2 diabetes within the FABP1
gene, the functional mutation rs2241883
) has been the most studied. According to our results while there was no association with the p.Thr94Ala
in our populations, haplotypes containing the allele T
(wild allele) were significantly associated with the risk of type 2 diabetes in both populations and in the pooled analysis. The p.Thr94Ala
polymorphism induces an amino-acid change that is located within the N-terminal region of the protein which is a component of the fatty acid binding site and therefore the binding capacity might be reduced in patients with the 94Ala
. In the study carried out by Brouillette et al, they found that carriers of the 94Ala
allele had higher baseline FFA, lower BMI and waist circumference than homozygotes Thr94Thr
. In our associated haplotypes, the direction of the association was driven by alleles of the rs2197076
and not by alleles of the functional mutation. Our results suggest that the most strongly associated SNP (rs2197076
), because of its location in the 3 prime UTR site and its association with a potential functional variant in one risk haplotype, could regulate the functional activity of the protein individually or in haplotypes. It is likely that the A
allele of the rs2197076
might therefore reduce FABP1
gene expression thereby contributing to enhance that situation of IR-lipotoxicity, IR and DM2.
Type 2 diabetes and insulin resistance are intimately related and therefore it is very difficult to separate one from the other. The association with the HOMA-IR index values disappeared after the exclusion of diabetic subjects what supports the association with type 2 diabetes. We cannot be sure if the association with type 2 diabetes is mediated through insulin resistance or not. It is well-known that both traits, insulin resistance and type 2 diabetes are strongly influenced by genetics with an estimated heritability for insulin resistance of around 60% and around 35–54% for the plasma insulin levels in familial and twin studies 
. Type 2 diabetes patients usually have lipid abnormalities with high levels of plasma triglycerides and free fatty acids eventually leading to increased FFA flow from the peripheral tissues (muscle, liver) causing lipotoxicity and hence IR.
We did not find an association of FABP1 variants with other diabetes related traits such as hypertriglyceridemia, hypertension, low HDL or high LDL cholesterol. Only there was a trend for the association with hypertriglyceridemia and hypercholesterolemia LDL but only in Segovia population and only with the qualitative trait but not with the quantitative trait. The same occurred for several variants of the intestinal FABP gene which were close to the significance after Bonferroni correction with hypertriglyceridemia but only in Segovia and only with the qualitative trait.
The main limitations of the present study are the lack of FFA levels in our populations and the absence of functional studies of the gene variants. Other limitation was related to the low genotyping call rate observed in many of the samples (around 19% in Hortega and 35% in Segovia). We decided to exclude those samples in an attempt to not compromise the results. We believe that the problems with those subjects with very low call rate were due to DNA problems although the amount of DNA and the ratio 260/280 were within the optimal range. After the exclusion of these problematic samples the genotyping call rate was very high as it has to be because of the high accuracy of the SNPlex platform 
. The major strength is that we were able to replicate the results in an independent Spanish population. Since the two populations belong to the same geographical region and that region has a low immigration rate, we do not expect bias in our results due to population stratification. For this reason as the genetic background was expected to be the same in both studies we decided the pooled strategy over other strategies such as meta-analysis in order to increase our sample size what is crucial to detect variants of low risk. The different metabolic profile observed between populations could be related with the different sampling methodology but especially with the different characteristics of the target populations. Individuals of Segovia population were selected in a primary care environment and the majority belonged to rural areas. They were also significantly younger than those from the Hortega study. The individuals from the latter study were recruited in the area covered by a tertiary hospital and majority of them lived in urban areas. This population was also even regarding to gender distribution compared with Segovia population which included mostly females. Because of the potential influence of the population to which each individual belongs, we also adjusted the analysis by this factor, and the association of the polymorphism rs2197076
and type 2 diabetes remained highly significant [OR 1.83 (1.38–2.43), Bonferroni p-value 0.0003 for the additive model and OR 2.11 (1.51–2.95), Bonferroni p-value 0.00013 for the dominant model] .
Also important is the inclusion of fat intake as a possible factor of interaction between the gene variants and metabolic traits.
In summary, our study supports the role of the liver FABP in the development of type 2 diabetes and insulin resistance in representative samples of Spanish general population. Functional studies may clarify in the future if the liver FABP may or may not be a potential target for treatment of type 2 diabetes and insulin resistance.