We evaluated interactions between dietary macronutrient intake and ANGPTL4[E40K] genotype for HDL-C and TG in 8511 White men and women from the ARIC study. For most macronutrients, the associations with TG and HDL-C were uniform across ANGPTL4[E40K] genotype groups in both men and women. However, we did note a significant interaction between percentage of energy from carbohydrate and ANGPTL4[E40K] genotype for HDL-C. In men only, percentage energy intake from carbohydrate showed a stronger inverse association with HDL-C in A allele carriers than in G allele homozygotes, essentially negating the beneficial effect of the ANGPTL4[E40K] mutation on HDL-C when percentage intake from carbohydrate was low.
It is well established that greater energy contribution from carbohydrate in the diet will lower HDL-C concentrations [7
]. However, research also indicates that the quality of carbohydrate may be equally important in determining the sum effect on HDL-C [23
]. Similarly, other dietary factors (nutrients and foods included in a single dietary pattern) may further influence associations with HDL-C ascribed to carbohydrate intake. In our study, dietary intake of several food groups significantly differed between men and women, including refined grain intake which was significantly greater in men (18 weekly servings in men vs. 16 in women, data not shown). Thus, it is possible that the significant interaction between percentage energy from carbohydrate and ANGPTL4
[E40K] genotype that we observed in men, but not in women, was partly due to gender differences in the foods comprised by carbohydrate as well as gender differences in overall dietary patterns (additional confounding dietary factors). We attempted to address this issue by adjusting for measures of carbohydrate quality (fiber and glycemic index), but we found neither attenuation in the interaction nor change in genotype-specific regression coefficients. Hormonal differences between men and women may also have played a role, although adjustment for use of hormone therapy did not change our results.
allele substitution in ANGPTL4
results in a substitution of lysine for glutamic acid, a loss of function of ANGPTL4, and a reversal of LPL inhibition [2
]. Greater LPL activity is associated with greater TG clearance and greater HDL-C concentrations. Consistent with this, the E40K ANGPTL4
mutation is associated with lower TG and higher HDL-C concentrations (shown here and demonstrated previously [5
]). In contrast, high carbohydrate intake is thought to decrease HDL-C concentrations by decreasing hepatic fatty acid oxidation and consequently increasing hepatic production of very low-density, triglyceride-rich LDL-C (VLDL-C) particles [25
]. The question of why would this effect would be magnified in E40K variants, as we observed in the current study, is uncertain and is inconsistent with the effect of the ANGPTL4
mutation on LPL activity (increased) and the subsequent effect of LPL activity on HDL-C concentrations (increased). However, it is important to acknowledge that these relationships relate to the postprandial state. Meal composition and timing (variables not collected in the current study) may affect these relationships. Furthermore, our samples were collected after a ≥8-h overnight fast, and our study did not measure LPL activity or mass.
Because carbohydrate intake is only a part of a larger landscape of dietary intake, we did attempt to account for other dietary factors by adjusting for intake of other macronutrients and measures of carbohydrate quality. Although these adjustments had little impact on our results, adjustment is likely incomplete due to residual confounding by imprecisely measured or unmeasured dietary factors. Similarly, residual confounding by other lifestyle factors is also possible, although we tried to account for differences in alcohol intake, education, smoking habits, and physical activity.
It is possible that the absence of interaction between other dietary macronutrients and ANGPTL4
[E40K] is due to imperfect assessment of dietary exposures. However, the fact that the main effects for the relations between macronutrient intake and plasma lipids were significant and in directions predicted by many controlled feeding studies [7
] suggests that for our purposes, diet was adequately measured. Nevertheless, additional study of this research question in cohorts with more comprehensive measures of dietary intake is warranted.
Ours is the first study to evaluate interactions between dietary macronutrient intake and ANGPTL4[E40K]. It is important that similarly designed analyses be conducted in other cohorts to verify whether the interaction observed in the current analysis is real or the result of chance. It may be of key importance that future work evaluate whether the gender differences observed in our sample were due to underlying biologic differences between men and women or due to differences in dietary patterns and food sources of carbohydrate between men and women.