Influence of dietary carbohydrates on plasma fatty acid composition : Results from interventional and observational studies

Given reporting bias in dietary questionnaires, biomarkers offer objective assessments. Measuring circulating fatty acid (FA) composition is an established method for evaluating dietary fat quality. However, the influence of carbohydrates and sugars on plasma FA composition is less clear. This thesis investigates how carbohydrates impact circulating FAs synthesized through de novo lipogenesis (DNL). We assessed palmitate and other FAs in circulating lipids. The hypothesis was that increased dietary carbohydrate intake elevates plasma palmitate levels. In Paper I, a randomized crossover study (with all meals provided) investigated the impact of three diets varying in carbohydrate amount and type on plasma palmitate levels in healthy adults. Results showed no influence of either carbohydrate quantity or quality on plasma palmitate. However, palmitoleate increased in higher-carbohydrate diets, and carbohydrate quality affected myristate levels. In Paper II, a parallel-groups study explored the impact of high intake of sugar-sweetened soda (SS) and semi-skimmed milk (milk) on plasma palmitate among Danish adults. Both SS and milk increased palmitate in phospholipids (PL) compared with water. Excessive SS, but not milk, increased palmitoleate in all lipid fractions. In Paper III, a prospective study in Swedish children found no association between plasma palmitate in PL and incident overweight. Palmitate did not correlate with carbohydrate or sugar intake. Only stearate was associated with overweight incidence. Again, stearate was not associated with carbohydrate or sugar intake. In Paper IV, a cross-sectional study in older men found no association between serum palmitate and carbohydrate or sucrose intake, even after considering BMI or insulin sensitivity. Instead, stearate was positively associated with carbohydrate and sucrose intake, while oleate was inversely associated with sucrose and fiber intake. In conclusion, this thesis suggests that moderate changes in carbohydrate quantity or quality do not alter plasma palmitate, although overfeeding with liquid sugar causes higher palmitate in plasma PL. Stearate, but not palmitate, was linked to incident overweight in children, but none of these FAs reflected higher carbohydrate or sugar intake. Overall, palmitoleate seems to be more responsive to increased carbohydrate intake than palmitate, whereas the latter does not appear as a useful biomarker of high carbohydrate intake in Nordic populations.