Trans-palmitoleate levels represented less than one percent of total fatty acids: mean±SD 0.18±0.05%, range=0.02, 0.55%. Trans-palmitoleate levels correlated strongly with fatty acid biomarkers of dairy fat consumption(34
) such as 15:0 (r=0.64) and 17:0 (r=0.66), but weakly with trans-fats frequently derived from partially hydrogenated oils(23
) such as trans-16:1n-9 (r=0.11), trans-18:1n-7 (r=0.25), total trans-18:1 (r=0.15), and total trans-18:2 (r=0.07), consistent with dairy foods rather than industrially produced trans-fats being a major source of trans-palmitoleate. In bivariate (unadjusted) analyses (), trans-palmitoleate was associated with slightly older age, white race, and modestly less prevalent CHD. Trans-palmitoleate was also associated with less alcohol use, modestly higher total fat intake, modestly lower carbohydrate and low-fat dairy consumption, and greater consumption of whole-fat dairy foods and red meat.
Baseline Characteristics According to Plasma Phospholipid Trans-Palmitoleic Acid in 3,736 US Adults.
We next elucidated and quantified the factors independently associated with circulating trans-palmitoleate levels in this large community-based cohort (). In multivariable analyses, older age, nonwhite race, and greater education were associated with higher levels, and female sex and prevalent CHD were associated with lower levels. Greater BMI and alcohol use were associated with slightly lower levels. Whole-fat dairy consumption had the strongest relationship, with 0.69 SD higher trans-palmitoleate levels among individuals consuming 15+ servings/week (2+ servings/day) versus ≤2 servings/week. Evaluating different dairy foods, this relationship appeared directly related to dairy fat content: it was strongest for whole milk (+0.49 higher SD of trans-palmitoleate per serving/day, P<0.001), then butter (+0.31, P<0.001), 2% milk (+0.21, P<0.001), cheese (+0.20, P=0.041), and ice cream (+0.18, P=0.093).
Multivariable-Adjusted Relationships of Demographic, Clinical and Lifestyle Factors with Plasma Phospholipid Trans-Palmitoleic Acid in 3,736 US Adults.
In multivariable-adjusted analyses, trans-palmitoleate levels significantly associated with several metabolic risk factors (). Higher trans-palmitoleate was associated with slightly lower BMI (across quintiles: −1.8%, P=0.058) and waist circumference (−1.8%, P=0.009). In multivariable-models further adjusted for adiposity, trans-palmitoleate was associated with slightly higher HDL-cholesterol (+1.9%, P=0.043), substantially lower triglycerides (−19.0%, P<0.001), lower total:HDL-cholesterol (−4.7%, P<0.001), and lower CRP (−13.8%, P=0.050), a marker of systemic inflammation related to metabolic syndrome and diabetes risk.(37
) Trans-palmitoleate was also associated with fibrinogen, an acute phase reactant elevated by insulin resistance(39
), but without dose-response: lower levels were seen only in the lowest quintile. Trans-palmitoleate was associated with lower fasting insulin (−13.3%, P<0.001) and insulin resistance (homeostasis-assessment-model: −16.7%, P<0.001). Trans-palmitoleate was unassociated with LDL-cholesterol, fasting glucose, or blood pressure. All relationships appeared similar by sex (not shown).
Multivariable-Adjusted Relationships of Plasma Phospholipid Trans-Palmitoleic Acid with Metabolic Risk Factors in 3,736 US Adults.
Among 2,985 participants free of prevalent diabetes, 304 new cases occurred during 27,866 person-years. In age- and gender-adjusted analyses, trans-palmitoleate was associated with lower risk of new-onset diabetes, including 2-fold lower risk in quintile 4 (HR=0.44, 95%CI=0.30–0.66) and 3-fold lower risk in quintile 5 (HR=0.36, 95%CI=0.23–0.57), versus quintile 1 (P-trend<0.001) (). Further adjustment for demographic, clinical, and lifestyle factors including dairy and red meat consumption did not appreciably alter results (). Lower risk appeared similar in men (extreme-quintile HR=0.29, 95%CI=0.11–0.74) and women (extreme-quintile HR=0.41, 95%CI=0.24–0.71) (P-interaction=0.87). When evaluated continuously per SD (0.05 percentage points of total fatty acids), each higher SD of trans-palmitoleate was associated with 28% lower risk of diabetes (multivariable-adjusted HR=0.72, 95%CI=0.61–0.86, P<0.001). To explore potential reverse causation (presence of subclinical disease altering trans-palmitoleate levels), we performed sensitivity analyses excluding 1,106 participants with fasting glucose≥5.56 mmol/l (100mg/dl) at baseline. Trans-palmitoleate remained inversely associated with incident diabetes (extreme-quintile HR=0.31, 95%CI=0.13–0.71).
Incidence of Diabetes Mellitus Between 1992 and 2006 According to Plasma Phospholipid Trans-Palmitoleic Acid in 3,736 US Adults without Prevalent Diabetes at Baseline.
Based on biologic considerations and present findings (), consumption of carbohydrate, protein, red meat, and dairy foods could each confound relations between trans-palmitoleate and incident diabetes, even in fully multivariable-adjusted analyses. We first evaluated whether these factors themselves were independently associated with incident diabetes. After multivariable-adjustment (covariates per ) but excluding adjustment for trans-palmitoleate, neither consumption of carbohydrate (P=0.81), protein (P=0.88), red meat (P=0.69), nor low-fat dairy (P=0.98) were associated with incident diabetes. Conversely, greater whole-fat dairy consumption was associated with lower risk (across 6 categories, P-trend=0.024). However, when further adjusted for trans-palmitoleate levels, this association was attenuated and no longer significant (P-trend=0.162). In contrast, the relationship of trans-palmitoleate with diabetes risk was robust to adjustment for dairy consumption ().
These findings suggested that trans-palmitoleate, rather than consumption of specific foods, was the principal factor related to diabetes risk. However, we recognized that trans-palmitoleate was measured objectively, whereas food intakes were estimated by questionnaire. We evaluated whether other fatty acid dairy biomarkers (15:0, 17:0) related to diabetes risk as well as whether associations of trans-palmitoleate with diabetes risk persisted following adjustment for these biomarkers; we similarly evaluated trans-18:1n-7 (vaccenic acid) that is present in both dairy fats(40
) and other food sources.(23
) After multivariable-adjustment (covariates per ), neither 15:0 (HR=0.99, 95%CI=0.89–1.10), 17:0 (HR=0.98, 95%CI=0.85–1.13), nor vaccenic acid (HR=0.97, 95% CI=0.93–1.01) were associated with diabetes risk (HR’s evaluated per 0.05 percentage points of total fatty acids, corresponding to 1.0, 0.7, and 0.3 SD differences, respectively). However, following simultaneous adjustment for levels of 15:0, 17:0, and vaccenic acid, trans-palmitoleate remained inversely associated with new-onset diabetes (extreme-quintile HR=0.43, 95%CI=0.25–0.75, P-trend<0.001).
In our separate validation cohort, higher erythrocyte trans-palmitoleate was associated with lower interleukin-6 (multivariable-adjusted levels across quartiles: 2.3 vs. 1.8 pg/ml, P-trend=0.020) and CRP (3.3 vs. 2.1 mg/l, P-trend=0.020) and trends toward lower total:HDL-cholesterol (4.21 vs. 3.87, P-trend=0.099) and hemoglobinA1c (5.92 vs. 5.66 percent, P-trend=0.056). In similar analyses, higher plasma trans-palmitoleate was associated with higher HDL-cholesterol (1.46 vs. 1.60 mmol/l [56.5 vs. 61.7 mg/dl], P-trend=0.033), lower total:HDL-cholesterol (4.28 vs. 3.63, P-trend<0.001), and lower hemoglobinA1c (5.88 vs. 5.64 percent, P-trend=0.028).