Should Dairy Products be Avoided? The Devil in the Details
Because self-reported dietary intakes are fraught with errors (under- and over-reporting of food groups with healthy perceptions, difficulties in calculating nutrient intakes from a restrictive food list, etc), biological measures of nutrient status are preferred.
Dawczyniski and colleagues evaluated the associations of circulating fatty acids on cardiovascular outcomes, taking issue with the conclusions of Chowdhury and colleagues on the health benefits of saturated fatty acids found in dairy products. Several important observations are made.
Differences in the turnover of fatty acids in the sn-1 (typically saturated fatty acids) and sn-2 (typically polyunsaturated fatty acids) in phospholipids occur because of enzyme specificity and position, phospholipase A1 for sn-1 and phospholipase A2 for the sn-2 positions respectively. These differences in enzyme efficacy change biological responses to dietary change.
Red blood cell (erythrocyte) membrane concentrations are more robust markers of dietary fatty acids which have to be ingested rather than synthesized endogenously – linoleic acid (LA), α-linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA).
Saturated fatty acids from dairy and ruminant meat sources may not be an independent CVD risk factor. Perhaps the meta-analysis by Chowdhury and colleagues was flawed because 2 studies (Pilz et al, 2006; Pilz et al, 2007) did not report data on total saturated fatty acid concentrations.
In their re-analysis, Dawczynski and colleagues report a cardioprotective effect of saturated fatty acids from milk (pentadecanoic, C15:0 and heptacecanoic, C17:0; Relative Risk = 1.21).
Bottom line: it is misleading to categorize certain foods (milk), or even fatty acid types (saturated fatty acids), as unhealthy. While we are what we eat (nutrient intake can change cellular composition and function), it is important to assess biological markers of nutrient status, not crude estimates of dietary intake over 2-3 days, to elucidate diet-health relationships.
Dawczynski C, Kleber ME, Marz W, Jahreis G, Lorkowski S. Saturated fatty acids are not off the hook. 2015 Nutr Metab CVD doi: 10.1016/j.numecd.2015.09.010
Hodson L, Eyles HC, McLachlan KJ, Bell ML, Green TJ, Skeaff CM. Plasma and erythrocyte fatty acids reflect intakes of saturated and n-6 PUFA within a similar time frame. 2014 J Nutr doi: 10.3945/jn.113.183749
Chowdhury R, Warnakula S, Kunutsor S, Crowe F, Ward HA, Johnson L, Franco OH, Butterworth AS, Forouhi NG, Thompson SG, Khaw K-T, Mozaffarian D, Danesh J, Di Angelantonio E. Association of dietary, circulating, and supplemental fatty acids with coronary risk: A systematic review and meta-analysis. 2014 Ann Intern Med doi:10.7326/M13-1788
Pilz S, Scharnagl H, Tiran B, Seelhorst U, Wellnitz B, Boehm BO, Schafer JR, Marz W. Free fatty acids are independently associated with all-cause and cardiovascular mortality in subjects with coronary artery disease. 2006 JCEM doi: 10.1210/jc.2006-0195
Pilz S, Scharnagl H, Tiran B, Wellnitz B, Seelhorst U, Boehm B, Marz W. Elevated plasma free fatty acids predict sudden cardiac death: a 6.85-year follow-up of 2215 patients after coronary angiography. 2007 Eur Heart J doi: 10.1093/eurheartj/ehm343