Plasma phospholipid very-long-chain saturated fatty acids: a sensitive marker of metabolic dysfunction or an indicator of specific healthy dietary components?

See corresponding article on page 1047.

The longstanding dogma with regard to the potential adverse effects of SFAs on metabolic functions and lifestyle diseases such as type 2 diabetes (T2D) and cardiovascular diseases has taken a new turn. During the past years, observational studies have shown inverse associations between intake of SFAs from dairy and reduced risk of lifestyle diseases (1), and new trials now investigate potential effects of very-long-chain SFAs (VLSFAs; i.e., SFAs with >20 carbon atoms). The article by Lemaitre et al. (2) in this issue of the Journal verifies the negative associations between plasma phospholipid VLSFAs [20:0 (arachidic acid), 22:0 (behenic acid), and 24:0 (lignoceric acid)] and risk of T2D shown previously in the EPIC (European Prospective Investigation into Cancer and Nutrition) study (3). In both studies, the underlying assumption is that different SFAs differ in biological activity and thus affect insulin sensitivity differently. The study by Lemaitre et al. is furthermore motivated by previous results in which plasma phospholipid VLSFAs were found to be associated with reduced risk of cardiac arrest, independently of plasma triglyceride and phospholipid palmitate (16:0) (4). However, this new study found that the negative association between plasma phospholipid VLSFAs and the incidence of T2D was practically completely abolished after adjustment for plasma phospholipid palmitate and total plasma triglyceride.

The dominant SFA in the diet is palmitic acid, and this is also the main end product of hepatic de novo lipogenesis and thus the dominant fatty acid in the circulation—especially in plasma phospholipids (8). Circulating concentrations of fatty acids have in numerous studies been shown only to work as biomarkers of intake for the non–endogenously synthesized fatty acids (8); thus, neither palmitic acid nor VLSFAs are expected to be good markers of dietary intakes of these fatty acids. The relative content of palmitic acid in plasma phospholipid has been shown to be associated with the consumption of red and processed meat and alcohol (9), and circulating palmitic acid is also associated with high intakes of simple carbohydrates (8)—all of which represent a typical Western diet. High plasma triglyceride is associated with excess energy intake, specifically from carbohydrates, and low physical activity, both of which are seen to be positively related to plasma phospholipid palmitic acid and negatively to plasma phospholipid VLSFAs in the study by Lemaitre et al. Plasma triglyceride is one of the best predictors of insulin resistance and a potent marker of metabolic dysfunction (10), which is well known to be caused by an overall unhealthy Western lifestyle pattern. Given the strong inverse association between plasma VLSFAs and palmitic acid, we suggest that low VLSFAs could reflect the risks associated with the underlying metabolic dysfunction and that it is likely that the observations could be explained by residual confounding from an overall unhealthy lifestyle, rather than from intake of VLSFAs.

The observed association between plasma phospholipid VLSFAs and insulin sensitivity in the study by Lemaitre et al. was attenuated by adjustment for plasma triglyceride and phospholipid palmitic acid. This indicates that the effects of these markers are linked, but not that the effect of plasma phospholipid VLSFAs is mediated by triglyceride and phospholipid palmitic acid, as suggested by the authors. Indeed, it could be the other way around—that a high lipid load (and thereby high liver and plasma triglyceride concentration) reduces the formation of VLSFAs from palmitic acid. The fact that a slight association was still evident with arachidic acid after the adjustment could indicate that low plasma phospholipid arachidic acid is a more sensitive marker of dysfunctional lipid metabolism than high phospholipid palmitic acid and high triglyceride.

When discussing associations between phospholipid VLSFAs and physiologic outcomes, it is essential to consider their highly specialized function. VLSFAs are almost exclusively found in sphingolipids (i.e., as acyl groups in ceramide, sphingomyelin, and glycosphingolipids). Thus, in the phospholipid fraction studied by Lemaitre et al. (2), VLSFAs will represent the concentration of sphingomyelin. This is supported by the fact that variations in plasma phospholipid VLSFAs are strongly associated with common single nucleotide polymorphism variants in 2 of the key enzymes in sphingomyelin synthesis, namely serine-palmitoyl-CoA transferase and the ceramide synthase that adds arachidic acid in the ceramide synthesis (5). The association between plasma phospholipid VLSFAs and reduced risk of T2D could therefore be explained by an association with a higher relative amount of sphingomyelin in the plasma phospholipid pool or an increase in the VLSFA content of the sphingomyelins [relative to those with palmitic acid or stearic acid (18:0)]. The latter is in accordance with 2 recent studies showing that knock-out of the ceramide synthase specific for synthesis of sphingolipids with myristic acid (14:0), palmitic acid, or stearic acid acyl groups protected mice from developing insulin resistance with a high-fat diet, whereas decreased expression of the ceramide synthase responsible for synthesis of VLSFA-containing sphingolipids rendered mice susceptible to insulin resistance (6, 7). Considering this, the attenuation of the association with T2D by triglyceride and palmitic acid adjustments could be interpreted as a downregulation of the synthesis of VLSFA-containing sphingolipids at high hepatic lipid loads.

On the basis of this, we agree with the authors that mechanistic studies are needed to explore dietary as well as metabolic determinants of plasma phospholipid VLSFAs as well as the biochemical links between circulating lipids. The above also emphasizes the weakness of disrupting the functional biochemical entity (in this case, the specific phospholipid structures) in epidemiologic association studies. A true understanding of the mechanisms at play will require lipidomic analysis of the intact native molecular structures because such methods could point to specific enzymes and/or pathways explaining the observed associations. Studies that take advantage of polymorphisms in genes related to the biochemical pathways will be an additional strong tool to be used in future studies in this field. Further insight into whether any specific dietary components associated with VLSFA concentrations could play a role in T2D prevention would require randomized controlled trials with the use of isocaloric diets and comparing the effect of these dietary components on lifestyle diseases such as T2D. This would be considerable work and effort, based on just a few, although well performed, observational studies. So, before we commence on a comparison of dietary palmitic acid and VLSFAs on lifestyle diseases, we should ask ourselves: Do we have a strong line of evidence for the biological plausibility?