I have read the publication by Pflimlin and colleagues (Pflimlin et al., 2018) testing the effects of palmitic acid esters of hydroxy-stearic acid (PAHSAs), the recently discovered lipids (Yore et al., 2014), on glucose control in mice. The authors developed an innovative enantioselective synthesis of the two key members of PAHSA family, namely 5-PAHSA and 9-PAHSA, and extensively tested their effects on murine models of insulin resistance. Their aim was to reproduce data published by the group of Kahn and Saghatelian (Yore et al., 2014) and focus on the therapeutic potential of PAHSA for the treatments of type 2 diabetes. Our group is also involved in the fatty acid esters of hydroxy-fatty acid (FAHFA) research. We have identified omega-3 fatty acid-derived FAHFAs with anti-inflammatory properties (Kuda et al., 2016) and explored the biology of PAHSAs in rats and humans (Brezinova et al., 2018; Kuda et al., 2018). Therefore, I would like to present an alternative interpretation of the data obtained by Pflimlin and colleagues. The authors have carefully designed their experiments to attempt to reproduce the original results (Yore et al., 2014) using mice of similar age,(R/S)-PAHSAs from their own lab or from Cayman Chemicals, etc., but changed one critical parameter. They dissolved the bioactive lipid (s) in olive oil instead of following the original procedure using the PEG-400/Tween-80 formulation (Yore et al., 2014). Although the gavage of PAHSA dissolved in olive oil resulted in higher bioavailability of the tested compounds, this approach completely changed the purpose of oral glucose tolerance test (OGTT). Technically, they performed an alternative version of mixed-meal tolerance test (MMTT): they gavaged mice with 150 μL olive oil per 30 g mouse 30 min before the glucose bolus used in OGTT. MMTT is considered the gold standard for measuring endogenous insulin production, but impaired glucose tolerance can’t be evaluated using MMTT since it doesn’t represent a standardized oral glucose challenge and the organism is exposed to a mixture of nutrients instead. It is rare to reverse obesity-related glucose intolerance by a single gavage of the tested compound 30 min before the OGTT (Yore et al., 2014). In the study by Pflimlin and colleagues, the chance to detect such PAHSA effect was much lower because the MMTT had a lower glycemic index (thus lower glucose excursions), and the previous olive oil gavage slowed entry of glucose into circulation. Moreover, the severity of insulin resistance in the additional high-fat diet models is hard to evaluate due to the lack of appropriate controls. Different high-calorie diets (eg, lipid composition) cause different effects on organs and whole-body insulin action and not all of them result in insulin resistance (Small et al., 2018). If one compares OGTT in control mice fed low-fat diet (Figure 2 in Pflimlin et al., 2018) with those fed lard-based diet (Figure 3 in Pflimlin et al., 2018), there is no difference in glucose and insulin tolerance (OGTT and insulin tolerance test). Although the authors used the PEG-400/Tween-80 formulation (Figure 3 in Pflimlin et al., 2018), the mice probably had normal glucose tolerance and no effect of PAHSA could be expected. Therefore, I believe that their interpretation of the data should be reconsidered and that the results aren’t in conflict with the original observations (Yore et al., 2014).

I was intrigued by the higher bioavailability of PAHSA in olive oil gavage because the original gavage formulation worked reasonably well in our own hands and we observed a linear dose response using 5-PAHSA in mice (Brezinova et al., 2018). Therefore, we have performed a simple comparative test …