Results and discussion

In this study, we investigated the effects of dietary PA on tissue accumulation of PA and on the tissue fatty acid profiles, using mice fed a diet containing a mixture of 3R, 7R, 11R- and 3S, 7R, 11R-PAs, the two isomers present in foods and the human body. Milk and dairy products are typical PA sources for human and contain PA with a few dozens of milligrams per 100 g of food (Brown et al., Reference Brown, Mei, Gibberd, Burston, Mayne, McClinchy and Sidey1993). Therefore, 0.05% PA was added to the experimental diet to approximate a situation where PA-containing milk or dairy products are consumed on a routine basis.

Dietary PA-induced changes in fatty acid compositions in each tissue are summarized in Table 1. Data on PA are expressed as the sum of compositions of 3R, 7R, 11R- and 3S, 7R, 11R-PAs because our gas chromatographic analysis could not discriminate between the two isomers. Our results on PA-fed mice clearly showed that dietary PA was absorbed into the mouse body and significantly accumulated in the parametrial adipose tissue. PA-fed mice also showed high composition of PrA in the adipose tissue compared with control mice, albeit without statistical significance (P = 0.06). Gas chromatographic analysis of the adipose tissue detected a total of 19 fatty acids in addition to PA and PrA. Two of the fatty acids exhibited different compositions between control and PA-fed mice, where the compositions of lignoceric acid (C24 : 0) and nervonic acid (C24 : 1, n-9) were decreased by PA intake. Since these two fatty acids can be de novo synthesized via a series of elongation and desaturation processes of palmitic acid which is the primary product of fatty acid biosynthesis, our results suggest that de novo fatty acid biosynthesis or elongation of synthesized fatty acids is inhibited by PA intake.

Table 1.

Dietary phytanic acid-induced changes in tissue fatty acid compositions (%)

Data are presented as means ± sem.

^#P < 0.05 compared with control in Mann–Whitney U test. *P < 0.05, **P < 0.01 and ***P < 0.001 compared with control in a non-paired Student's t test.

The liver should be the organ most responsible for PA catabolism and PrA formation because the α-oxidation-mediated conversion of PA into PrA is a hepatic microsomal-peroxisomal process (Van Veldhoven et al., Reference Van Veldhoven, Mannaerts, Casteels and Croes1999). However, our results demonstrated that the hepatic concentrations of both PA and PrA in PA-fed mice were below the detection limit of gas chromatographic analysis, which may imply that a part of dietary PA was converted into PrA in the liver and the remaining PA and the resultant PrA were translocated to the adipose tissue. Our gas chromatographic analysis detected 18 fatty acids in the liver, among which 8 fatty acids showed different compositions between groups. Regardless of the undetectable levels of PA in the liver, dietary PA modified the liver fatty acid profile, with a higher impact than in adipose tissue. Our results demonstrated that the compositions of myristic acid (C14:0), palmitic acid (C16:0), palmitoleic acid (C16:1, n-7) and nervonic acid were significantly decreased by PA intake, which emphasizes the notion, that de novo fatty acid biosynthesis is inhibited by PA intake. On the other hand, dietary PA significantly increased the levels of linoleic acid (C18:2, n-6), γ-linolenic acid (C18:3, n-6) and docosahexaenoic acid (C22:6, n-3). As a consequence, the hepatic ratio of polyunsaturated fatty acids to saturated and monounsaturated fatty acids was significantly increased by PA intake (control: 16.06 ± 1.28%, PA: 21.48 ± 1.01%, P < 0.05). Because PPARα and PPARγ, which are preferentially activated by PA (Heim et al., Reference Heim, Johnson, Boess, Bendik, Weber, Hunziker and Fluhmann2002), play a critical role in lipid metabolism, these activations could be involved in PA-induced changes in the tissue fatty acid profiles, although the precise mechanism remains unclear.

Our results indicated that neither PA nor PrA accumulated in mouse brain. Among 19 fatty acids which were detected in the brain, only palmitoleic acid and linoleic acid showed significantly different compositions between groups, suggesting that dietary PA may have a relatively low impact on the fatty acid profile of the brain, at least compared with the liver.

Previous in vitro studies have suggested that improvement of glucose and lipid metabolism is a typical example of the health-promoting effects of PA. Our study supports the notion that the in vitro actions of PA can be also elicited in in vivo physiological situations by showing that PA as well as PrA accumulated in the adipose tissue and that PA modified the hepatic fatty acid profiles. Furthermore, our results showed evidence regarding the beneficial effects of PA, indicated by the significant decline of the hepatic palmitoleic acid/palmitic acid ratio in PA-fed mice (control: 0.14 ± 0.00, PA: 0.11 ± 0.01, P < 0.05). Because the palmitoleic acid/palmitic acid ratio positively correlates with symptoms of liver diseases such as non-alcoholic steatohepatitis (Yamada et al., Reference Yamada, Mizukoshi, Seike, Horii, Terashima, Iida, Kitahara, Sunagozaka, Arai, Yamashita, Honda, Takamura, Harada and Kaneko2019), our results imply that PA has beneficial properties for the treatment of these diseases. On the other hand, excess PA intake is considered a risk factor for health problems such as Refsum disease. Since neurological symptoms are commonly observed in patients with Refsum disease, many studies have evaluated the potential toxicity of PA on neuronal cells (Roca-Saavedra et al., Reference Roca-Saavedra, Marino-Lorenzo, Miranda, Porto-Arias, Lamas, Vazquez, Franco and Cepeda2017). However, PA could not be detected in the brain of PA-fed mice in our study, which indicates that the PA-induced neurotoxicity is negligible, at least in a case where PA can be normally catabolized by α-oxidation. Since this study cannot completely reject the possibility of the involvement of dietary PA in brain functions, further studies are needed to determine the overall physiological action of PA and to discuss comprehensively its therapeutic and toxic effects for human health.

In conclusion, this study demonstrated that dietary PA accumulated in the adipose tissue and modified the hepatic fatty acid profile, although the brain was relatively less affected by dietary PA. These findings provide basic information to cultivate a better understanding of the in vivo physiological role of PA, and to produce milk and dairy products with a modified PA concentration.