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Comparative Study
. 2025 Jul 15;39(13):e70805.
doi: 10.1096/fj.202501283R.

Comparing Soy and Milk Protein Regulation of Hepatic Omega-3 Fatty Acid Biosynthesis

Affiliations
Comparative Study

Comparing Soy and Milk Protein Regulation of Hepatic Omega-3 Fatty Acid Biosynthesis

Melissa Gonzalez-Soto et al. FASEB J. .

Abstract

The omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) have numerous benefits, including strong anti-inflammatory and triglyceride-lowering properties. EPA and DHA are primarily obtained by consuming fatty fish; however, they are also endogenously synthesized primarily in the liver from α-linolenic acid (ALA) through a pathway mediated by the delta-6 desaturase (D6D) enzyme. Previous reports in rodents and humans suggest that dietary proteins such as soy and dairy may impact this pathway differently. The primary aim was to investigate the effects of diets containing either soy or milk protein on the expression, abundance, and enzymatic activity of the desaturases and elongases regulating hepatic omega-3 fatty acid biosynthesis. Male C57BL/6N mice (n = 16 per group) were fed a moderate-fat diet for 8 weeks containing either 1% or 3% energy from ALA. Protein content (15% energy) corresponded to either skim milk powder (SMP) or soy protein isolate (SPI). Hepatic fatty acid content was quantified by gas chromatography-flame ionization detection. Gene expression and protein expression were assessed by RT-qPCR and western blotting, respectively. D6D activity was measured in isolated hepatic microsomes. Fat oxidation was examined using a high-resolution respirometer. Hepatic omega-3 fatty acids (ALA, SDA, EPA, DPAn-3) were lower in SPI-fed mice compared to SMP-fed mice. Fads1, Fads2, Elovl2, and Elovl5 expression was higher in SPI-fed mice compared to those fed SMP, while Srebp-1c expression was lower and Cpt1a expression higher in SPI-fed mice. Consistent with the changes seen at the gene expression levels, FADS2 protein abundance was higher in SPI-fed mice, whereas ELOVL5 protein expression was lower in the SPI groups. Little to no differences in microsomal D6D activity and mitochondrial respiration were detected. Our findings suggest that SPI-related reductions in hepatic omega-3 fatty acid content occur independent of changes in desaturase gene expression, protein expression, enzymatic activity, or mitochondrial respiration. Further studies investigating the influence of dietary proteins on ALA metabolism are therefore warranted.

Keywords: DHA; EPA; delta 6 desaturase; milk protein; omega‐3; soy protein.

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Conflict of interest statement

The authors declare no conflicts of interest.

Figures

FIGURE 1
FIGURE 1
Body weight and food intake measurements in mice fed experimental diets. (A) Total body weight, (B) weekly body weight, (C) relative liver weight, (D) total food intake, (E) weekly food intake, and (F) feed efficiency in mice fed experimental diets for 8 weeks are shown. Data are reported as mean ± SD. n = 16 per group.
FIGURE 2
FIGURE 2
Total fatty acid levels in liver. Pie charts depicting the percentage breakdown of fatty acid classes (SFA, MUFA, n‐3, and n‐6) in mice fed the different dietary groups are shown (A–D). Data are reported as mean ± SD (see Table S2 for values). n = 9 per group.
FIGURE 3
FIGURE 3
Relative n‐3 PUFA composition in liver. Hepatic levels of the main n‐3 PUFA are shown (A–E). Small blue insert graphs depict the content of the same fatty acid in the four experimental diets. Post hoc test significance: *represents a p ≤ 0.05, **represents a p ≤ 0.01, and ***represents a p ≤ 0.001. n = 9 per group.
FIGURE 4
FIGURE 4
Hepatic mRNA levels of genes involved in the biosynthesis pathway. (A) Fads2, (B) Fads1, (C) Elovl2, (D) Elovl5, (E) Srebp‐1c, and (F) Cpt1a. Post hoc test significance: *represents a p ≤ 0.05, **represents a p ≤ 0.01, and ***represents a p ≤ 0.001. n = 7 per group.
FIGURE 5
FIGURE 5
Hepatic protein expression of desaturases and elongases. (A) FADS2 (D6D), (B) FADS1 (D5D), (C) ELOVL2, and (D) ELOVL5. Experimental groups are indicated at the top. The molecular weight is shown on the righthand side. Ponceau S was used as a loading control and for data normalization. n = 7 per group.
FIGURE 6
FIGURE 6
Delta‐6 desaturase activity in isolated hepatic microsomes. Microsomes were obtained from liver tissue and incubated with ALA. D6D activity is reported as μg/min/ng of protein. n = 6 per group.
FIGURE 7
FIGURE 7
Hepatic mitochondrial respiration. (A) Graphs for complex I and II supported respiration, (B) PCoA supported respiration, and (C) MCoA capacity to inhibit CPTI. PM, pyruvate + malate; +D, PM + ADP; +G, PMD + glutamate; +S, PMDG + succinate; MD, malate + ADP; +LC + PCoA, l‐carnitine + palmitoyl‐CoA; MCoA, malonyl‐CoA. Statistical significance (main effect) is indicated with a horizontal line accompanied by the associated p‐value. n = 5–6 per group.

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