Does the deletion of adipose triglyceride lipase (Atgl) gene impair male fertility?The deletion of Atgl gene impaired male fertility but the effect was partially reversed by a low long-chain triglyceride (TG) diet.ATGL specifically hydrolyses long-chain fatty acid TG to diacylglycerol and a high level of expression of ATGL in testes has been reported. However, the role of ATGL in male fertility is unknown.To investigate the effect of deletion of Atgl gene on male fertility, cauda epididymides and testes were collected from wild-type, heterozygous and homozygous Atgl-deficient mice at 10 weeks of age and epididymal sperm analysis and histological analysis of the testes were performed. To investigate whether a medium-chain triglycerides (MCTs) replacement diet mitigated the impaired male fertility by deletion of Atgl gene, homozygous Atgl-deficient mice were fed a MCT replacement diet, or a standard diet including long-chain triglycerides (LCTs) in a control group, for 6 weeks from 5 weeks of age (n = 22). The systematic and local effects of the MCT replacement diet on spermatogenesis and sperm maturation in the epididymis were analyzed at 10 weeks of age.Hematoxylin and eosin staining in paraffin-embedded sections of testes and Oil Red O staining in frozen sections of testes were performed. The epididymal sperm concentrations were analyzed. Statistical analyses were performed using the Student's t-test or Mann-Whitney U test with Shapiro-Wilk Normality test.Although heterozygous mice were fertile and showed a similar number of epididymal total and motile sperm concentrations to wild-type mice, the deletion of Atgl gene in homozygous mice led to accumulation of TG deposits in testes and impaired spermatogenesis. The deletion of Atgl gene also impaired the sperm maturation process required for sperm to acquire the ability to move forward in the epididymis. The MCT replacement diet for 6 weeks increased the plasma level of non-esterified fatty acid (NEFA) (1.5-fold, P = 0.005), but not the plasma total cholesterol (T-Cho) and TG levels. In testes, the MCT replacement diet decreased the number of Oil Red O stain positive vacuoles (-40%, P < 0.001) and increased testis tissue weight (1.1-fold, P = 0.012), total sperm concentration (1.5-fold, P = 0.011) and motile sperm concentration (2.1-fold, P < 0.001) compared to the control group. However, there was no significant change in the sperm survival rate between the two groups.None.One previous study reported that Atgl-deficient male mice were fertile. In most studies heterozygous Atgl(+/-) mice were used to generate homozygous Atgl-deficient Atgl(-/-) mice. Although the same gene targeting mice were used in this study and the formation of vaginal plugs were observed after mating with Atgl(-/-) male mice, there were no pregnant wild-type mice observed after mating with Atgl(-/-) male mice.Local TG metabolism in the male reproductive system could affect spermatogenesis and sperm motility in men. The MCT replacement diet could be an effective therapy for idiopathic non-obstructive oligozoospermia or asthenozoospermia in men with low levels of serum NEFA.This study was supported in part by the Japan Society for the Promotion of Science JSPS KAKENHI Grant (Nos. JP24249080, JP25462557, JP16K11086). The authors declare no conflict of interest.