**5. Lipid Peroxidation**

The sperm plasma membrane contains a high proportion of polyunsaturated fatty acid (PUFAs) to generate the fluidity needed in order to accomplish the membrane fusion events associated with fertilization. This high PUFAs content makes spermatozoa especially susceptible to suffer LPO [32,33]. The highly reactive hydroxyl radical (OH−) is an inductor of LPO produced through two consecutive reactions (Figure 3): the first is the Haber–Weiss reaction in which a ferric ion (Fe<sup>3</sup>+) in the presence of a superoxide radical (O2−) is reduced to ferrous ion (Fe<sup>2</sup>+), followed by Fenton reaction, where Fe2<sup>+</sup> reacts with hydrogen peroxide (H2O2), forming Fe3<sup>+</sup> and a hydroxyl radical (OH−).

$$\cdot \bullet \bullet ^{-}\_{2} \bullet \bullet ^{-}\_{2} \bullet \xrightarrow{} \xrightarrow{} \bullet \bullet ^{-} \bullet \bullet ^{-} \bullet \bullet ^{+}$$

**Figure 3.** Haber–Weiss Reaction and Fenton reaction.

Secondary products are formed during LPO: malondialdehyde (MDA), propanol, hexanol, and 4-hydroxynonenal (4-HNE) [34], which are highly reactive and may attack other nearby PUFAs, thus initiating a chain reaction with harmful effects that eventually disrupts membrane fluidity. These secondary products are used as lipid oxidative stress biomarkers.

Nowadays, cryopreservation is becoming an important issue for the success of ART in humans and livestock. Although cryopreservation is routinely used, it is a tough procedure associated with deleterious effects on sperm function due to an increase of ROS production linked to LPO and thus an increase of membrane permeability [35–37]. In this context, the use of antioxidants as additives during cryopreservation/thawing procedure is a common strategy to counteract negative effects of ROS on sperm function.

### **6. E**ff**ects of Oral Antioxidant Intake on Male Reproductive Outcome**

Currently, there is a growing trend of oral antioxidant intake to counteract high levels of ROS found in spermatozoa and seminal plasma of subfertile or infertile men. This hypothesis is supported by several works that describe an improvement of sperm parameters after oral antioxidant intake. Among those improvements, sperm concentration, motility, or decrease of DNA damaged are reported (Reviewed by [38]). However, only a few works have shown the effect of antioxidant therapy on fertility outcomes. Here, we discuss the major findings of oral antioxidant intake in reproduction outcome and its endpoints, such as fertility and live birth (summarized in Table 1).





NI: natural insemination, IVF: in vitro fertilization, ICSI: intracytoplasmic sperm injection, IU: international unit, PVE: prostate-vesiculo-epididymitis, LC: L-carnitine, LAC: L-acetyl-carnitine, LPO: lipid peroxidation, ↑ increase, ↓ decrease.
