Cell membrane systems are particularly vulnerable to reactive oxygen and nitrogen species(RONS) attacks due to their content of polyunsaturated fatty acids (PUFAs) and their metabolic activities, which endogenously generate other oxidizing metabolites.

Astaxanthin protects cell membranes against RONS and oxidative damage. Due to its chemical structure, its polar groups overlap the polar regions of the cell membrane. In contrast, the central non-polar region of the molecule fits into the inner non-polar region of the membrane.

Thus, astaxanthin may take on a trans-membrane alignment in biological membranes, helping maintain the membrane structure, decreasing membrane fluidity, and acting as an antioxidant.

Astaxanthin scavenges RONS and other reactive species (sulfur and carbon) directly by donating electrons and bonding with the free radical to form a non-reactive product.

Additionally, the presence of a series of conjugated bonds in the central non-polar region of astaxanthin enables the molecule to remove free radicals (high-energy electrons) from the cell interior by transporting them along its carbon chain so that these are neutralized by other antioxidants located outside the cell membrane, such as vitamin C.

The increased susceptibility of membrane lipids and low-density lipoprotein (LDL) to oxidation may trigger the formation of thrombi and the development of atherosclerosis. One of the reactive species that induces lipid peroxidation and LDL oxidation is peroxynitrite (ONOO−), which is neutralized by astaxanthin to form 15-nitroastaxanthin, a compound that also has important antioxidant action.

The LDL oxidation time in the presence of astaxanthin has been analyzed in vitro and ex vivo. In the in vitro assays, astaxanthin prolonged LDL oxidation in a dose-dependent manner, in addition to being more effective than lutein and α-tocopherol.

In turn, the blood samples of individuals who were supplemented daily with 1.8, 3.6, 14.4, or 21.6 mg astaxanthin for 14 days showed a significant delay in LDL oxidation compared to samples collected before supplementation. The greatest effect was obtained with the dose of 14.4 mg (oxidation time increased by 5.0, 26.2, 42.3, and 30.7% with 1.8, 3.6, 14.4, and 21.6 mg astaxanthin, respectively).

Thus, it was demonstrated that the intake of astaxanthin delayed LDL oxidation, one of the critical factors in atherosclerosis.

LDL oxidation is also related to developing endothelial dysfunction in patients with diabetes mellitus, which increases the risk of cardiovascular complications. Endothelial dysfunction consists of impaired vessel relaxation dependent on endothelial factors, such as nitric oxide (NO) production by endothelial NO synthase (eNOS).

One of the pathways responsible for this type of dysfunction is the binding of oxidized LDL to its endothelial receptor, lectin-like ox-LDL receptor 1 (LOX-1), thus favoring oxidative stress, which leads to increased lipid peroxidation and eNOS inactivation.

Supplementation with 10 mg/kg astaxanthin for 42 days in a diabetic rat model increased artery relaxation by significantly lowering oxidized LDL, LOX-1 receptor, and lipid peroxidation levels in the aorta.

Additionally, the increase of eNOS demonstrated that astaxanthin might have therapeutic potential for treating endothelial dysfunction in diabetic patients.

Erythrocytes have a large amount of PUFAs and high oxygen and ferrous ions (Fe2+), making these cells more susceptible to oxidative changes in the lipid bilayer, leading to compromised cell stability, oxygen transport, and blood rheological properties. The oxidation of erythrocytes is related to the formation of atheromas and the occurrence of intraplate hemorrhage during the development of atherosclerosis.

As for lipid peroxidation in erythrocytes, daily supplementation with 6 or 12 mg of astaxanthin for 12 weeks in healthy individuals demonstrated that this carotenoid is incorporated and distributed into these blood cells. When incorporated into erythrocytes, astaxanthin exerted antioxidant effects on the cell membrane by significantly reducing the levels of phospholipid hydroperoxides, which are the primary products of phospholipid oxidation (14.9 pmol/ml for the placebo group and 8.0 and 9.7 pmol/ml for the 6 and 12 mg astaxanthin groups, respectively).

In that study, the two doses of astaxanthin had similar effects compared to placebo, suggesting that the intake of 6 mg of this carotenoid is sufficient to inhibit oxidative stress in erythrocytes.

A significant reduction in oxidative damage by lipid peroxidation was also observed, as the plasma levels of 12- and 15-hydroxy fatty acids (P=0.048 and P=0.047, respectively) decreased after three months of supplementation with 8 mg astaxanthin in healthy men.

Blood rheology is important for cardiovascular homeostasis. Astaxanthin reduced blood transit time in a hypertensive rat model and humans. In the latter study, individuals receiving supplementation with 6 mg astaxanthin for ten days had a significantly faster blood transit time than before and in the placebo group (47.6±4.2 vs. 54.2±6.7 sec for the treated and placebo groups, respectively; P<0.05.).

One hypothesis for the improvement of blood rheology by astaxanthin is its antioxidant effect on the intra and extracellular environment and the consequent increase in the flexibility of the erythrocyte membrane conferred by the structural arrangement of astaxanthin in the membrane.

Other factors that affect the blood flow velocity, such as plasma viscosity and vasodilation, affect peripheral vascular resistance and may contribute to hypertension and its main cardiac complication, myocardial hypertrophy.

Studies of spontaneously hypertensive rats (SHR) reported that astaxanthin supplementation significantly reduced systolic pressure and induced significant histological changes in the aorta associated with decreased vascular stiffness and blood pressure. This response was caused by increased endothelial cell-dependent vasodilation due to the greater bioavailability of NO and the remodeling of the arteries. The increase in NO was caused by the reduced production of superoxide anion radicals released by NADPH oxidase, which is one of the antioxidant effects of astaxanthin.

Astaxanthin also contributed to remodeling the smooth muscle cells of the vessels, reducing their proliferation and the damage caused by oxidative stress. Astaxanthin lowered RONS levels by increasing the activity of antioxidant enzymes and regulating mitochondrial dynamics, mitophagy, and mitochondrial biogenesis, which are important for maintaining mitochondrial and cellular metabolism.

The high bioavailability of NO due to lower oxidative stress promoted by astaxanthin was also associated with its anti-thrombogenic effects. In a stroke-prone SHR model, astaxanthin significantly downregulated the oxidative stress marker 8-hydroxy-2′-deoxyguanosine (8-OHdG) in the urine, lowered systolic blood pressure, and suppressed thrombogenesis in the cerebral veins.

The observed anti-thrombogenic effect may have been due to vasodilation and platelet aggregation inhibition caused by increased NO’s bioavailability. In a murine model of thrombosis treated with astaxanthin in the form of the CDX-085 prodrug, an increase of ~20% in the blood flow of the carotid artery was observed before the occurrence of endothelial dysfunction, as was a delay in the formation of occlusive thrombi.

These results were due to the increase in NO and the decrease in ONOO−. The increase in blood flow was due to vasodilation caused by increased release of NO by endothelial cells and reduced platelet activation triggered by the antioxidant effect.

Some of the vascular benefits promoted by the antioxidant effect of astaxanthin were reported in an open study of 20 postmenopausal women with a high oxidative stress rate.

After eight weeks of supplementation with 12 mg astaxanthin, there was a significant reduction of 4.64 and 6.93% in the systolic and diastolic pressure values, respectively, possibly resulting from the decrease in the vascular tone due to the action of the carotenoid in the endothelium.

Reduced vascular resistance in the lower limbs (3.7% increase in the ankle-brachial pressure index), a 4.58% increase in antioxidant capacity, and improvement of some physical and mental symptoms, such as tired eye sensation and difficulty sleeping, were also observed.

In addition to the protective role of astaxanthin in the lipid oxidation process, this carotenoid affects the activity of antioxidant enzymes involved in lipid metabolisms, such as thioredoxin reductase (TrxR) and paraoxonase-1. TrxR is an antioxidant enzyme involved in reducing thioredoxin, lipid hydroperoxides, and hydrogen peroxide.

A previous study demonstrated that thioredoxin in its oxidized form was associated with the degree of severity of chronic heart failure and the resulting oxidative stress. Paraoxonase-1 binds to serum high-density lipoprotein (HDL) and is responsible for protecting both LDL and HDL from oxidation and breaking down oxidized lipids.

The effect of astaxanthin on these two enzymes was evaluated in rabbits fed a cholesterol-rich diet. The authors found that astaxanthin (100 and 500 mg/100 g of feed) reduced the amount of oxidized protein, possibly due to changes in the activities of TrxR and paraoxonase-1. Still, the in vitro evaluation only demonstrated the direct action of this molecule on TrxR.

Two randomized and double-blind clinical studies on overweight or obese individuals were carried out to demonstrate the antioxidant effects of astaxanthin.

In one study, volunteers who received 5 and 20 mg astaxanthin for three weeks exhibited lower oxidative stress biomarkers associated with lipid peroxidation compared with before treatment, with a 34.6 and 35.2% reduction in malondialdehyde (MDA) levels, and 64.9 and 64.7% reduction in isoprostane (ISP) levels, respectively.

An increase in the activity of the antioxidant defense system was also observed, with a 193 and 194% increase in superoxide dismutase (SOD) and a 121 and 125% increase in total antioxidant capacity at the doses of 5 and 20 mg, respectively, compared with the data before treatment. No significant differences were observed between the results obtained with the two doses, indicating that the clinical effects of this carotenoid are not dose-dependent.

The authors analyzed lipid profile, oxidative stress, and antioxidant system parameters. After 12 weeks of supplementation with 20 mg astaxanthin, the same results in oxidative stress and the antioxidant system as in the previous study were observed. Regarding the lipid profile, there was a significant reduction of 10.4% in the LDL concentration, 7.59% in ApoB, and 8.22% in the ApoA1/ApoB ratio (considered an index of the risk of heart attack) compared to the placebo group values.

Therefore, these studies demonstrated that astaxanthin reduces oxidative stress and modulates the lipid profile in overweight and obese individuals, mitigating the risk of developing cardiovascular diseases.

Astaxanthin also has potent detoxifying and antioxidant effects in smokers. The free radicals induced by smoking have been strongly associated with increased oxidative stress, contributing to the increased susceptibility of smokers to the pathogenesis of cardiovascular diseases. This group of individuals requires a higher daily intake of antioxidants compared with non-smokers to reduce the consequences of prolonged exposure to cigarette toxins.

After three weeks, supplementation with different doses of astaxanthin (5, 20, and 40 mg) in active smokers prevented oxidative damage by suppressing lipid peroxidation and stimulating the activity of the antioxidant system. This effect was confirmed by the significant reduction in serum MDA and ISP levels and the increased SOD activity and total antioxidant capacity in the three astaxanthin groups compared with the indices before treatment.

The authors also observed that the serum concentration of astaxanthin in the groups treated with 20 and 40 mg was similar, showing that there was saturation of its absorption and that smaller doses, such as 5 mg, may have the necessary antioxidant effect for these individuals. However, placebo-controlled studies with larger groups and longer interventions may help determine the optimal dosage for smokers.

Several preclinical studies have demonstrated that astaxanthin also exerts an indirect antioxidant effect by activating transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) and increasing the expression of its antioxidant target genes, such as phase II biotransformation enzymes.

A study with a model of coronary microembolization in rats revealed that supplementation with astaxanthin drastically attenuated the induction of cardiac dysfunction, myocardial infarction, and cardiomyocyte apoptosis, which was associated with the suppression of oxidative stress via activation of Nrf2/heme oxygenase-1 signaling.

Thus, astaxanthin may accumulate in the blood plasma and, through its antioxidant action, it helps reduce the levels of RONS responsible for LDL oxidation and lipid peroxidation; it increases the bioavailability of NO, enabling its vasodilator and anti-thrombogenic effects; it increases the activity of antioxidant enzymes; and it ensures the stability of blood rheological properties, thus avoiding the loss of erythrocyte flexibility and the increase in plasma viscosity, factors that affect the blood flow velocity. These actions of astaxanthin against early events of atherosclerotic plaque formation and arterial dysfunction may delay the progression of cardiovascular diseases.