Does astaxanthin slow ageing?
Astaxanthin demonstrably improves skin moisture levels and elasticity. The studies are small and were carried out largely in one population group, which says something about how broadly the findings apply.
Astaxanthin, a red pigment from microalgae, shows its clearest results for the skin. People who take it regularly can expect a noticeably better moisture level and improved skin elasticity compared with those taking a dummy pill. The effect on wrinkle depth, however, has not been demonstrated: several controlled studies found no meaningful difference there.
For UV protection the signals are positive but less convincing. Astaxanthin appears to limit sun-related skin damage, but the studies are small and were carried out mainly in healthy Japanese women, making it unclear whether the findings apply to everyone. In addition, most of those studies were funded (in part) by commercial parties, which may colour the picture. Astaxanthin is in any case no substitute for sunscreen.
If you want to try it for your skin, capsules containing astaxanthin from the microalga Haematococcus pluvialis are available at a dose of 4 mg per day; allow at least 9 to 10 weeks before expecting any effect. That applies whether your goal is better skin texture or extra protection against sun damage.
Astaxanthin also looks reasonably promising for blood markers related to damage from free radicals: it lowers blood levels of malondialdehyde, a breakdown product that forms when cells are damaged by free radicals. This effect was most pronounced in people with type 2 diabetes. On commonly used markers of how much internal irritation and swelling the body is producing, the supplement had no measurable effect. One specific substance the body makes when it is inflamed did fall, but only in people with type 2 diabetes.
The effect on mental performance, such as memory, has not been studied enough to draw any conclusions. One study saw some improvement in verbal memory in middle-aged adults, but that alone is not enough to justify a recommendation.
This answer draws on several overviews and combined analyses of previously published studies. The findings on skin moisture and elasticity come from controlled studies with a dummy pill, and the direction of effect is consistent across them. The authors of the skin studies themselves point out that the studies are too small and too short to support firm conclusions. The researchers also explicitly note that most studies were carried out in one specific population and that commercial interests were difficult to correct for. For free-radical damage markers the results are somewhat stronger, but they too apply mainly to people with a specific condition. Research into mental performance is still very early stage.