biochemistry - How do antioxidants affect human metabolism?

Anti-oxidants affect human metabolism by altering the redox states of the cell and redox-regulated functions and signaling mechanisms.

The following quotes are from The Redox Stress Hypothesis of Aging (Free Radic Biol Med. Feb 2012)

More recently, in a major conceptual shift, ROS have been found to be
physiologically vital for signal transduction, gene regulation, and
redox regulation, among others, implying that their complete
elimination would be harmful. An alternative notion, advocated here,
termed the "redox stress hypothesis," proposes that aging-associated
functional losses are primarily caused by a progressive pro-oxidizing
shift in the redox state of the cells, which leads to the
overoxidation of redox-sensitive protein thiols and the consequent
disruption of the redox-regulated signaling mechanisms.

Many proteins contain cysteine residues which can undergo reversible modifications such as S-nitrosylation and S-glutationylation.

The redox state affects the oxidation levels of these redox-sensitive protein thiols and the consequent disruption in protein activity and the redox-regulated signaling mechanisms.

Under normal conditions the usual types of oxidation modifications of these protein thiols are reversible, and is a way to protect the thiol in transient periods of oxidative stress.

When the redox state shifts to a more oxidised point then more irreversiable protein thiol modifications occurs, i.e. damages the protein.

As shown in Figure 6, prolonged exposure of sulfenic acid, formed at
the active or regulatory sites, to H2O2, sequentially leads to the
formation of sulfinic- (−SO2H) and sulfonic-acids (−SO3H), which are
deemed to be largely irreversible reactions. In addition, protein
disulfides may undergo over-oxidation, leading to the formation of
thiosulfenate and thiosulfonate, which are also believed to be
relatively irreversible reactions. The level of oxidation of protein
cysteinyl thiolates depends upon the strength (concentration) and the
duration of exposure to H2O2.

Protein activity and enzyme catalytic efficiency which affects metabolism, such as sirtuins, are affected by the these thiol redox modifications.
(eg SIRT1 is a redox-sensitive deacetylase that is post-translationally modified by oxidants and carbonyl stress and A redox-resistant sirtuin-1 mutant protects against hepatic metabolic and oxidant stress.)

The primary arbiter of redox state is glutathione, i.e. the GSH/GSSG couple. The effect of anti-oxidants appears to vary depending on whether it affects the redox state.

The antioxidant activities of SOD and catalase neutralize the ROS species, O2− and H2O2, without having a direct impact on glutathione redox state. However this frees up glutathione from having to neutralize the ROS species, which it can do so with the glutathione peroxidase enzyme.