Acts as an electrogenic sodium (Na(+)) and chloride (Cl-)- dependent sodium-coupled solute transporter, including transport of monocarboxylates (short-chain fatty acids including L-lactate, D- lactate, pyruvate, acetate, propionate, valerate and butyrate), mocarboxylate drugs (nicotinate, benzoate, salicylate and 5- aminosalicylate) and ketone bodies (beta-D-hydroxybutyrate, acetoacetate and alpha-ketoisocaproate), with a Na(+):substrate stoichiometry of between 4:1 and 2:1 (PubMed:14966140, PubMed:15090606, PubMed:16729224, PubMed:16805814, PubMed:17178845, PubMed:17245649, PubMed:17526579, PubMed:20211600, PubMed:30604288). Catalyzes passive carrier mediated diffusion of iodide (PubMed:12107270). Mediates iodide transport from the thyrocyte into the colloid lumen through the apical membrane (PubMed:12107270). May be responsible for the absorption of D- lactate and monocarboxylate drugs from the intestinal tract (PubMed:17245649). Acts as a tumor suppressor, suppressing colony formation in colon cancer, prostate cancer and glioma cell lines (PubMed:12829793, PubMed:15867356, PubMed:18037591). May play a critical role in the entry of L-lactate and ketone bodies into neurons by a process driven by an electrochemical Na(+) gradient and hence contribute to the maintenance of the energy status and function of neurons (PubMed:16805814). Mediates sodium-coupled electrogenic transport of pyroglutamate (5-oxo-L-proline) (PubMed:20211600). Can mediate the transport of chloride, bromide, iodide and nitrate ions when the external concentration of sodium ions is reduced (PubMed:19864324). Part of D-serine transport systems. May contribute to D-serine reabsorption at the brush border of renal proximal tubules thus modulating the dynamics of urinary versus blood D-serine levels. {ECO:0000269|PubMed:12107270, ECO:0000269|PubMed:12829793, ECO:0000269|PubMed:14966140, ECO:0000269|PubMed:15090606, ECO:0000269|PubMed:15867356, ECO:0000269|PubMed:16729224, ECO:0000269|PubMed:16805814, ECO:0000269|PubMed:17178845, ECO:0000269|PubMed:17245649, ECO:0000269|PubMed:17526579, ECO:0000269|PubMed:18037591, ECO:0000269|PubMed:19864324, ECO:0000269|PubMed:20211600, ECO:0000269|PubMed:30604288, ECO:0000269|PubMed:38650461}. This is the function of ENSG00000256870 (SLC5A8, solute carrier family 5 member 8).