Acts as an electroneutral and low-affinity sodium (Na(+))-dependent sodium-coupled solute transporter. Catalyzes the transport across the plasma membrane of many monocarboxylates such as lactate, pyruvate, nicotinate, propionate, butyrate and beta-D-hydroxybutyrate. May be responsible for the first step of reabsorption of monocarboxylates from the lumen of the proximal tubule of the kidney and the small intestine. May play also a role in monocarboxylates transport in the retina. Mediates electroneutral uptake of lactate, with a stoichiometry of 2 Na(+) for each lactate. Detected at the brush border membrane of the kidney. Colocalizes with vimentin in Mueller cells. Expressed in the cortical region of the kidney corresponding to the proximal tubule. Expressed in Mueller cells of the inner retina (at protein level). Isoform 1 is expressed in the retina, kidney, small intestine and skeletal muscle. Isoform 2 is not detected in the kidney, small intestine and skeletal muscle. In the kidney, expressed predominantly in tubular epithelial cells of the cortical region and in the convoluted portions of the proximal tubule (pars convoluta). In the small intestine, its expression is highest in the proximal part and gradually decreased towards the distal end. Expressed in the neural retina. Not detected in the caecum and colon. Up-regulated by CEBPD. Belongs to the sodium:solute symporter (SSF) (TC 2.A.21) family.