Luis Federico Leloir ForMemRS (September 6, 1906 – December 2, 1987) was an Argentine physician and biochemist who received the 1970 Nobel Prize in Chemistry for his discovery of the metabolic pathways by which carbohydrates are synthesized and converted into energy in the body. Although born in France, Leloir received the majority of his education at the University of Buenos Aires and was director of the private research group Fundación Instituto Campomar until his death in 1987. His research into sugar nucleotides, carbohydrate metabolism, and renal hypertension garnered international attention and led to significant progress in understanding, diagnosing and treating the congenital disease galactosemia. Leloir is buried in La Recoleta Cemetery, Buenos Aires.

Leloir's parents, Federico Augusto Rufino and Hortencia Aguirre de Leloir, traveled from Buenos Aires to Paris in the middle of 1906 with the intention of treating Federico's illness. However, Federico died in late August, and a week later Luis was born in an old house at 81 Víctor Hugo Road in Paris, a few blocks away from the Arc de Triomphe. After returning to Argentina in 1908, Leloir lived together with his eight siblings on their family's extensive property El Tuyú that his grandparents had purchased after their immigration from the Basque Country of northern Spain: El Tuyú comprises 400 km² of sandy land along the coastline from San Clemente del Tuyú to Mar de Ajó which has since become a popular tourist attraction.

During his childhood, the future Nobel Prize winner found himself observing natural phenomena with particular interest; his schoolwork and readings highlighted the connections between the natural sciences and biology. His education was divided between Escuela General San Martín (primary school), Colegio Lacordaire (secondary school), and for a few months at Beaumont College in England. His grades were unspectacular, and his first stint in college ended quickly when he abandoned his architectural studies that he had begun in Paris' École Polytechnique.

It was during the 1920s that Leloir invented salsa golf (golf sauce). After being served prawns with the usual sauce during lunch with a group of friends at the Ocean Club in Mar del Plata, Leloir came up with a peculiar combination of ketchup and mayonnaise to spice up his meal. With the financial difficulties that later plagued Leloir's laboratories and research, he would joke, "If I had patented that sauce, we'd have a lot more money for research right now."

After returning again to Argentina, Leloir obtained his Argentine citizenship and joined the Department of Medicine at the University of Buenos Aires in hopes of receiving his doctorate. However, he got off to a rocky start, requiring four attempts to pass his anatomy exam. He finally received his diploma in 1932 and began his residency in the Hospital de Clínicas and his medical internship in Ramos Mejía hospital. After some initial conflicts with colleagues and complications in his method of treating patients, Leloir decided to dedicate himself to research in the laboratory, claiming that "we could do little for our patients... antibiotics, psychoactive drugs, and all the new therapeutic agents were unknown [at the time]."

In 1933, he met Bernardo Houssay, who pointed Leloir towards investigating in his doctoral thesis the suprarenal glands and carbohydrate metabolism. Houssay happened to be friends with Carlos Bonorino Udaondo, the brother-in-law of Victoria Ocampo, one of Leloir's cousins. Following the recommendation of Udaondo, Leloir began working with Houssay, who in 1947 would later win the Nobel Prize for Physiology or Medicine. The two would develop a close relationship, collaborating on various projects until Houssay's death in 1971; in his lecture after winning the Nobel Prize, Leloir claimed that his "whole research career has been influenced by one person, Prof. Bernardo A. Houssay".

After only two years, Leloir received recognition from the medical department at the University of Buenos Aires for having produced the best doctoral thesis. Feeling that his knowledge in fields such as physics, mathematics, chemistry, and biology is lacking, he continued attending classes at the university as a part-time student. In 1936 he traveled to England to begin advanced studies at the University of Cambridge, under the supervision of another Nobel Prize winner, Sir Frederick Gowland Hopkins, who had obtained that distinction in 1929 for his work in physiology and in revealing the critical role of vitamins in maintaining good health. Leloir's research in the Biochemical Laboratory of Cambridge centered around enzymes, more specifically the effects of cyanide and pyrophosphate on succinic dehydrogenase; from this moment Leloir began to specialize in researching carbohydrate metabolism.

Leloir returned to Buenos Aires in 1937 after his brief stay at Cambridge. 1943 saw Leloir marry; Luis Leloir and Amelia Zuberbuhler (1920-2013) would later have a daughter also named Amelia. However, his return to Argentina was amidst conflict and strife; Houssay had been expelled from the University of Buenos Aires for signing a public petition opposing the Nazi regime in Germany and the military government led by Pedro Pablo Ramírez. Leloir fled to the United States, where he assumed the position of associate professor in the Department of Pharmacology at Washington University School of Medicine in St. Louis, collaborating with Carl Cori and Gerty Cori and thereafter worked with David E. Green at the Columbia University College of Physicians and Surgeons as a research assistant. Leloir would later credit Green with instilling within him the initiative to establish his own research in Argentina.