Ragnar Berg (September 1, 1873 – March 31, 1956) was a Swedish-born biochemist and nutritionist who worked most of his adult life in Germany. He is best known for his theories on the importance of acid-base balance and inorganic minerals like calcium in the diet; later in life he endorsed vegetarianism and ways to prolong the human life span. He promoted an alkaline rich diet and also invented the alkaline dietary supplement Basica, which Volkmar Klopfer manufactured and marketed from 1925.

Ragnar Berg was the son of the respected Swedish historian and archaeologist Wilhelm Berg (1839-1915) and his first wife, Ulrika Charlotta Emerentia "Emy" Gumaelius (1846-1902). He married Ella Buscher in 1902, and they had two sons, Gunnar Wilhelm Emil (1907-1974) and Alf Ragnar Wilhelm (1912-1994).

Berg was recruited by Karl Lingner to the Dresden Center for Dental Hygiene (Zentralstelle für Zahnhygiene) in 1902, where he met dentist Carl Röse (1864–1947), his long-time experimental partner. From 1909 to 1921 Berg headed the physiology lab at the homeopathic sanatorium founded by Heinrich Lahmann at Weisser Hirsch near Dresden, researching vitamins, trace elements and the metabolism of minerals. A fire damaged the laboratory at the end of December 1914.

In 1921, he was dismissed from Lahmann's Sanatorium, since business had dried up during World War One, and its new directors wanted to focus on the more lucrative fields of psychoanalysis and gynecology. Historians have noted the dismissal occurred because the "directors did not value his scientific approach to nutrition." Berg continued conducting experiments on himself and analyzing foodstuffs from a home laboratory. From 1927 to 1932, he headed his own nutrition department at the Dresden-Friedrichstadt Hospital. In 1934, he became head of the nutrition department at the Rudolf Hess Hospital in Dresden. However, his funding ran out two years later. Only during the 1940s was he able to get federal funds for his "war-related" work. Berg belonged to the Nazi Party's Main Office for People's Health.

In March 1945, Berg and his wife, Ella, fled bombed-out Dresden for Berlin and then to Stockholm, Sweden. (Neither their house nor his lab in the hospital had been damaged, however.) They lived in his native Sweden until her death from a heart attack at the end of 1954. Berg was very lonely, his health deteriorated, and he spent many months in the hospital before moving to his son's home north of Hamburg, where he died a few months later of old age and metastatic prostate cancer. He was nearly blind by this time.

Berg's most notable work was his book Vitamins: A Critical Survey of the Theory of Accessory Food Factors translated from the German by Cedar and Eden Paul, in 1923. The book has been cited as one of the earliest on vitamin research in Europe. It has a huge bibliography of 1500 entries.

A 1923 review in The British Medical Journal praised Berg for documenting scientific knowledge of vitamins and summarizing data from a huge mass of original literature but noted that some of his ideas about inorganic metabolism "have not met with general acceptance". In contrast, nutritionist Katharine H. Coward negatively reviewed the book, stating that "Berg is blinded by his own ideas and work on the importance of the inorganic salt content of a diet... Altogether it is a confused, and, in many cases, an inaccurate, account of the subject."

Although the term "vitamins" was included in the title of the book, Berg did not like this term. Instead, he used the term "complettins" which was criticized for confusing readers. Berg also coined the term "acomplettinoses" for vitamin deficiencies A, B and C.