Nobel Prize in Physiology or Medicine · 1953
Hans Krebs
Driven out of Nazi Germany, he mapped the chemical cycle that turns food into energy in almost every oxygen-breathing cell.
The Nobel citation: “for his discovery of the citric acid cycle”
- Born
- August 25, 1900, Hildesheim, Germany
- Died
- November 22, 1981, Oxford, United Kingdom
- Shared with
- Fritz Lipmann
- Affiliation at the time
- Sheffield University, United Kingdom
Medicine prize
1953
Shared with 1 other laureate.
Age that year
53years
Born in 1900.
Sources cited
19
Fact-checked September 24, 2026.
- Nature turned down his 1937 paper announcing the citric acid cycle because of a backlog of letters, so he sent a longer version to the Dutch journal Enzymologia.
- A July 1933 warning of his final dismissal from the University of Freiburg was signed by its rector, the philosopher Martin Heidegger.
- When he left Germany in 1933 he took 16 wooden boxes of lab apparatus with him, and within a month he was running experiments again in Cambridge.
- In 1934 Cambridge gave him a junior teaching post, making him the first German refugee scientist in England to get a regular university job.
- In 2015 his Nobel medal sold for £225,000, and the money went to a new trust that supports refugee scientists and student researchers.
The breakthrough
The citric acid (Krebs) cycle: how cells burn food for energy (1937)
Every cell that uses oxygen gets its energy by slowly burning food. By the mid-1930s biochemists had found scattered pieces of this process. Albert Szent-Györgyi showed that small amounts of certain four-carbon acids kept minced pigeon breast muscle breathing, acting like helpers rather than fuel, and Carl Martius and Franz Knoop worked out the first steps in how citric acid is broken down. Nobody could say how the pieces fitted together. In Sheffield in 1936 and 1937, Krebs and his student William Arthur Johnson found that muscle builds citric acid from oxaloacetic acid plus a product of sugar breakdown, and that a pinch of citric acid made the tissue take up far more oxygen than burning that citric acid could explain. So citric acid was being remade again and again. Krebs joined the reactions into a loop. A two-carbon piece of food joins a four-carbon molecule to make citric acid. Step by step, the cell strips off hydrogen to feed its energy machinery, releases two carbons as carbon dioxide and rebuilds the four-carbon molecule for the next turn. It works like a water wheel whose buckets keep coming round, so a little of each acid can process a great deal of fuel. Work by Fritz Lipmann, Feodor Lynen and others later identified the two-carbon piece as acetyl coenzyme A, and many laboratories showed that fats and proteins feed the same wheel. In animals and other higher organisms, Krebs estimated, the cycle releases about two-thirds of the energy in food.[1],[3],[4],[5],[8]
“It is indeed remarkable that all foodstuffs are burnt through a common terminal pathway.”
What it meant for humanity
The citric acid cycle gave biology its central map of energy. Before Krebs, the Nobel presenter said, many separate reactions were known but no one could link them. His cycle, filled out by many other laboratories, showed how carbohydrates, fats and proteins are all burned in one shared pathway, and how the same cycle supplies raw materials for building amino acids and other parts of the cell. It has been found in living things from bacteria to mammals, and researchers still describe it as the hub of cell metabolism. That map now guides medicine. Some cancers are driven by faulty cycle enzymes. Broken fumarate hydratase or succinate dehydrogenase lets tumour-promoting molecules pile up, and mutant IDH enzymes turn a cycle molecule into a cancer-promoting one. In 2024 the US Food and Drug Administration approved vorasidenib, the first targeted drug for grade 2 gliomas, a kind of brain tumour, that carry IDH mutations. Krebs's earlier urea cycle explains how the liver gets rid of toxic ammonia. Inherited defects in it affect roughly 1 in 8,000 to 1 in 44,000 babies, and about half of those with the severe newborn form die. Doctors treat these disorders with low-protein diets, drugs that clear nitrogen by other routes, and arginine or citrulline, two molecules in the cycle Krebs described. The salt solutions his lab designed to match body fluids became standard in physiology and pharmacology labs. In wartime Sheffield he also led nutrition studies on volunteers. By his own account, tests of the National Wheatmeal loaf and calcium absorption helped shape government food policy, and his vitamin C study produced an estimate of the minimum daily dose that prevents scurvy, a figure that still feeds into international advice.
- The Nobel presenter credited Krebs with finding, among a jumble of known reactions, the main pathway by which cells burn food, and said his work had laid a lasting foundation.[4]
- In his Nobel lecture Krebs estimated that in animals and other higher organisms, about two-thirds of the energy released from food is set free in the citric acid cycle.[3]
- Mutant IDH enzymes turn a cycle molecule, alpha-ketoglutarate, into one that promotes cancer; in 2024 the FDA approved vorasidenib, the first targeted drug for grade 2 gliomas with these mutations.[16],[18]
- European guidelines describe urea cycle disorders as defects of the Krebs-Henseleit cycle, affecting about 1 in 8,000 to 1 in 44,000 births.[17]
- His wartime Sheffield experiment estimated the minimum vitamin C intake that prevents scurvy; the World Health Organization's recommendation is partly based on it.[7],[9]
- The £225,000 raised by selling his Nobel medal in 2015 went to the new Sir Hans Krebs Trust, which supports refugee scientists and student researchers, echoing the help he received in 1933.[12],[19]
Impact in numbers
Krebs's impact is foundational rather than countable. The citric acid cycle is the core of how cells get energy from food, and the urea cycle explains how the body disposes of ammonia. Later work on inherited metabolic diseases, cancer metabolism and nutrition builds on these maps, but many other scientists turned them into tests and treatments. The nearest countable outcomes, such as care for urea cycle disorders or drugs aimed at faulty IDH enzymes in cancer, sit at the end of long causal chains with many contributors and no published totals that we could find, so any number credited to Krebs would be guesswork. His wartime vitamin C study still shapes official intake advice, but we found no published count of scurvy cases it prevented. We therefore record no quantified claims, for benefits or for harms. The Krebs family's sale of his Nobel medal in 2015 now funds refugee scientists, a small echo of the rescue that made his own career possible.
Fundamental scienceHealthFood
No number is given here on purpose. Some contributions cannot be counted honestly, and we would rather describe them than invent a figure.
The double edge
No harm is documented from the citric acid cycle itself. The controversy concerns how some of Krebs's wartime nutrition research was done. From 1944 he directed a Medical Research Council experiment at Sheffield's Sorby Research Institute in which about 20 volunteers, most of them conscientious objectors who took part instead of military service, were given zero, 10 or 70 mg of vitamin C a day for an average of nine months. Researchers made experimental wounds to measure healing, and two volunteers developed life-threatening heart problems. None was permanently harmed, and several later said they would volunteer again. Still, a dental researcher who re-analysed the data in 2021 called the study shocking and said it would not be approved today. His re-analysis also argued that the original finding, that 10 mg a day wards off scurvy, was judged by eye, and suggested that advice built on it, such as the WHO's 45 mg a day, is too low.
- Moderate
Vitamin C deprivation experiment on conscientious objectors
Krebs directed a Medical Research Council study, begun in 1944, in which about 20 volunteers, mostly conscientious objectors, were kept on zero, 10 or 70 mg of vitamin C a day for an average of nine months and given experimental wounds. Two developed life-threatening heart problems. None was permanently harmed and all had volunteered, but a 2021 re-analysis called the study shocking by today's standards.[6],[9],[10]
- Minor
Vitamin C advice that may be set too low
The Sheffield team concluded that 10 mg of vitamin C a day was enough to prevent scurvy, and the WHO's 45 mg recommendation is partly based on the study. A 2021 statistical re-analysis argued that about 95 mg a day is needed to prevent weak scar strength in 97.5% of people, so advice built on the original reading may be too low.[9]
Against the odds
In 1933 Germany had about 505,000 Jews, less than 1% of its people. Krebs's father, a patriotic surgeon, raised his sons outside Judaism because he saw assimilation as the answer. It made no difference. On 7 April 1933 the Nazi government passed its first major law aimed at Jews, which reached into state universities and hospitals. Within days the University of Freiburg sent Krebs on compulsory leave and told him his post would end, just after his urea cycle had made his name; a July warning of final dismissal carried the signature of the rector, Martin Heidegger. Krebs wrote to Frederick Gowland Hopkins in Cambridge, who wanted him but had no money to pay him. A Rockefeller Foundation fellowship filled the gap. He left in June 1933 and restarted his experiments within a month, but he had no assurance he could stay: he received an unconditional residence permit only in late 1935, and British citizenship in 1939. His family scattered. In 1936 his sister emigrated to Palestine and his brother came to England, where he was interned for a time during the war. Krebs kept an anti-refugee leaflet that circulated in Sheffield in 1938. Even so, he found England largely free of the class, race and religious prejudice he had known in Germany.
1933
Dismissal
Under the April 1933 civil service law, the first major Nazi law aimed at Germany's roughly 505,000 Jews, Freiburg put him on compulsory leave and ended his appointment. A warning of final termination dated 15 July 1933 was signed by the rector, Martin Heidegger.[2],[5],[6],[14],[15]
1933
Exile
He left for Cambridge in June 1933 on a one-year Rockefeller fellowship, bringing 16 wooden boxes of lab apparatus, with no assurance he could stay in Britain. In May 1934 he became a Cambridge demonstrator, the first German refugee scientist in England to get a regular academic post. An unconditional residence permit came only in October 1935.[5],[6],[8]
1936
Exile
His family scattered: his sister Elisabeth and her husband emigrated to Palestine in 1936, and his brother Wolfgang came to England the same year and was interned for a time during the war.[6]
1938
Discrimination
An anti-refugee leaflet was circulated in Sheffield, where he worked, in 1938; Krebs kept a copy among his papers.[6]
1940
War
In June 1940 he submitted a letter to the Manchester Guardian about the internment and restrictions imposed on aliens officially classed as victims of Nazi oppression. His own brother was interned for a time during the war.[6]
Jewish background
Both of Krebs's parents came from Jewish families. His father Georg was a Hildesheim ear, nose and throat surgeon; his mother Alma Davidson was a banker's daughter from a family long settled in the area. The family had no religious ties, and Georg, who believed assimilation was the answer for German Jews, raised his children outside the faith. The Nazis dismissed Krebs as a Jew anyway in 1933. In exile he explored a post in Palestine and visited in 1936, the year his sister settled there. He later held an honorary degree from Jerusalem, and in 2013 the Association of Jewish Refugees chose him for its first plaque.[2],[5],[6],[7],[13]
Key dates
August 25, 1900
Born in Hildesheim, Germany, to Georg Krebs, an ear, nose and throat surgeon, and Alma Krebs, née Davidson.[1],[2]
December 1918
After a brief call-up into the German army at the end of World War I, begins medical studies at the University of Göttingen.[5],[8]
1926
Becomes research assistant to Otto Warburg at the Kaiser Wilhelm Institute for Biology in Berlin-Dahlem, where he learns the methods he will later use.[2],[3],[5]
1932
With medical student Kurt Henseleit at Freiburg, publishes the urea (ornithine) cycle, showing how the liver makes urea from ammonia.[5],[11]
April 1933
Dismissed from the University of Freiburg under the Nazi civil service law because he is Jewish.[2],[5],[6]
June 1933
Emigrates to England to work in Frederick Gowland Hopkins's biochemistry laboratory at Cambridge on a Rockefeller Foundation fellowship.[2],[5]
October 1935
Moves to the University of Sheffield as lecturer in pharmacology; in 1938 he becomes head of its new Department of Biochemistry.[2],[5]
June 1937
With William Arthur Johnson, proposes the citric acid cycle; Nature returns the paper, citing a backlog, so he publishes a longer version in Enzymologia.[5],[6]
1938
Marries Margaret Cicely Fieldhouse; they have three children, Paul, Helen and John.[2],[5]
September 1939
Becomes a naturalised British citizen, days after World War II begins.[5],[6]
1944
Directs a wartime vitamin C deprivation experiment on volunteers, mostly conscientious objectors, at Sheffield's Sorby Research Institute.[9],[10]
1953
Shares the Nobel Prize in Physiology or Medicine with Fritz Lipmann; his half honours the discovery of the citric acid cycle. He also wins a Lasker Award.[1],[11]
1954
Becomes Whitley Professor of Biochemistry at Oxford, bringing his Medical Research Council unit; he is knighted in 1958.[2]
November 22, 1981
Dies in Oxford at 81, having kept up his research until a few months before his death.[1],[5]
Sources
- 1.Hans Krebs - Facts · NobelPrize.org (Nobel Prize Outreach)
- 2.Hans Krebs - Biographical · NobelPrize.org (from Nobel Lectures, Physiology or Medicine 1942-1962, Elsevier, 1964), 1964
- 3.The citric acid cycle (Nobel Lecture, 11 December 1953) · NobelPrize.org, 1953
- 4.Award ceremony speech, Nobel Prize in Physiology or Medicine 1953 (Professor E. Hammarsten) · NobelPrize.org, 1953
- 5.Krebs, Hans Adolf (Complete Dictionary of Scientific Biography entry by Frederic L. Holmes; World of Microbiology and Immunology entry) · Encyclopedia.com
- 6.Catalogue of the papers and correspondence of Sir Hans Adolf Krebs, FRS (1900-1981), CSAC 113/4/86, compiled by Jeannine Alton and Peter Harper · Centre for Scientific Archives (originally Contemporary Scientific Archives Centre; papers held at the University of Sheffield Library), 1986
- 7.Sir Hans Adolf Krebs (Inspiring Physicians) · Royal College of Physicians Museum
- 8.A lucky man (Elizabeth Willcocks) · Chemistry World (Royal Society of Chemistry), 2003
- 9.New analysis of landmark scurvy study leads to update on vitamin C needs · UW News, University of Washington, 2021
- 10.Medical experiments carried out in Sheffield on conscientious objectors to military service during the 1939-45 war (John Pemberton), International Journal of Epidemiology 35(3):556-558 · Oxford University Press, 2006
- 11.Krebs Cycle for converting food into energy (1953 Albert Lasker Basic Medical Research Award) · Lasker Foundation
- 12.Krebs Nobel Auctioned (Amanda B. Keener) · The Scientist, 2015
- 13.AJR Blue Plaques · Association of Jewish Refugees
- 14.Law for the Restoration of the Professional Civil Service · United States Holocaust Memorial Museum, Holocaust Encyclopedia
- 15.Germany: Jewish Population in 1933 · United States Holocaust Memorial Museum, Holocaust Encyclopedia
- 16.Mitochondrial TCA cycle metabolites control physiology and disease (Martínez-Reyes and Chandel), Nature Communications 11:102 · Nature Portfolio, 2020
- 17.Suggested guidelines for the diagnosis and management of urea cycle disorders (Häberle et al.), Orphanet Journal of Rare Diseases 7:32 · BioMed Central (via PubMed Central), 2012
- 18.FDA Approval Summary: Vorasidenib for IDH-mutant Grade 2 Astrocytoma or Oligodendroglioma following surgery (Barbato et al.), Clinical Cancer Research 31(21):4412-4418 · American Association for Cancer Research (via PubMed Central), 2025
- 19.Sheffield academic's Nobel Prize medal sells for £225,000 · BBC News, 2015
Fact-checked on September 24, 2026 by a separate AI fact-checking pass that re-opened the sources, with 12 corrections made. How we check
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