Muriel Emma Bell was born on 4 January 1898 at Murchison, the daughter of Thomas Bell, a farmer, and his wife, Eliza Sheat. She began her education at Murchison School, where she was a reluctant pupil. Her childhood was shaped by family tragedy. During 1905–6 she lost two siblings and in 1907 her mother was killed and her father badly injured in a tramcar accident in Wellington. He was unable to continue farming and the family moved to Nelson, where he became the mayor of Richmond.
Thomas Bell married Jessie McNee, the postmistress at Murchison, in 1909, and she encouraged Muriel and her younger sister to further their education. Muriel attended Nelson Girls' Central School, won a scholarship and progressed to Nelson College for Girls in 1911, where she became head girl. The winner of a Junior Scholarship in 1916, she began a BA degree at Victoria University College.
Muriel Bell transferred to the University of Otago Medical School in 1917. She obtained a public health bursary in 1921, graduated MB, ChB in 1922 and in 1926 was awarded an MD by the University of Otago. Her thesis, on basal metabolism in goitre, was supervised by Professor John Malcolm. This early research work established her reputation for doing research for the public good; it contributed to the introduction of iodised salt. She was appointed assistant in physiology to Malcolm in 1922, house surgeon at Dunedin Hospital, and lecturer in physiology from 1923 to 1927, thereby becoming one of the first women academics to work at Otago Medical School.
Muriel Bell married James Saunders, a labourer who was active in Labour Party politics, on 10 January 1928 at Wellington. The Labour politician Peter Fraser was a witness at their wedding. There were no children of the marriage. She continued to use her maiden name throughout her life, having published and built up her research reputation before her marriage. In 1929 Bell was awarded the William Gibson Research Scholarship for Medical Women of the British Empire. This allowed her to conduct research on bush sickness in sheep for the newly established DSIR, which sparked her interest in soil deficiencies, and to travel to London to undertake research on vitamins with the nutritionist Professor Jack Drummond at University College from 1930 to 1932. Her husband, James, accompanied her. They remained in England, where Muriel worked as a pathologist, including one year at a hospital run by women, the Elizabeth Garrett Anderson Hospital.
In 1935 the couple returned to Dunedin so that Muriel could take up a lectureship in physiology and experimental pharmacology at Otago Medical School. When the first Labour government established the Medical Research Council in 1937, Muriel Bell became a foundation member and she served for the following two decades. She was a member and later chair of its nutrition committee and also represented women and children on the Board of Health, of which she was the sole woman member, from 1937 to 1965. The nutrition committee conducted surveys into the diets of basic wage earners in the tramway and boot and shoe industries, and into Maori diets in the late 1930s and early 1940s. It was of particular concern to Bell that the dietary survey of basic wage earners in 1939 showed that these families drank an average of 0.7 pints of milk per person a day, which she thought was not enough. This reinforced her advocacy of school milk.
James Saunders died in 1940. That year the Labour government appointed Bell to the post that made her a public figure, as the first nutrition officer in the Department of Health. From this point until her retirement in 1964 she held two posts simultaneously, director of nutrition research at the Otago Medical School and state nutritionist, and she devoted all her energy to these responsibilities. Both jobs were managed from Dunedin with periodic trips to Wellington. In Dunedin, laboratory facilities were available at the Medical School and co-operation was possible with the School of Home Science, which was headed by her friend Dr Elizabeth Gregory. Bell married Alfred Ernest Hefford, chief inspector of fisheries and director of fisheries research, on 20 June 1942. Muriel became stepmother to Hefford's seven children, six of whom were grown-up, while the youngest son was the beneficiary of her insistence on a healthy diet. Alfred Hefford died 15 years later.
For many years Muriel Bell was active in numerous campaigns to improve people's health and well-being by improving their diet. She made a vital contribution to public health, by conducting and supervising a wide range of research and making the results accessible and understandable. Appointed government nutrition officer to advise on problems arising out of the Second World War, she made recommendations concerning rationing and set the national ration scales for food items. Bell's forte was applied research into subjects of practical everyday importance, such as the vitamin content of New Zealand fruit, vegetables, fish and cereals. She provided information to the public through the Department of Health, the Plunket Society, and the press. For example, during the war she published in the New Zealand Listener a recipe for rose-hip syrup to provide vitamin C for babies and children. She wrote over 100 articles for the Listener from 1941, taking turns with H. B. Turbott, the director general of health. She also gave radio broadcasts and wrote numerous articles for trade journals, magazines and newspapers.
One of her many goals was a safe, cheap milk supply. A champion of school milk to build strong bones and teeth, she was the only woman member of the Central Milk Council, established in 1945 after an inquiry into the supply of milk to the four main centres exposed its shortcomings. For the rest of her life she worked to advertise the benefits of drinking milk, 'our best single food'. She advocated it not just as a children's food but for adults as part of a balanced diet, maintaining that it would improve their work and reduce accidents and the risk of osteoporosis. She educated the industry and the public about the effect of sunlight on milk in glass bottles, and was instrumental in requiring it to be delivered in covered trucks. As a member of the Milk Council she campaigned for pasteurised milk and the destruction of cows with tuberculosis. She experimented with culturing yoghurt and homogenised milk. With Dr Helen Deem of the Plunket Society, she updated Plunket's feeding tables for bottle-fed babies to increase the protein and reduce the fat content, recognising that high-fat infant formulas were more likely to cause digestive upsets, and devised new mixtures for babies with milk allergies.
Bell paid particular attention to dental caries. She discovered that New Zealanders' teeth had little fluorine. After a sabbatical in the United States, based at Harvard University in 1952, when she interviewed doctors about experiments with fluoridated water supplies, she returned to fight for fluoridation in New Zealand. This campaign, in particular, prompted her to describe herself as 'Battle-axe Bell' because of the struggle she had against formidable opposition headed by Dove Myer Robinson, the mayor of Auckland, in the 1950s. She won the battle locally and from 1958 was a member of the Fluoridation Committee of the Department of Health.
As part of her duties she gave lectures in home science at Otago and at the postgraduate school for nurses in Wellington. On behalf of the nutrition committee of the Medical Research Council she edited the textbook Good nutrition: principles and menus, first published in 1939 or 1940. She also wrote Lecture notes on normal nutrition, first published in 1948, to prepare nurses for their state examination. From the 1940s to the 1960s she advised the public to eat more fruit and vegetables and to cut down on sugar, fat and meat, while continuing her practical efforts to improve food quality. One of her successes was to produce better quality bread. Working with scientists, she educated bakers into using flour made by improved methods of extraction to increase its vitamin B1 content.
In the 1950s she conducted research into cholesterol and heart disease. She tried in vain to persuade insurance companies to collect statistics on obesity. Long interested in Maori and Pacific island diets, she was invited to undertake nutrition surveys in Fiji and Western Samoa in the 1950s. Such was her reputation that she prepared the food rations for the men and dogs of Edmund Hillary's trans-Antarctic expedition in 1956–57.
Muriel Bell was recognised in her lifetime, but belatedly. She became a fellow of the New Zealand Institute of Chemistry in 1941, the Royal Society of New Zealand in 1952, the Royal Society of Medicine, and the Royal Australasian College of Physicians in 1959. That year she was appointed a CBE. She was also a member of the Physiological Society of New Zealand, the Nutrition Society of New Zealand, the New Zealand Dietetic Association and the New Zealand Dental Association. She co-operated with an influential network of professional women, especially Gregory and Deem, and executives of the New Zealand Medical Women's Association and the New Zealand Federation of University Women. She was not supported by the medical establishment, who were largely ignorant about nutrition. Turbott appeared to be jealous of her. However, her public-spirited efforts were acknowledged when in 1968 she was awarded an honorary doctorate from the University of Otago: 'Dr Bell campaigned with unexampled energy to make the findings of research available for the common benefit'.
Kind, warm and caring, Bell had 'infinite patience' with people and was supportive to her staff. Always innovative, she entertained her students with her ingenuity and sense of fun. Unusual, at times bizarre in her behaviour, she startled visitors, for example, by offering them a cup of tea while she was testing rats. If the kitchen bench was full, she would work on the floor. She wore plastic sandals because she had hammer toes, and amused and perplexed friends with her unreliable cars. She died on 2 May 1974 in Dunedin, active to the end: an article on the karaka berry was found in her typewriter.