Muriel Bell was born in Murchison, daughter of Thomas Bell later Mayor of Richmond. She was twice married – to James Saunders in 1928, and to Alfred Ernest Hefford in 1942 – but throughout her working life was known by her maiden name. She joined Professor Malcolm as an assistant in physiology in 1922, returning in 1923, after a period as a house physician and surgeon in Dunedin Hospital to be lecturer in physiology until 1927. In this period, she introduced the measurement of basal metabolic rate as a diagnostic test into New Zealand and in 1926, was awarded her MD degree for her thesis on basal metabolism in goitre.

In 1929, the Royal Society of Medicine awarded her for three years the William Gibson Research Scholarship for Medical Women of the British Empire. She worked at University College, London, with Professor Sir Jack Drummond, whose inspiration and enthusiasm helped to set her on a course as a nutritional scientist for the remainder of her life. She extended her overseas experience as assistant pathologist at the Elizabeth Garrett Anderson Hospital in London (1932 to 1933), and at the Sunderland Royal Infirmary in 1934, before returning to New Zealand to be lecturer in physiology and experimental pharmacology from 1935 to 1940.

During the tenure of this academic appointment, Dr Bell was already reaching out into aspects of public health. She became a member of the Board of Health, and the Central Milk Board in 1937. She was also a foundation member of what was to be the Medical Research Council of New Zealand when it was set up in 1937, as the Medical Research Committee of the Board of Health. The old Medical Research Council had 21 full meetings before it was replaced by a newly constituted council in 1951, and Dr Bell attended every meeting. She was not a member of the new council but was appointed at its first meeting to the nutrition research committee (as chairman), and to the dental research committee. Meanwhile in 1940, she was appointed nutritionist to the Department of Health, a post she held, together with the directorship of the Medical Research Council's nutrition research unit in the medical school, until her retirement when the unit was disbanded in 1964. Dr Bell faithfully preserved the original chair of physiology which Dr Malcolm purchased to sit upon in 1905, and gave over to her safekeeping on his retirement in 1944. She handed it back in 1961, and suitably labelled, it is still in the department.

The main part of Dr Bell's dedicated work for the nutritional welfare of the people of New Zealand occupied the period following 1940, and with the band of devoted helpers she trained to work with her, ranged widely over the field of fundamental and applied nutrition. She had done earlier work on bush sickness in sheep and was among the first to realise the importance of mineral trace elements for agriculture in this country, recognising the deficiency of cobalt in pastures at Morton Mains in Southland. Her research unit collaborated with workers from the soil bureau and the dental unit in work, which showed the importance of fluoride as a protective agent against dental caries, as well as with the Wheat Research Institute on methods of wheat extraction and with the fats research laboratory of DSIR on many aspects of nutrition, with special relevance to cardiovascular disease. She directed work on the nutrients contained in foods commonly eaten in New Zealand and the Pacific area. She gave good advice to the Government when difficult decisions had to be taken about rationing during World War II, and promoted the introduction of rosehip syrup. Though human nutrition was her primary concern, she covered the whole field of nutrition, and had an extraordinary facility for finding relevant information among its voluminous and contradictory literature. One result of her expertise was the great success of the diets she devised for the dogs as well as the men on Sir Edmund Hillary's trans-Antarctic expedition.

Dr Bell was as concerned with the application as with the acquisition of nutritional knowledge, giving advice and instruction formally and informally to doctors, nurses and dietitians, to mothers through the Plunket Society, and to countless individuals who called on her personally or by letter or telephone. She had a natural curiosity and warm interest in people, and a physician's concern for their welfare. One of her most striking characteristics was the way she would respond to an enquiry or any human problem that came her way. No matter how busy she was, she would drop everything to spend often many hours on the immediate new problem, and then work all hours of the day and night attempting to catch up. She had no use for humbug or ostentation, but her warm heart could never turn away a genuine enquirer.

The award of the honorary degree of DSc by the University of Otago in 1968, was a fitting recognition of a life devoted to the pursuit of science for the benefit of humanity. The University's public orator in presenting her remarked: 'All who have been brought up in this country or have brought up their children here... are indebted to Dr Bell's persistent and inquiring interest in the welfare of women and children during her long active career... If you seek for her monument, you have only to look about you.