Mackay specialised in
paediatrics. She was the first female physician at the Queen Elizabeth Hospital for Children and was later one of first women to be appointed as consultant there. In 1919, Mackay moved to
Vienna where she studied
rickets and other
nutritional diseases at the
Lister Institute of Preventative Medicine as part of the Beit Research Fellowship. She remained in Vienna until 1922 when she continued her research in the United Kingdom. Mackay was a part of a British team led by
Harriette Chick whose studies displayed the importance of cod liver oil and sunlight in preventing and potentially curing rickets. While working on finding the cause of rickets, Mackay noticed all of the infants in her studies were anaemic, and thus she became interested in finding the cause of this. Mackay returned to London and conducted the first investigation on anaemia in infants with the help of medical statistician
Major Greenwood. Mackay discovered that iron deficiency had an important role in causing childhood anaemia. She further discovered that breastfed infants had a lower chance of becoming anaemic than those fed with artificial milk. Her research also provided compelling evidence that showed infants who were given iron developed fewer infections, gained more weight, and were overall healthier. The findings of her studies were summarised in 'Nutritional Anaemia in Infancy' and published in 1931. She was the first person to try to define anaemia by defining the lower limit of normal haemoglobin concentration. The
World Health Organization's definition of anaemia closely resembles Mackay's definition. Mackay was awarded the
Dawson Memorial Prize in Paediatrics and won the
British Medical Association Ernest Hart Memorial Research Scholar fellowship for her preventative medicine research. She became the first woman fellow of the Royal College of Physicians in recognition of her research into
nutritional anaemia in 1934. Mackay continued to work on research focusing on
breastfeeding, formula feeding, and dietary deficiency diseases. ==Later life and death==