Maintaining a healthy diet rich in plant-based foods, with low to moderate intake of healthy animal-based foods and lower intake of ultra-processed foods, was linked to a higher likelihood of healthy ageing -- defined as reaching age 70 free of major chronic diseases and with cognitive, physical, and mental health maintained, according to a new study by researchers at Harvard T.H. Chan School of Public Health, University of Copenhagen, and University of Montreal. The study is among the first to examine multiple dietary patterns in midlife in relation to overall healthy ageing.

"Studies have previously investigated dietary patterns in the context of specific diseases or how long people live. Ours takes a multifaceted view, asking, how does diet impact people's ability to live independently and enjoy a good quality of life as they age?" said co-corresponding author Frank Hu, Fredrick J. Stare Professor of Nutrition and Epidemiology and chair of the Department of Nutrition at Harvard Chan School.

The study will be published March 24 in Nature Medicine.

The researchers used data from the Nurses' Health Study and the Health Professionals Follow-Up Study to examine the midlife diets and eventual health outcomes of more than 105,000 women and men ages 39-69 over the course of 30 years. Participants regularly completed dietary questionnaires, which the researchers scored on how well participants adhered to eight healthy dietary patterns: the Alternative Healthy Eating Index (AHEI), the Alternative Mediterranean Index (aMED), the Dietary Approaches to Stop Hypertension (DASH), the Mediterranean-DASH Intervention for Neurodegenerative Delay (MIND), the healthful plant-based diet (hPDI), the Planetary Health Diet Index (PHDI), the empirically inflammatory dietary pattern (EDIP), and the empirical dietary index for hyperinsulinemia (EDIH). Each of these diets emphasises high intake of fruits, vegetables, whole grains, unsaturated fats, nuts, and legumes, and some also include low to moderate intake of healthy animal-based foods such as fish and certain dairy products. The researchers also assessed participants' intake of ultra-processed foods, which are industrially manufactured, often contain artificial ingredients, added sugars, sodium, and unhealthy fats.

The study found that 9,771 participants -- 9.3% of the study population -- aged healthfully. Adhering to any one of the healthy dietary patterns was linked to overall healthy ageing and its individual domains, including cognitive, physical, and mental health.

The leading healthy diet was the AHEI, which was developed to prevent chronic diseases. Participants in the highest quintile of the AHEI score had an 86% greater likelihood of healthy ageing at 70 years and a 2.2-fold higher likelihood of healthy ageing at 75 years compared to those in the lowest quintile of the AHEI score. The AHEI diet reflects a diet rich in fruits, vegetables, whole grains, nuts, legumes, and healthy fats and low in red and processed meats, sugar-sweetened beverages, sodium, and refined grains. Another leading diet for healthy ageing was the PHDI, which considers both human and environmental health by emphasising plant-based foods and minimising animal-based foods.

Higher intake of ultra-processed foods, especially processed meat and sugary and diet beverages, was associated with lower chances of healthy ageing.

"Since staying active and independent is a priority for both individuals and public health, research on healthy ageing is essential," said co-corresponding author Marta Guasch-Ferré, associate professor in the Department of Public Health at the University of Copenhagen and adjunct associate professor of nutrition at Harvard Chan School. "Our findings suggest that dietary patterns rich in plant-based foods, with moderate inclusion of healthy animal-based foods, may promote overall healthy ageing and help shape future dietary guidelines."

"Our findings also show that there is no one-size-fits-all diet. Healthy diets can be adapted to fit individual needs and preferences," added lead author Anne-Julie Tessier, assistant professor in the Department of Nutrition at the University of Montreal, researcher at the Montreal Heart Institute, and visiting scientist at Harvard Chan School.

The study had some limitations, notably that the study population was composed exclusively of health professionals. The researchers noted that replicating the study among populations with diverse socioeconomic statuses and ancestries would offer further insights into the findings' generalisability.