Long fasting windows are often promoted as a way to improve metabolic health, but a new study suggests that the picture may look different in older adults.
In a 15-year observational study of nearly 3,000 adults age 60 and older in Sweden, people who routinely went 14 to 24 hours between eating occasions accumulated chronic diseases faster than those whose longest daily gap was six to 11.5 hours. The association was most apparent among adults age 78 and older.
The study, published in the Journal of Internal Medicine, does not show that fasting caused disease. It also did not test an intentional intermittent-fasting program. Researchers defined habitual fasting as the longest gap between any two eating occasions reported during a typical day, whether or not that gap was deliberate.
That distinction matters because long stretches without food in later life can reflect very different circumstances. Some people may intentionally restrict their eating window, while others may skip meals because of reduced appetite, illness, mobility limitations, living alone or other factors.
The researchers analyzed data from 2,981 participants in the Swedish National Study on Aging and Care in Kungsholmen. At the start of the study, participants were an average of 74 years old, and most lived in the community rather than institutional settings. Their longest daily gap between eating occasions was estimated from food-frequency questionnaires that asked about the timing of meals and snacks.
Participants were divided into four groups based on their longest fasting interval: 6 to 11.5 hours, 11.5 to 12.75 hours, 12.75 to 14 hours and 14 to 24 hours.
Compared with the shortest-gap group, people in the 14- to 24-hour group had a 0.119 higher annual rate of chronic disease accumulation after the researchers adjusted for a wide range of health, social and dietary factors. Those included age, sex, living arrangement, education, social network, smoking, physical activity, sleep, diet quality, energy intake, protein intake and reduced food consumption.
The association also differed by age.
Among adults younger than 78, the longest fasting interval was not significantly associated with faster overall disease accumulation. Among those age 78 and older, however, people in the 14- to 24-hour group had a significantly faster rate of accumulating chronic conditions than those in the shortest-gap group.
The pattern was also seen for cardiovascular and neuropsychiatric diseases in the overall sample, but not for musculoskeletal conditions. In age-stratified analyses, the strongest and most consistent pattern was for total chronic disease accumulation among adults 78 and older.
One important complication is that the people who fasted the longest already looked different at the beginning of the study.
They were older, had more chronic conditions, took more medications, ate fewer meals and were more likely to live alone. They also tended to have lower diet quality, energy intake and protein intake and were more likely to report eating less because of appetite loss or other factors.
Those differences raise the possibility of reverse causation: declining health could lead to longer gaps between meals rather than the other way around.
The researchers tried to address that concern with a series of sensitivity analyses. They repeated the models after accounting for frailty, functional limitations, breakfast skipping, meal frequency, late-night eating, medication burden, cognitive impairment and other factors. They also excluded people with high disease burden at baseline in one analysis. Across 21 sensitivity analyses, the main association between longer fasting and faster total disease accumulation remained largely consistent.
Still, observational research cannot completely rule out the possibility that unmeasured differences between participants help explain the findings.
The results also should not be interpreted as evidence that all forms of intermittent fasting are harmful in later life.
Previous studies have found that time-restricted eating and other intermittent-fasting approaches can produce modest improvements in weight, blood pressure and glucose regulation in some middle-aged and older adults. But those studies have generally been shorter and often involved planned dietary interventions rather than naturally occurring eating patterns over many years.
The authors suggest several reasons prolonged gaps between meals could potentially become less advantageous with advancing age.
Older adults can experience changes in digestion and nutrient absorption, and older muscle responds less efficiently to dietary protein. Longer fasting periods also mean fewer opportunities throughout the day to stimulate muscle protein synthesis. The researchers note that reduced metabolic flexibility and the need to take some medications with food could also make prolonged fasting more complicated in older adults.
Those explanations are plausible, but the study did not directly test them.
The findings are also most applicable to populations similar to the one studied. Participants were predominantly highly educated, urban-dwelling and community-living older adults in Sweden. Meal timing, food access and reasons for skipping meals could differ considerably in other countries and socioeconomic settings.
For now, the study adds an important wrinkle to the fasting conversation: a long eating break that may be manageable or even beneficial in some younger adults may not carry the same implications in advanced age.
Rather than suggesting that older adults should avoid fasting altogether, the findings point to a need for more age-specific research on meal timing, nutrition and the long-term accumulation of chronic disease.
Data collection for the Swedish National Study on Aging and Care in Kungsholmen was supported by the Swedish Research Council, Swedish Ministry of Health and Social Affairs and participating county councils and municipalities. Individual researchers also received support from the Swedish Research Council, Swedish Research Council for Health, Working Life and Welfare, Karolinska Institutet research programs, Alzheimerfonden and several Swedish foundations.