Going for much of the day without eating is often promoted as a way to improve health or lose weight. But a new study suggests that long gaps between meals may have different effects on older adults – especially those in the oldest age groups.
For the study, my colleagues and I followed nearly 3,000 people aged 60 and over in Sweden and found that those who usually went longest without eating developed chronic diseases faster than those who had shorter gaps between meals. The difference was clearest among people aged 78 and over.
This does not mean that fasting causes disease in old age, nor that every older person should avoid long intervals without food. But it does raise questions about whether advice based on studies of younger and middle-aged adults can be safely applied to older age groups.
Fasting helps the body switch from using readily available sugar to using stored energy, such as fat. Studies in younger adults have linked some forms of fasting to better control of blood sugar and other potentially beneficial changes in metabolism. A recent review found that intermittent fasting led to modest weight loss and some improvements in metabolic health compared with eating without restrictions.
Older bodies, however, work differently in important ways. Ageing changes how people digest food, absorb nutrients, control blood sugar and maintain muscle. Older adults are also more likely to live with several long-term health conditions and take multiple medications. All these factors could change how fasting affects health.
Our study used data from 2,981 adults aged 60 and over who took part in a long-running Swedish study of ageing. When the study began between 2001 and 2004, participants were 74 years old, on average.
Participants reported their usual times for eating meals, snacks and drinking. We calculated the longest gap between any two times they ate or drank during a typical 24-hour day. This gave us a measure of their usual fasting time, rather than whether they followed a particular fasting diet.
We did not know whether people deliberately limited the hours in which they ate or simply went for long periods without food because of their appetite, health, personal preferences or other circumstances. Our analysis, though, took several of these factors into account.
We then tracked diagnoses of 60 chronic diseases for up to 15 years. The diagnoses came from severals, medication use, and patient registers. This allowed us to examine not only how many conditions people had, but how quickly that number increased
Participants were divided into four groups, based on how long they typically went without eating each day. The shortest-fasting group went between six and 11.5 hours, while those in the longest-fasting group habitually did so for between 14 and 24 hours.
People who went 14 to 24 hours without eating developed chronic diseases faster than those with shorter fasting periods. This included cardiovascular diseases, such as heart failure and stroke, and neuropsychiatric conditions, such as depression and dementia. But it made no difference for musculoskeletal conditions like arthritis and osteoporosis.
Age appeared to matter. Among participants aged 78 and over, longer fasting was associated with faster accumulation of chronic diseases. We found no clear association among those aged 60 to 77.
Why age might change the picture
Several biological processes might explain why fasting has different effects at different ages. The digestive system can become less efficient in later life, reducing the absorption of essential nutrients such as vitamin B12, folate, calcium, and iron. Longer fasting compresses the time available for eating, potentially making it harder for older people to obtain and absorb enough nutrients.
Muscles also respond less effectively to protein with age. Eating protein regularly throughout the day provides repeated signals that help the body maintain muscle. Long gaps between meals may reduce these opportunities. Previous studies have linked muscle loss in older adults to ill health, including cardiovascular and neuropsychiatric diseases.
Medication use may also be important. Some medicines need to be taken with food to improve absorption or reduce side-effects, while others can affect blood sugar if taken far from meals. These explanations are plausible, but our study was not designed to identify the biological mechanisms behind the association of fasting with chronic conditions.
People who usually went longest without eating also tended to be older, eat fewer meals, consume less energy and protein, exercise less, and have more diseases when the study began. We took these and many other differences into account, including diet quality, smoking, sleep, education, living arrangements, social connections and care needs. The results were also similar when we repeated the analysis using different assumptions.
Even so, this was an observational study, so we cannot say that long fasting periods caused people to develop diseases faster. Poor health could instead lead people to skip meals, lose their appetite or change their eating habits. We took frailty and difficulties with everyday activities into account and still found similar results, but we cannot completely rule out the possibility that poor health caused the longer fasting periods rather than the other way around.
Participants were mainly highly educated, urban, community-dwelling older adults in Sweden. Eating patterns may differ in other countries, rural or disadvantaged communities, and among older people living in institutions. The meal-timing data were also collected more than two decades ago, and dietary habits may since have changed.
Taken together, these findings do not point to one ideal number of hours between meals. Instead, they suggest that fasting advice may need to account for age, nutritional vulnerability, medication use, and existing health problems. An eating pattern that offers some metabolic benefit earlier in adulthood may carry additional risks later in life, when getting enough nutrients, maintaining muscle and staying physically independent can become harder.
