August 28, 2026
Intermittent fasting may carry risks after age 78
A 15-year study found that older adults with the longest gaps between meals accumulated chronic illnesses faster, particularly after age 78
ByLuis Arrais
Author
Plenty of people go 14 hours or more between dinner and breakfast without ever calling it fasting. In younger adults, trials have tied windows that long to modest weight loss and steadier blood sugar. Researchers in Stockholm followed older adults for up to 15 years and found the reverse
The people with the longest daily gap between meals accumulated chronic illnesses fastest, and the association appeared only among those 78 and older
This was not an experiment. The team followed people’s own habits, so the study can’t show that long gaps caused the illnesses. In nearly 3,000 people, longer gaps went with faster disease accumulation, and in the younger half of the group they didn’t
Luis Kalmbach and Adrián Carballo-Casla ran the analysis at the Aging Research Center, part of Karolinska Institutet‘s Department of Neurobiology, Care Sciences and Society. Neither of them was expecting this answer
“Given fasting’s strong reputation for metabolic benefits, if anything we might have expected a neutral or even positive picture among older adults too,” they told Earth.com
The longest fasters skipped breakfast
All 2,981 participants lived on Kungsholmen, an island district in central Stockholm. They joined between 2001 and 2004, at 60 or older, and the SNAC-K study has brought them back for medical exams ever since
Each of them filled in a questionnaire listing up to eight typical eating occasions in a day. The team then took the longest gap between any two of them. Eat at 8 a.m., 1 p.m. and 6 p.m., and you land at 14 hours
That sorted everyone into rough quarters, from 702 people at 6 to 11.5 hours up to 657 people at 14 to 24 hours
The two ends looked like different populations. The long-gap quarter ate about three times a day, against about five at the short-gap end. They were eight years older on average, and 10.8% skipped breakfast, compared with just 0.1% of people with the shortest gaps between meals
Age decided whether the gap mattered
Chronic illness here means a count, out of 60 disease categories, drawn from physician exams, medical records, national registry data and prescriptions. Almost everyone’s count climbed over the follow-up. The question was how fast
People at the 14-to-24-hour end added 0.119 more conditions a year than people at the 6-to-11.5-hour end. Each extra hour of gap came with another 0.019 a year. Those read like rounding errors until you stretch them out, and at that rate the long-gap quarter picks up an extra diagnosis every eight years
Participants under 78 showed none of this. Among the 1,362 people aged 78 and older, those with the longest gaps between meals accumulated chronic illnesses faster, adding about one extra condition per decade compared with those with the shortest gaps
Among the 1,619 people under 78, there was no meaningful difference between the two groups
So the habit only looks costly once people reach their late seventies. Past 78 the threshold dropped, too, and a gap of 12.75 to 14 hours, an ordinary overnight fast, was enough to matter
Joint disease showed no link
Not every kind of illness behaved the same way, and that unevenness is what the authors keep coming back to
Heart and blood vessel disease separated only at the far end, among the longest fasters, with no steady rise hour by hour. Brain and mental health conditions, a group that includes depression, dementia and Parkinson’s disease, climbed with every additional hour. For arthritis, osteoporosis and back problems, the researchers found no link at all
Asked by Earth.com what surprised them most, Kalmbach and Carballo-Casla pointed at that split rather than the headline result
“Cardiovascular disease only showed up at the very longest fasting durations, while the neuropsychiatric association scaled gradually with every additional hour, and musculoskeletal disease showed no relationship at all,” they said. “That unevenness suggests something more specific than ‘fasting is generally bad for older adults’, though we can only speculate as to why.”
Eating less didn’t explain it
The obvious objection is that long-gap participants were simply eating less, or were already sick. They were certainly different: more often living alone, less physically active, and lower on diet quality, calories and protein
So the models were adjusted for all of that, plus age, sex, education, living arrangement, social contact, care at home, smoking, sleep problems, and loss of appetite. After every one of those adjustments, the longest fasters still accumulated disease fastest
The authors then reran the analysis 21 different ways and got the same answer each time, including a version that dropped everyone who died within three years
“The association wasn’t just about participants who fasted longer also simply eating less overall. Even after adjusting for diet quality, energy intake, and protein intake, the link between fasting duration and chronic disease accumulation remained,” Kalmbach and Carballo-Casla said
Older bodies absorb less from food
Nobody knows why the same habit would help at 45 and hurt at 85. The authors offer three possible explanations
Start with absorption. Stomach acid and pancreatic enzymes both decline with age, and vitamin B12, folate, calcium and iron get harder to take up. Fit a day’s food into a shorter eating window and there are fewer meals left to make up the difference
Muscle is the second. Older muscle responds weakly to the protein in a meal, so fewer meals means fewer chances to trigger repair. Spread too little protein unevenly across a day and older adults tend to carry less muscle
Then there’s medication. Many common drugs need to be taken with food, and medication use tends to increase with age
No trial has tested this yet
Mealtimes were self-reported for a typical day, and a late-night snack counted the same as a full dinner. The authors also couldn’t separate people who fasted on purpose from people who missed meals because of illness or appetite, though adjusting for frailty didn’t change the answer
A 6-to-11.5-hour gap is normal meal spacing rather than a fasting plan, and 76.5% of this cohort ran longer than 11.5 hours anyway. Everyone here was Swedish, urban and mostly well educated
What’s missing is the experiment. Population studies have twice linked a later breakfast and a shorter eating day to worse health in older people. But the trials that made intermittent fasting popular, including tests of an eight-hour window, were mostly run in younger and middle-aged people
“This study wasn’t designed to test causation,” Kalmbach and Carballo-Casla said in an email interview with Earth.com, “and a proper test would require a randomized controlled trial specifically in older adults, which doesn’t currently exist to our knowledge.”
For now, the researchers say the findings support caution rather than firm recommendations
“Based on our findings, very long habitual fasting windows may not be advisable in this age group, and anyone in their eighties with specific health concerns should discuss their eating pattern with their physician,” they said
The full study was published in the journal Journal of Internal Medicine
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