A week on a wrist tracker told researchers who was frailer: 1,203 older villagers with jumbled days and poorer diets had more of the signs

Frailty is the slow slide in strength, speed and stamina that makes falls, hospital stays and loss of independence more likely in later life. It can also go backwards, which is why researchers keep looking for everyday habits linked to it. Diet is one of the usual suspects. A cross-sectional study published in Nutrients on 23 October looked at a second one alongside it: the shape of a person's day.

The data came from the baseline survey of a healthy-ageing cohort in rural Shandong Province, China. Of 1,906 people aged 65 and over who were surveyed, 1,203 had complete data, with an average age of 72.5 and 56% women. Each wore a research-grade accelerometer on the non-dominant wrist for seven days and nights. From the recordings the team scored three things: how alike one day was to the next, how often activity and rest broke up within a day, and how sharply the busiest ten hours stood out from the quietest five. Diet came from a food frequency questionnaire scored against China's national dietary guidelines on a 0 to 100 scale. Frailty was measured with the standard five-part check: grip strength, walking speed over 4.57 metres, exhaustion, low physical activity and unplanned weight loss.

Most people were not frail: 54.7% were robust, 42.5% pre-frail (one or two of the five signs) and 2.8%, or 34 people, frail. A statistical sorting of the rhythm scores gave five groups. One in five had a robust rhythm: regular days, few breaks, a clear day and night. Half had a typical rhythm. About one in five had fragmented, flattened days. Two small groups, around 5% each, had either very irregular days alone or the lot: irregular, broken up and flat, labelled severely disrupted.

After allowing for age, sex, education, living alone, BMI, smoking, chronic illness, calorie intake and the tracker's own sleep estimates, the severely disrupted group scored about half a point higher on the five-point frailty scale than the robust group, with roughly two and a half times the odds of being in a more frail category. The fragmented, flattened group had about 50% higher odds, though that result was borderline and did not survive a correction for multiple comparisons. Irregular days on their own showed no link. Diet mattered separately: each 10 points higher on the diet score went with 29% lower odds of being in a more frail category. The robust group averaged 64 points, the frail 57.

Putting the two together, compared with people who had both a robust rhythm and an above-median diet, those with a typical rhythm and a below-median diet had about twice the odds of more frailty, and the fragmented, flattened group with a poorer diet had 2.2 times the odds. The severely disrupted groups had the highest estimates whether their diet was better or worse, though with only 61 people split two ways the ranges were wide. Formal tests found no sign that diet changed the rhythm link; the two looked like separate pieces of the same picture.

The limits are real. This is one snapshot, so it cannot tell whether a jumbled day leads to frailty or frailty breaks up the day; someone who tires easily will rest more often. Diet rested on a year's recall, the cohort came from one rural province, and the people left out for missing data differed in sex and smoking from those kept. With only 34 frail people, the estimates for small groups are rough. A wrist tracker sees activity and stillness, not the body clock itself. One of the seven authors works for a health management company; the work was funded by the National Natural Science Foundation of China.