The longer 1,027 European children had gone without food before a morning blood test, the calmer some inflammation markers looked. But only in the children of normal weight

Intermittent fasting is usually discussed as an adult habit, with eating windows and skipped breakfasts. Every child already fasts once a day, though: overnight, between the last snack of the evening and the first food of the morning. A study published in Nutrition Journal on 28 October asks whether the length of that gap shows up in children's blood, and whether a child's weight changes the answer.

The data come from the I.Family study, a European project that followed children and their families in eight countries: Belgium, Cyprus, Estonia, Germany, Hungary, Italy, Spain and Sweden. In 2013 and 2014, 1,027 children and adolescents (average age 11.4, 44% girls) gave a fasting blood sample. At the visit, the children or their parents said when they had last eaten anything with calories, and the researchers counted the time from then to the moment the blood was drawn.

Each sample was tested for seven markers of inflammation: high-sensitivity C-reactive protein (hs-CRP), the interleukins IL-6, IL-8 and IL-15, tumour necrosis factor alpha (TNF-α), interferon gamma-induced protein 10 (IP-10) and the interleukin-1 receptor antagonist (IL-1Ra), a molecule the body makes to damp down inflammation. The analysis took account of age, sex, country, physical activity, sleep, daily energy intake, fasting insulin and BMI.

In children of normal weight, a longer fast went with higher IL-1Ra (coefficient 5.34, 95% CI 0.80 to 9.87) and slightly lower IL-6 (coefficient -0.01, 95% CI -0.02 to -0.00), the combination the authors call a favourable, if subclinical, anti-inflammatory profile. IL-15 fell a little as the fast grew longer in the whole group, and again when only children who had fasted at least six hours were counted. In children with overweight or obesity there was no consistent link between fasting time and the markers.

The authors read this as a hint that excess weight may blunt the body's normal overnight switch from running on food to running on its stores, and the quiet immune adjustment that seems to come with it. They are careful to call the results exploratory. Each child gave one blood sample, so the study can't show that a longer fast caused the differences; the time of the last meal was self-reported; and the effects are small, below anything a doctor would act on. Studies that follow children over time would be needed to know whether it matters for their health later.

For families, nothing here suggests putting children on fasting plans. It is a reminder that when a child last ate can nudge blood test results, and that the overnight gap is a normal part of a child's daily rhythm. The I.Family work was funded by the European Commission's Seventh Framework Programme, and the authors declared no competing interests.