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Fasting's biggest benefits may require patience. A new study found that while the body starts burning fat within the first couple of days of a prolonged fast, the broader molecular changes linked to health benefits don't appear until around day three. Researchers hope the findings could eventually lead to treatments that mimic fasting's effects for those who can't safely go without food.
Fasting's biggest benefits may require patience. A study published in Nature Metabolism by researchers at Queen Mary University of London and the Norwegian School of Sports Sciences tracked 12 healthy volunteers through a seven-day water-only fast, measuring nearly 3,000 blood proteins daily. While the body predictably shifted from burning glucose to burning fat within the first two to three days, the more dramatic — and potentially health-relevant — molecular changes didn't show up until around day three.
About one-third of all proteins measured changed significantly during the fast, with effects linked to all major organs. Notably, some changes involved proteins associated with neuronal structural support in the brain — suggesting fasting's reach extends well beyond metabolism. Three days after resuming eating, fat loss largely persisted while lean mass had mostly recovered.
By the numbers:
Why it matters: The findings challenge the assumption that shorter fasting windows (like intermittent fasting) deliver the same benefits as prolonged fasts. More importantly, mapping these molecular changes could help scientists develop drugs or therapies that replicate fasting's effects — a potential breakthrough for patients who can't safely fast.
