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The science behind post-meal sluggishness
Overindulging in a lavish holiday meal triggers a complex chain of signals between the gut and brain, leaving many feeling drowsy afterward. Experts explain why this happens-and how the body copes with sudden caloric excess.
The satiety cascade
When we eat, hormones released by the digestive tract and metabolites from food breakdown communicate with the brain to signal fullness. This process, known as the satiety cascade, also prompts the pancreas to release insulin, regulating blood sugar levels.
Tony Goldstone, a clinical associate professor at Imperial College London and consultant endocrinologist, notes that these signals originate from different parts of the gut and operate over varying timeframes. However, their exact role in post-meal drowsiness-often called a food coma-remains unclear.
Aaron Hengist, a postdoctoral fellow at the U.S. National Institutes of Health, adds that while blood flow to the brain doesn't decrease after eating, the hormonal response may still contribute to fatigue. "The gut hormone response is a cocktail; we don't yet know which specific hormones affect which brain regions," he says.
Short-term overeating: surprisingly resilient
Occasional overindulgence appears to have minimal immediate metabolic consequences, according to a 2020 study led by Hengist. Fourteen healthy men consumed pizza until comfortably full in one session and until painfully full in another-doubling their intake in the latter. Despite the excess, their blood sugar and fat levels remained stable, thanks to heightened insulin and gut hormone secretion.
"We were surprised that, despite double the energy intake, the body regulated blood sugar remarkably well."
Aaron Hengist, National Institutes of Health
However, Hengist cautions that the findings, limited to young, healthy men, may not apply to broader populations, including women or individuals with obesity.
Prolonged feasting: strain on the system
A 2021 study, dubbed the Tailgate Study, revealed starkly different results. Eighteen overweight but healthy men consumed 5,087 calories-mostly high-fat, high-sugar foods and alcohol-over five hours. Liver scans afterward showed increased fat accumulation, a precursor to metabolic dysregulation.
Hengist warns that such prolonged overconsumption strains the body, potentially reducing oxygen to the brain and triggering inflammation linked to long-term cognitive risks. "Passively consuming food and alcohol for hours overwhelms the body's regulatory systems," he explains.
Evolutionary clues: why we crave excess
Goldstone suggests that humans evolved to endure occasional overeating as a survival mechanism. Hunger drives us to seek calorie-dense foods, and once found, the brain's appetite circuitry quiets-even before eating begins. "Hunger is an aversive state; we eat to escape it," he says, noting that much of this process occurs subconsciously.
Yet modern feasts are a recent phenomenon. Historically, excess was rare, and its effects-like obesity-develop gradually, unlike starvation, which is immediately life-threatening.
Brain changes: short-term feasts, lasting impact
Stephanie Kullmann, a metabolic neuroimaging expert at the University of Tübingen, found that even brief periods of overeating can alter brain function. In a 2021 study, 18 healthy men added 1,200 extra calories of ultra-processed snacks to their daily diet for five days. Though their weight remained stable, brain scans revealed insulin resistance in regions governing appetite and memory-changes typically seen in long-term obesity.
"The brain changes before the body. Participants' brain responses resembled those of someone who had been overweight for years."
Stephanie Kullmann, University of Tübingen
One week after returning to normal diets, participants' cognitive and memory functions remained impaired, suggesting that short-term indulgence may have lingering effects.
The bottom line: moderation matters
While a single holiday meal is unlikely to harm the brain, repeated overindulgence-even over days-can strain metabolism and disrupt neural pathways. Kullmann's research underscores that the brain is particularly vulnerable to dietary excess, even before physical changes like weight gain occur.
Hengist offers a reassuring takeaway: "Enjoy your Christmas dinner. A one-off indulgence isn't as damaging as you might fear." But he advises caution for those planning multi-day feasts, as the cumulative effects may extend beyond the plate.
