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Older people, no matter how well they eat, have more of a fullness hormone in their blood than young people—but that doesn’t explain why some older people eat less.
Descriptive
When people eat a small snack before lunch, most people eat less at lunch—but undernourished older adults don’t cut back, even when they’ve already eaten something.
Even though undernourished older women have more of the hunger hormone in their blood, they still don’t eat more—so high hunger signals don’t always make people eat.
Older women who aren't eating enough feel less hungry than younger women, even though their bodies are making more of the hunger hormone, which might explain why they eat less as they age.
After eating the same meal, older people’s bodies release more insulin than younger people’s, which could mean their metabolism works differently as they age.
When people have more of the fullness hormones (CCK and insulin) after eating, they feel less hungry — this is true whether they’re young or old.
Correlational
When young people eat, their body releases more of the fullness hormone than older people do, which might help them feel satisfied faster.
Frail older people have less of the hunger hormone in their blood even before they eat, which might make them less interested in food.
Older people don’t feel hungry after eating because their body doesn’t release the right hunger hormone the way younger people do.
Mice given GLP-1 drugs tend to avoid nicotine, suggesting these drugs might reduce the appeal of nicotine.
When mice get both nicotine and liraglutide, several parts of their brain light up with activity, suggesting the two drugs work together in the brain.
When mice are given liraglutide, the usual dopamine spike caused by nicotine is reduced in the brain’s reward center.
A drug that activates GLP-1 receptors makes certain brain cells in mice more active — specifically ones that control appetite and reward.
Giving mice both nicotine and a diabetes drug called liraglutide together helps them lose weight by making them eat less and burn more calories.
Even small changes in how much protein is in a meal — not huge amounts — can help people feel fuller and eat less, whether they’re lean or obese.
Fat doesn’t make obese men feel as full or eat less as it does in lean men, meaning their bodies respond differently to fatty foods.
Meals with enough or lots of protein make two key hunger hormones (CCK and ghrelin) stay active longer in both lean and obese men, which might help them feel full longer.
Obese men feel less hungry after eating high-protein meals and eat less afterward when they eat meals with more protein, no matter if it's high-protein or just enough protein.
When lean men eat meals with more fat or protein instead of lots of carbs, they feel less hungry, fuller longer, and end up eating less food later.
Whether a rat is hungry or just ate, and what it’s been eating before, changes the levels of two key brain chemicals in its reward center.
If a rat has been eating lots of sugary treats, and then is given regular food after being hungry, its brain’s serotonin activity goes down—no matter if it was stressed or not.
When rats are stressed and eat sugary food, their brain’s chemical balance changes differently when they’re hungry versus when they eat again—especially in the area linked to pleasure and reward.
When stressed rats eat sugary food after being hungry, their brain's serotonin activity goes up—but if they’ve been eating sugary food for a while and then eat normal food, serotonin activity drops.
When rats eat lots of sugary, fatty food for a long time, their brain's pleasure center becomes more active with dopamine, which might make them keep eating even when they don't need to.