While young people can eat everything in sight, older people often lose appetite. It's important to know why to stay healthy at any age.

"Our body tells us more or less what to eat and when to eat, and maybe we should listen," said Susanne Klaus, a chief researcher at the German Institution of Human Nutrition.

That makes sense. But the way our body demands energy of us isn't the same throughout our lifetimes.

From being spoon-fed by parents when we're infants to sneaking sugary sweets when we're children, to eating everything in sight as teenagers and then pecking on small, simple plates in later stages of life, our appetites change as the years roll on. Understanding why can help ensure good health in our later years.

The hunger hormones and how they work

At its core, eating food is functional: Without the energy we draw from it, we simply wouldn't survive.

Carbohydrates in our food are converted into energy while fats and amino acids help create the vital proteins and other structures that help the body function.

To ensure these processes run like clockwork, the body has special systems to ensure a regular energy supply.

"These [are self-regulating] mechanisms that drive hunger and satiety," said Klaus. "They're mainly signals from the stomach and intestine but also from hormones, such as leptin, which is secreted by adipose tissue [or body fat] and which signals to the main hypothalamus [a control center in the brain]. This is an autonomic system, like breathing."

These chemical drivers compelling us to seek (or stop seeking) food are sometimes called hunger hormones. Aside from leptin, ghrelin is perhaps the best-known hunger hormone.

Ghrelin is released into the bloodstream by the stomach and tells the brain to get us eating. When you're full, ghrelin release slows, giving a sensation of fullness.

Other hormones regulate feelings of fullness and emptiness, too. These include insulin and other pancreatic hormones that inhibit hunger, such as GLP-1, which the diabetes drug, Ozempic, mimics.

The mechanics of digestion

When these hormones prompt you to stick food in your mouth, the body uses digestive processes to, quite literally, eat away at your meal.

Mechanical digestion begins in your mouth, where you grind your food into smaller, mushier forms you can swallow. This process continues as this swallowed slurry is forced down the esophagus into your stomach — a process known as peristalsis.

Alongside this process is chemical digestion. This begins in the mouth where amylase enzymes in saliva begin to break down starches in food. More of these digestive enzymes are in the stomach to finish the job so water and nutrients can then be absorbed from the intestines into the bloodstream.

How your appetite changes as you age

This impulse for food hits overdrive when you reach adolescence. The body craves energy to fuel its most important growth stage — puberty — spurring it toward physical and sexual maturity.

But lifelong nutrition can be a challenge. For older people, there is a risk of the body becoming less effective at prompting the necessary intake of nutrients. Some studies have shown changes to hunger hormone secretion patterns in later life.

"When people age, on average, [they] lose muscle mass and muscle is the compartment which uses the most energy," said Klaus.

A major driver of muscle mass reduction is a failure to consume sufficient protein.

"Protein intake in later life is lower than what is recommended and even the recommendations, according to various scientific groups, should actually be higher for protein intake in later life, because it is very important to maintain muscle mass," said Daniel Crabtree, a late life-stage nutrition researcher at the University of Aberdeen.

Despite the advice, Crabtree said older people's protein intake does tend to be below what's recommended, and that can include physiological factors and other signs of an ageing body — from problems with your teeth to changes in taste or smell.

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