Why do you feel less hungry when it's hot out?

In the sweaty, sticky, dog days of summer, you may notice that you're not as hungry as you are in cooler weather. But what's the connection between temperature and appetite? Why do we feel less hungry when it's hot out?

The influence of temperature on appetite has long been observed by scientists. "What we know is, people in colder environments eat more calories," Allison Childress, a registered dietitian and associate professor at Texas Tech University, told Live Science.

There's a basic biological reason for this. Calories are a unit of energy; burning them off can release heat, helping people maintain their body heat in colder climates. But as the winter gives way to warmer weather, "people notice that they are a lot less hungry" — a trend Childress has seen in both her clinical practice and in the broader scientific literature.

The mechanisms behind this phenomenon, however, are unclear. Many factors influence caloric intake, Childress said.

Matt Carter, a neuroscientist at Williams College in Massachusetts, agreed. Many variables — including hormones, proteins and environmental factors — affect how and why we feel hungry and, ultimately, why that feeling diminishes on hotter days, Carter said.

Related: What's the hottest temperature the human body can endure?

Our bodies are always trying to keep internal conditions stable. This is called homeostasis. It's why we sweat in the scorching sun or drink water after a strenuous workout. Hunger is also homeostatic; we feel hungry when our body is low on calories and feel full after eating a meal, keeping our inner physiological state balanced.

Many homeostatic processes are maintained by hormones, which serve as chemical messengers in the body. With appetite and fullness, two hormones play a large role: ghrelin, which is released by the stomach when it is empty, and leptin, which is secreted by fat cells and tells the brain when the body is full.

To influence our feelings and behavior, these hormones then signal the hypothalamus, a part of the brain that works to regulate aspects like body temperature, hunger and thirst. At the bottom of the hypothalamus lies a mass of specialized neurons that "are orchestrating a sense of hunger and fullness," Carter said. There, ghrelin stimulates the neurons associated with hunger, called AgRP neurons, making you feel hungry. Leptin, in contrast, inhibits these neurons and stimulates POMC neurons, which make you feel full.

But how temperature influences this intricate system is "still an open area of investigation," Carter said. The brain has sensors for temperature — proteins that change shape once your body hits a certain level of warmth. A study published in the journal eLife in 2020 found that in mice, certain brain cells send information to the AgRP neurons when temperatures are cold, which increases the feeling of hunger.

On the other hand, when it's hot out, POMC neurons have a heat-sensing protein that is activated when body temperature increases, which then activates the neurons associated with fullness, according to a 2018 study in the journal PLOS Biology.

"But there's probably more to it than that," Carter said; other brain circuits likely also work together to influence how much we eat.

Childress also noted that there are likely other factors. "While we have these biological mechanisms in place and the heat and the cold, it's also important to know that we can kind of out-eat those biological mechanisms," she said. We sometimes lose that ability to listen to our body cues — for example, we eat past the point of fullness or don't eat when we're hungry.

Regardless of cues, Childress noted that in the summer, it's important to stay hydrated, whether that's eating water-rich foods such as vegetables and fruits or by drinking fluids. Counterintuitively, frozen treats can actually increase body temperature because they are often high in calories.

Hot or not, appetite is an intricate balance — a way that our bodies sync with the environment. "Eating and drinking are things that seem like they just happen," Carter said. "But really, behind the scenes, the brain is precisely measuring the need for calories, for water, for an optimal body temperature. And I think that's amazing."