Odors affect what we eat. That is so obvious that we seldom give it any thought. Food aromas send signals to the brain that prepare us for digestion, secreting saliva and gastric juices in preparation for digestion. When we lose the sense of smell, food loses much of its taste and appeal, and we eat less.
A recent study looks deeper than that, however. It turns out, that in mice at least, odors also affect how much energy is burned by brown fat, the tissue that keeps the mouse (and humans) warm by burning calories to create heat.
The researchers damaged the neurons in the mouse brain that are responsible for processing olfactory sensations (smells). As you might expect, the mice ate less. But they also burned more calories in their brown fat than the control mice did. Moreover, they were resistant to diet-induced obesity, and when mice made obese by diet were given the same procedure, they lost fat mass and regained insulin sensitivity.
In another set of mice, they made changes that enhanced the sense of smell. These mice gained weight, even when fed the same amount of food as before the treatment, and the same amount of food as controls.
So the effect of food odors is not limited to changes in food intake. It directly affects the rate at which calories are burned to keep the mouse warm.
The effects of odor don’t stop at weight loss. In worms and flies, disabling olfaction leads to longer lifespans. As we have seen in other studies, metabolism and lifespan are inter-related. The smell of food (in flies and worms) decreases lifespan, but only when they are calorie restricted. So perhaps when using intermittent fasting to prolong your life, you might want to avoid the smell of food. Of course, doing so makes fasting easier, so maybe you are already avoiding Cinnabon on your fast days.
Food deprivation activates cells in the brain known as agouti-related protein neurons. These hunger neurons activate food intake, but also energy expenditure and metabolism. The smell of food can inhibit these neurons in seconds if the mouse is hungry.
Of course, mice are not people. People have a third as many odor receptors as mice, and humans lack the organ that allows mice to sense pheromone odors. But measuring the brown and beige fat activity of fasting and non-fasting humans to odors might not be a difficult experiment to try.