Along with oxygen and water, food is life sustaining. Food provides macronutrients including fats, proteins and carbohydrates, which are used as sources of energy, as well as building blocks for everything from healing injured muscles to producing enzymes and hormones. When you go without food for extended periods, your body quickly realizes the food deficit and responds accordingly to create hunger. One of the main areas of your body involved in and affected by this response is your brain.
Hypothalamus
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The main part of your brain that plays a central role in body reaction after not eating is the hypothalamus. The hypothalamus is located just above the brain stem and is responsible for regulating homeostasis, or balance, in your body. Food is necessary for body function, which means that when you're not consuming calories, the hypothalamus works overtime to restore balance to the body through the process of hunger.
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Recognizing Hunger
Three main areas of the hypothalamus are associated with hunger, the lateral, paraventricular and ventromedial hypothalamus. The paraventricular hypothalamus helps regulate your hunger. The ventromedial hypothalamus helps you recognize you're full after eating. Not eating mostly stimulates the lateral hypothalamus, which helps you recognize that you're hungry through a series of reactions in the body.
Ghrelin
The feeling of hunger revolves around a hormone called ghrelin. Ghrrelin is produced in both the stomach and the hypothalamus and in smaller amounts through the placenta, pituitary gland and kidneys. Since food is needed to create energy in the body, ghrelin is created when energy levels get too low from lack of food. Ghrelin then signals the energy imbalance to the brain. The brain then responds by creating the feeling of hunger to alert you to the need to eat. However, it's important to note that feeling hungry does not always mean you're actually in need of energy. Hunger may also be initiated due to a lack of water, or may be initiated due to learned patterns of eating -- such as feeling hungry at noon because you always eat lunch at noon, even if you had an over-sized breakfast.
Malnutrition and the Brain
Another way not eating can affect the brain is through malnutrition. The cells of your brain rely on energy from food just like any other cell in the body. Without the nutrition that food supplies, both your neural function and brain chemistry can be negatively impacted. This in turn affects just about every facet of your bodily function that the brain is involved in, including concentration, memory, sleep patterns, mood and your motor skills.