8.3 Hunger and Eating

LEARNING OBJECTIVES

  • Describe how hunger and eating are regulated in the human body.

Understanding Hunger

Hunger and eating behaviors are regulated through several physiological mechanisms in the human body. These mechanisms are initiated primarily through the following processes:

Physiological Mechanisms of Hunger

  1. Stomach Contractions

    • When the stomach is empty, it begins to contract, leading to sensations known as hunger pangs.
    • These contractions stimulate the secretion of chemical signals that travel to the brain, indicating the need to initiate feeding behavior.

  2. Blood Glucose Levels

    • A drop in blood glucose levels triggers the pancreas and liver to produce various chemical signals.
    • These signals act to induce hunger, prompting the initiation of feeding behavior.
    • References: (Konturek et al., 2003; Novin, Robinson, Culbreth, & Tordoff, 1985)

Regulation of Satiation

Once food is consumed, the feeling of satiation, which refers to the state of fullness and satisfaction, typically suppresses further eating behavior. Several physiological mechanisms regulate this satiation:

  1. Blood Glucose Levels

    • As blood glucose levels rise after eating, the pancreas and liver issue signals aimed at shutting off hunger and stopping eating.
    • References: (Drazen & Woods, 2003; Druce, Small, & Bloom, 2004; Greary, 1990)

  2. Gastrointestinal Signals

    • The passage of food through the gastrointestinal tract provides critical satiety signals to the brain.
    • Reference: (Woods, 2004)

  3. Hormonal Influence

    • Leptin, released by fat cells, acts as a satiety hormone that plays a significant role in regulating hunger and eating behavior.

Brain Integration of Hunger and Satiety Signals

The integration of various hunger and satiety signals occurs in specific areas of the brain, primarily within the hypothalamus and hindbrain. Research suggests these regions are crucial for regulating eating behavior:

  • Reference: (Ahima & Antwi, 2008; Woods & D'Alessio, 2008)
  • The brain's activity ultimately dictates whether an individual will engage in feeding behavior.

Hunger and Eating Regulation
Figure 1. Hunger and eating are regulated by a complex interplay of hunger and satiety signals that are integrated in the brain.

Metabolism and Body Weight

Several factors influence body weight, including:

  1. Gene-Environment Interactions
  2. Caloric Balance
    • The relationship between the number of calories consumed and the number of calories expended in daily activities defines body weight regulation.
    • If caloric intake exceeds caloric expenditure, the body stores the excess energy as fat. Conversely, consuming fewer calories than burned leads to the conversion of stored fat into energy.

Activity Levels and Metabolic Rate

  • Energy Expenditure
    • A person’s energy expenditure is affected by their activity levels, but it is also dependent on their metabolic rate.
    • Metabolic Rate
      • Defined as the rate of energy expended over a certain period of time.
      • There exists significant individual variability in metabolic rates; those with higher metabolic rates can more easily burn calories than those with lower rates.

Weight Fluctuations and Set-Point Theory

While fluctuations in body weight are normal, most individuals maintain their weight within a narrow range unless there are extreme changes in diet or physical activity.

  • This phenomena has led to the proposal of the set-point theory of body weight regulation, which posits that:
    • Each individual has a genetically predetermined ideal body weight, known as their set point.
    • Significant deviations from this set point are resisted by compensatory changes in energy intake and/or expenditure.
    • Reference: (Speakman et al., 2011)

Body Mass Index (BMI) and Health

The Body Mass Index (BMI) is a common metric used to assess health and classify body weight categories, including:

  • Underweight: BMI < 18.5
  • Normal weight: BMI 18.5 - 24.9
  • Overweight: BMI 25 - 29.9
  • Obese: BMI ≥ 30
  • Morbid Obesity: BMI > 40
  • According to the Centers for Disease Control and Prevention (CDC), individuals with a BMI within the overweight classification (25-29.9) are at higher risk for various health complications. An adult with a BMI of 30 or higher is considered obese.

Criticism of BMI

While BMI has been utilized by the World Health Organization (WHO), CDC, and others as an indicator of healthy body weight, its limitations are noted:

  • BMI does not differentiate between fat and muscle mass, potentially misclassifying athletes with high muscle mass.

Health Consequences of Obesity

Obesity is associated with several negative health outcomes, such as:

  • Increased risk for cardiovascular disease
    e.g. strokes
  • Type 2 diabetes
  • Liver disease
  • Sleep apnea
  • Certain cancers (e.g. colon, breast)
  • Infertility
  • Arthritis

Given that approximately one-third of the adult U.S. population is classified as obese, alongside nearly two-thirds of adults and one in six children qualifying as overweight (CDC 2012), there is significant public health interest in understanding and combating obesity as a widespread health crisis.

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