Hunger and Eating

Key Learning Goals

• Summarize evidence of the physiological factors implicated in the regulation of hunger.

• Explain how food availability, situational factors, culture, and learning influence hunger.

• Describe evidence of the prevalence, health consequences, and roots of obesity.

The Motivation of Hunger and Eating

• Hunger is a complex motivational system, not a simple one.

Biological Factors in the Regulation of Hunger

• Early theories linked stomach contractions to hunger, based on observations.

• Walter Cannon and A. L. Washburn (1912) verified the association between stomach contractions and hunger.

• Cannon theorized that stomach contractions cause hunger.

• This theory was discredited because people still experience hunger after stomach removal (Wangensteen & Carlson, 1931).

• Current theories focus on:

  • The role of the brain

  • Digestive factors

  • Hormones

Brain Regulation

• Research with lab animals suggested that hunger is controlled in the brain, specifically the hypothalamus.

• The hypothalamus regulates biological needs related to survival.

• Lateral Hypothalamus (LH):

  • Destroying the LH in rats led to a decrease or loss of interest in eating

• Ventromedial Nucleus of the Hypothalamus (VMH):

  • Destroying the VMH in rats led to excessive eating and rapid weight gain - ability to recognise satiety (fullness) had been neutralised

• Early conclusion: LH and VMH were the brain's on-off switches for hunger (1940s and 1950s)

• The dual-centres model of hunger was later undermined .

• Current thinking: LH and VMH are elements in the neural circuitry that regulates hunger, but are not key on-off centers.

• Arcuate Nucleus and Paraventricular Nucleus:

  • These areas are believed to play a larger role in modulating hunger.

  • The arcuate nucleus is considered especially important.

• Contemporary theories focus on neural circuits that pass through areas of the hypothalamus, not anatomical centres.

• These circuits depend on neurotransmitters and are more complicated than previously thought.

• The neural circuits regulating hunger are interconnected with extensive parallel processing.

• This complex circuitry is sensitive to a diverse range of physiological processes.

Digestive and Hormonal Regulation

• The digestive system has mechanisms that influence hunger.

• Walter Cannon's hypothesis that the stomach regulates hunger was partially correct.

• The stomach sends signals to the brain to inhibit further eating after food consumption (Woods & Stricker, 2013).

• Vagus Nerve: Carries information about the stretching of stomach walls, indicating fullness.

• Other nerves carry satiety messages based on the nutrient content of the stomach.

• Hormones contribute to the regulation of hunger.

• Ghrelin: Secreted by the stomach when the body goes without food, causing stomach contractions and promoting hunger.

• CCK: Released by the upper intestine after food consumption, delivering satiety signals to the brain and reducing hunger.

• Leptin: Contributes to long-term regulation of hunger and other bodily functions.

  • Produced by fat cells and released into the bloodstream.

  • Provides the hypothalamus with information about the body's fat stores.

  • High leptin levels diminish hunger; low leptin levels promote increased hunger.

• Insulin: Secreted by the pancreas and sensitive to fluctuations in body fat stores.

• Hormonal signals (insulin, ghrelin, CCK, and leptin) converge in the hypothalamus, especially the arcuate and paraventricular nuclei..

Environmental Factors in the Regulation of Hunger

• Eating is influenced by social and environmental factors.

Food Availability and Related Cues

• Physiological regulation of hunger assumes homeostatic mechanisms are at work.

• Incentive Value: Humans are motivated to eat by the anticipated pleasure of eating, not just energy deficits.

• Environmental variables influence food consumption:

  • Palatability: The better food tastes, the more people consume.

  • Quantity Available: People consume what is put in front of them

    • Larger plates and dinnerware increase consumption.

  • Variety: Increased variety leads to increased consumption

    • Sensory-Specific Satiety: Incentive value of a specific food declines as you eat it.

    • Buffets lead to overeating due to the availability of many foods.

  • According to incentive models, availability and palatability are key factors regulating hunger.

  • Presence of Others: Individuals eat 44\% more when eating with others.

    • People use each other as guides and eat similar amounts.

    • Women may reduce intake in the presence of an unfamiliar opposite-sex person.

Eating and Weight: The Roots of Obesity

• Obesity is assessed by Body Mass Index (BMI): weight (in kilograms) divided by height (in metres) squared (kg/m^2).

• BMI of 25.0-29.9 is overweight, and over 30 is obese.

• About 31\% of adults are obese and 58\% are overweight in South Africa (Chooi et al., 2019).

• The obesity epidemic is a global problem (Lobstein, 2017).

Evolutionary Perspective

• Humans evolved to consume more food than necessary due to competition for limited resources.

• Excess calories were stored as fat for future shortages.

• Modern societies have abundant, reliable supplies of palatable food, leading to chronic overconsumption.

Health Consequences of Obesity

• Obesity increases mortality risk.

• Associated with coronary disease, stroke, hypertension, diabetes, respiratory problems, gallbladder disease, arthritis, muscle and skeletal pain, sleep apnea, and some cancers.

• May promote inflammatory and metabolic changes contributing to Alzheimer's disease.

Factors Contributing to Obesity

Genetic Predisposition

• Obesity is partly hereditary.

• Identical twins reared apart are more similar in BMI than fraternal twins reared together (Stunkard et al., 1990).

• Genetic factors account for 61\% of weight variation in men and 73\% in women (Allison et al., 1994).

Excessive Eating and Inadequate Exercise

• Overweight people eat too much in relation to their exercise level.

• Modern society provides tasty, high-calorie foods everywhere.

• Marketing efforts increase consumption of unhealthy foods.

• Highly processed, high-fat, high-sugar foods may be addictive.

• Modern societies create an 'obesogenic' environment.

• Decline in physical activity due to modern conveniences.

• Overeating contributes more to obesity than lack of exercise.

• Food industry downplays overconsumption and frames the issue as balancing consumption with exercise.

• About one-half of the public believes exercise is more influential than diet in determining weight.

• People who think exercise is more influential are more likely to be overweight.

The Concept of Set Point

• People tend to regain weight after losing it, suggesting homeostatic mechanisms defend against weight loss.

• Metabolic and neuroendocrine processes resist weight loss.

• Energy expenditure declines after weight loss.

• Reduced fat stores result in reduced leptin levels, fueling increased hunger.

• The body is also wired to resist weight gain.

• Adaptive mechanisms maintain a stable body weight, suggesting a set point for weight.

• The set point is an individual's natural point of stability for weight.

Food Availability in South Africa

• South Africans experience two extremes: poverty leading to underfeeding and rising obesity rates.

• Many individuals lack access to healthier food options.

• Affordable and easily available food is often unhealthy.

• Many South Africans cannot afford a healthy diet.

• People may choose unhealthy food due to palatability, variety, and convenience.

• The 'sugar tax' is an attempt to reduce intake of unhealthy foods.

Learned Preferences and Habits

• Food preferences are acquired through learning.

• People prefer familiar foods.

• Geographical, cultural, religious, and ethnic factors limit exposure to certain foods.

• Repeated exposure to a new food increases liking.

• Forcing a child to eat a specific food can backfire.

• Taste preferences are learned associations formed through classical conditioning.

• Youngsters can be conditioned to prefer flavors paired with pleasant events.

• Taste aversions can be acquired through conditioning.

• Eating habits are shaped by observational learning.

Set Point and Weight

• The set point concept suggests the body has a natural weight range.

• A key question is why obesity has increased if the body defends a weight range.

• The body defends against weight loss more strongly than weight gain.

• This is likely because in ancestral times, defending against weight loss was more crucial for survival due to food scarcity.