50

key concepts

50.1

Food Provides Energy As Well As Materials for Biosynthesis

50.2

Diverse Adaptations Support Ingestion and Digestion of Food

50.3

The Vertebrate Gastrointestinal System Is a Disassembly Line

50.4

Nutrient Availability Is Controlled and Regulated

Thrifty Phenotypes

For thousands of years the Pima of southwestern North America were hunters and gatherers, supplementing their diet with subsistence agriculture. Their environment was arid, so they developed sophisticated irrigation systems; even so, they frequently encountered drought and subsequent starvation. Today most individuals of the ethnic Pima population in North America are clinically obese. As a population they are one of the heaviest in the world.

Obesity contributes to health problems such as diabetes, high blood pressure, and heart disease. Diabetes incidence in the Pima is seven times the national average. Two-thirds of Pima adults over the age of 40 are diabetic, and diabetes is occurring in younger individuals than previously seen. What has caused such a radical health change in the population? Two interacting factors are involved: genetics and lifestyle.

Geneticists hypothesize that recurring episodes of starvation produce strong selective pressure for “thrifty genes”—particular alleles of the genes involved in digestion, absorption, and energy storage that result in greater-than-average efficiency in converting food into fat. Thrifty genes would carry a strong selective advantage when food is scarce. The Pima display a “thrifty” phenotype. They have low resting metabolic rates and convert food into fat readily. Insulin facilitates conversion of dietary sugar into fat. For many Pima, consuming a standard amount of glucose causes their insulin levels to rise three times higher than it does in people of European ancestry.

Modern lifestyle also plays a role in the Pima obesity epidemic. Instead of eating their traditional diet, the Pima now eat a high-calorie, high-fat Western diet, and they engage in less physical activity than their ancestors did. A population of Pima living in the Sierra Madre of northern Mexico are genetically the same as the Arizona population. However, they eat traditional foods and live a traditional lifestyle that involves much physical activity. Obesity and diabetes are not prevalent among the Mexico Pima.

A high-calorie diet and sedentary lifestyle affect not just the Pima but contribute to the overall increase in obesity throughout the U.S. population. Recently an additional factor that might contribute to obesity and diabetes has been discovered—the type of food that is eaten. The composition of the diet influences the populations of gut microbes, and those microbes influence outcomes of digestion. Apparently calories are not all equal; their impact depends on their origin.

If the nutritional values were the same, could different foods have different impacts on health?