Notes on Thermal Regulation and Body Temperature Measurement

Measurement Methods of Body Temperature

  • Oral Thermometers:

    • Problem: Breath can confound the values due to increased ventilation and ambient room temperature.

  • Rectal Thermometers:

    • Problem: Exercise increases blood flow from lower limbs to the rectum, potentially skewing temperature readings.

  • Tympanic Thermometers:

    • Problem: Increased blood flow to skin and tympanic membrane can alter temperature readings, making it less reliable.

Thermoregulation

  • Concept: Balance between heat production and heat loss.

  • Heat production occurs during exercise due to inefficiencies.

  • Non-shivering Thermogenesis: Refers to heat production without movement, primarily involving brown fat (adipose tissue).

    • Lean individuals tend to have more brown fat compared to those with obesity.

Cold Exposure and Brown Fat

  • Cold exposure leads to an increase in brown fat, as evidenced by studies where participants sat in cold conditions (17°C) for two hours a day over six weeks.

  • Results showed measurable increases in brown fat and improvements in metabolic markers like blood glucose and lipids.

Medications Affecting Brown Fat

  • Mirabegron:

    • Medication that stimulates beta-three adrenergic receptors in brown fat and the bladder, leading to weight loss and increased energy expenditure.

    • Side effects include improvements in glucose tolerance.

  • Skeletal Muscle Adaptation:

    • Mirabegron promotes switching of muscle fibers to type I, slow oxidative fibers, enhancing metabolic efficiency.

Role of Exercise in Brown Fat Activity

  • Traditionally believed that exercise could increase brown fat levels, but recent studies show no significant increase after moderate to vigorous exercise over six months.

  • Lean individuals tend to have more brown fat, but it is not influenced by exercise.

Mechanisms of Heat Exchange

  • Heat Exchange Mechanisms:

    • Conduction: Direct contact between tissues.

    • Convection: Movement of air or liquid across the skin.

    • Radiation: Absorption of heat from sources such as the sun.

    • Evaporation: Key for heat loss through sweat; evaporation is affected by humidity levels.

    • Each liter of evaporated sweat loses approximately 580 kcal of heat.

Sweating Mechanism and Performance

  • Sweating helps regulate body temperature during exercise, and athletes generally sweat more efficiently than non-athletes.

  • Sweat Glands: Two types - eccrine (sweat for cooling) and apocrine (sweat leading to body odor).

Fluid Loss and Exercise in Heat

  • Prolonged exercise leads to cardiac drift, where the heart rate increases over time, typically in hot conditions without fluid intake.

  • This is due to decreased plasma volume reducing stroke volume, requiring an increase in heart rate to maintain cardiac output.

  • Hydration strategies are critical, but simply replenishing fluids may not always aid performance due to blood flow redistribution.

Training Adaptations in Heat

  • Regular exercise in hot conditions can improve blood volume, lactate thresholds, and overall performance.

  • Exercising in heat prompts physiological adaptations that can enhance subsequent performance.

Tips for Exercising in Heat

  • Use common sense strategies such as hydrating with slushies or wearing chilled garments.

  • Acclimatization by gradual exposure to heat is effective for improving performance; adaptations may last for weeks after cessation of heat training.

Summary of Heat Regulation Strategies

  • Evaporation through sweating is the main mechanism to cool the body.

  • Increased heart rate due to decreased plasma volume affects cardiovascular performance.

  • Use cooling strategies to enhance performance during hot conditions, while balancing the need for acclimatization for future heat exposures.