Specific dynamic action (SDA)

Specific dynamic action (SDA): the power of food to stimulate metabolic rate and induced thermogenesis.

• starts one hour after digestion of food

• reaches a maximum in 4 - 5 hours

• declines to the original level in 12 - 14 hours

Cause of SDA:

• due to metabolic process of absorbed food in the liver

  • e.g. assimilate and synthesize proteins or deaminate amino acids and urea formation

    • evidence: SDA is absent in hepatectomized animals

• Not due to digestion of absorption of food

Factors affecting SDA:

1. Amount of food

   1. more amount of food intake, more SDA will be

2. Type of food

   1. protein: a meal of pure protein 25%

   2. carbohydrate: 6%

   3. fat: 4% of total caloric value of protein

3. External temperature (environmental temperature)

   1. SDA maximum at 33°C

   2. SDA absent at 3°C

SDA represents a waste heat, except when the body is exposed to cold, as the SDA helps keep the body temperature constant

Respiratory Quotient (RQ)

Respiratory Quotient (RQ)

Respiratory Quotient (RQ): the volume of carbon dioxide released over the volume of oxygen absorbed during respiration.

Excess RQ = the volume of carbon dioxide released over the volume of oxygen absorbed during exercise - during rest

Respiratory exchange ratio: indicates the actual gas exchange at the lungs

Measurement of RQ

1. For the whole body: by analysis of respired air

2. For a certain organ: by multiplying its blood-flow per unit multiplied by the arterio-venous difference for CO2 and O2 across the organ.

Importance of RQ

1. indicates the nature of food substance oxidized

2. shows the transformation of one food substance into another

3. determines the energy equivalent of O2

Factors affecting RQ

1. factors increasing RQ above 1:

   • early phase of severe muscular exercise

   • lipogenesis

   • hyperventilation

   • metabolic acidosis

   • fever

2. factors decreasing RQ below 1:

   • recovery from muscular exercise

   • hypoventilation

   • metabolic alkalosis

   • gluconeogenesis