Specific dynamic Action (SDA), explain it

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I now have comprehensive information from multiple authoritative textbooks. Here is a complete explanation:

Specific Dynamic Action (SDA)

Definition and Synonyms

Specific Dynamic Action (SDA) is the increase in metabolic rate (heat production) that occurs after ingestion of food. The term is now largely replaced by its modern equivalents:

Diet-Induced Thermogenesis (DIT) - preferred biochemistry term
Thermic Effect of Food (TEF) - preferred nutrition/physiology term

These all describe the same phenomenon. As stated in Basic Medical Biochemistry (6e): "DIT was formerly called the specific dynamic action (SDA)."

What Causes SDA?

After eating, the metabolic rate rises because energy is consumed by several processes:

Secretion of digestive enzymes - energy cost of synthesis and secretion
Active transport of absorbed nutrients across the intestinal epithelium
Digestion and absorption of macronutrients
Synthesis of metabolic reserves - building glycogen, triacylglycerols, and proteins from absorbed nutrients (this is the major part of the cost)
Distribution and storage of nutrients throughout the body

Per Harper's Illustrated Biochemistry (32e): "A small part of this is the energy cost of secreting digestive enzymes and of active transport of the products of digestion; the major part is the energy cost of synthesizing reserves of glycogen, triacylglycerol, and protein."

Magnitude of SDA by Macronutrient

This is the most exam-important aspect:

Macronutrient	Rise in Metabolic Rate	Duration
Protein	~30% above normal	3-12 hours
Carbohydrate	~4% above normal	Shorter
Fat	~4% above normal	Shorter
Mixed diet (typical)	~8-10% of total daily intake	-

"After a meal high in protein, the metabolic rate usually begins rising within an hour, reaching a maximum of about 30% above normal, and this rate lasts for 3 to 12 hours. This effect of protein on the metabolic rate is called the specific dynamic action of protein." - Guyton & Hall Medical Physiology
Proteins have the highest thermic effect because amino acid catabolism and protein synthesis (including urea cycle activity) are energetically expensive processes.

Why Does Protein Have the Highest SDA?

Several biochemical reasons:

Deamination and urea synthesis are ATP-consuming processes
Gluconeogenesis from amino acids requires energy
Protein synthesis itself has a high ATP cost (~4 ATP equivalents per peptide bond)
Storage of amino acids as protein or conversion to fat/glucose is metabolically intensive

Contribution to Total Daily Energy Expenditure (TDEE)

The three major components of TDEE are:

Resting/Basal Metabolic Rate (BMR/RMR) - largest component (~60-70%)
Physical Activity - variable component
SDA/DIT/TEF - ~8-10% of total daily energy expenditure

Per Guyton & Hall: "The thermogenic effect of food accounts for about 8% of the total daily energy expenditure in many persons."

Per Basic Medical Biochemistry: "The energy required to process the types and quantities of food in the typical American diet is probably equal to approximately 10% of the kilocalories ingested."

Because DIT is roughly ~10% of intake, it is often ignored in clinical caloric calculations (it approximately cancels out the rounding error in assigning 4/4/9 kcal/g to protein/carbohydrate/fat).

Clinical Relevance

Obesity assessment: In very sedentary individuals, DIT can represent a meaningful fraction of TDEE. Conditions that blunt DIT (e.g., hypothyroidism, insulin resistance) can contribute to weight gain.
Thyroid disease: Thyroid hormone is a major regulator of thermogenesis - hyperthyroidism increases BMR and thermogenesis overall.
Cachexia/cancer: Increased protein catabolism and futile metabolic cycles elevate thermogenesis pathologically.
Nutritional calculations: DIT is typically not separately calculated in clinical practice; formulas like Harris-Benedict incorporate it implicitly.

Summary Table

Feature	Detail
Old name	Specific Dynamic Action (SDA)
Modern names	Diet-Induced Thermogenesis (DIT), Thermic Effect of Food (TEF)
Trigger	Food ingestion (all macronutrients)
Highest SDA	Protein (~30% rise)
Lowest SDA	Fat and carbohydrate (~4% rise)
Overall contribution	~8-10% of total daily energy expenditure
Mechanism	Enzyme secretion, active transport, synthesis of metabolic reserves

Sources:

Guyton and Hall Textbook of Medical Physiology - "Thermogenic Effect of Food" section
Basic Medical Biochemistry (6e) - "Diet-Induced Thermogenesis" section
Harper's Illustrated Biochemistry (32e) - energy reserves section
Medical Physiology (Boron & Boulpaep) - "Energy expenditure" section

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