Canine Digestion, Metabolism & Nutrient Requirements

Canine Gastrointestinal Anatomy & Carnivore Heritage

• Dentition
  • Large canines for seizing prey; carnassial cheek teeth act like scissors (slice flesh/bone rather than grind).
• GI tract layout (simple-stomach animal)
  • Stomach → duodenal loop (pancreas nestled) → long small intestine → short large intestine/colon → tiny cecum.
  • Intestinal loops hang freely (unlike humans where loops are partly fixed to dorsal body wall).
• Dogs are evolutionary carnivores BUT functionally omnivorous
  • Wolves consume entire carcass: muscle, bones, skin, fur, feathers, fins, viscera (except rumen contents).
  • Stomach analyses: also eat seasonal fruits (apples, persimmons, plums), grasses, etc.
  • Practical implication: “raw-meat-only” diets are nutritionally incomplete—need viscera, bone, etc.

Nutrient vs. Ingredient Mind-set

• Pet-food advertising focuses on ingredients; nutritionists focus on nutrients (proteins, fats, carbs, vitamins, minerals).
• Ingredients are merely vehicles to deliver balanced nutrients.

Scaling of Energy & Nutrient Needs

• Surface-area principle: bigger animals have lower heat loss per kg → curved relationship between size & requirements.
• Metabolic Body Weight (MBW): MBW = BW^{0.75}
• Resting Energy Expenditure (REE) for dogs: REE\,(\text{kcal·day}^{-1}) \approx 70 \times BW^{0.75}
• For most nutrients the requirement per 1000\,\text{kcal} is constant across dog sizes & across many species.

Protein Metabolism

• Ileal digestibility range 50–100 %; whole-tract digestibility even higher.
  • Meat & casein nearly 100 %; soy highly digestible; field-bean moderate; keratin (feather/fur) lowest.
• Arginine is essential in dogs (and cats)
  • Required to maintain urea cycle (detoxifies ammonia → urea).
  • Dogs need less arginine than cats → can safely eat human foods; cats must NOT eat dog food.

Lipid Digestion & Systemic Handling

• Digestibility 85–100 %; gastric + pancreatic lipase (no lingual lipase); bile salts emulsify.
• Dogs metabolize fat ≈2× faster than humans; skeletal muscle fibres are universally oxidative/aerobic.
• Lipoprotein profile in the fasted state
  • Packages: chylomicron > VLDL > LDL > HDL (size descending).
  • Humans: ~⅔ cholesterol in LDL ("bad"), ⅓ in HDL ("good").
  • Dogs & cats: reverse – ~⅔ in HDL, ⅓ in LDL → atherosclerosis & myocardial infarction rare (unless endocrine disease).

Fatty Acid Families (n-6 vs n-3)

• n-6 pathway: Linoleic acid → γ-linolenic → dihomo-γ-linolenic → arachidonic acid (AA) → potent eicosanoids.
• n-3 pathway: α-linolenic acid → EPA → DHA → milder eicosanoids (anti-inflammatory).
• Dogs have poor conversion of α-linolenic → EPA/DHA.
  • Choose supplements listing EPA & DHA, not generic "omega-3" or just ALA.

Carbohydrate & Fiber

• No salivary amylase; robust pancreatic amylase + disaccharidases + monosaccharide transporters = efficient starch digestion.
• Bond specificity
  • \alpha(1\rightarrow4) linkages (starch) → digestible.
  • \beta(1\rightarrow4) linkages (cellulose) or complex branching → indigestible.
• Fiber categories
  • Insoluble/non-fermentable (cellulose, lignin) → acts as sponge, firms stool.
  • Soluble/fermentable (pectin, gums, certain hemicelluloses) → bacterial fermentation → volatile fatty acids (VFAs).

Do Dogs NEED Dietary Carbohydrate?

• If adequate protein provided, dogs can gluconeogenically meet glucose needs.
  • Study 1: Pregnant bitches on low-protein, zero-carb, high-fat diet → hypoglycaemia, ketosis, fetal death.
  • Same protocol with high protein → normal.
  • Study 2: Sled dogs trained months on high-protein/high-fat/no-carb diet → normal performance.

Digestibility Data (representative)

Fermentation & Volatile Fatty Acids (VFAs)

• VFAs produced: acetate, propionate, butyrate (preferred fuel for colonocytes).
• Dog colon short; turnover low → only ~5 % of total energy comes from VFAs (vs. 60–70 % in ruminants/horse).
• Despite small colon, VFA concentrations are highest among surveyed species.
• Adequate fermentation prevents diversion colitis, aids Na⁺/water absorption.
• Excess soluble fiber or non-absorbed sugars (e.g., therapeutic lactulose) → VFA overload → osmotic diarrhoea.

Vitamins & Minerals

• In most cases, requirement per 1000\,\text{kcal} parallels other mammals.
• Notable species specifics
  • Vitamin C: NOT required (dogs synthesise).
  • Vitamin D: MUST be dietary (cutaneous synthesis inadequate).
  • Sodium/Chloride: low need (dogs pant, don’t sweat). During exercise supply water ± glucose, NOT electrolyte drinks.

Exercise Metabolism

Fuel Hierarchy (rate of ATP provision)

1. ATP + creatine-P
2. Anaerobic glycolysis (muscle glycogen → lactate)
3. Aerobic oxidation of glucose/glycogen
4. β-oxidation of fatty acids (slowest rate)

Endurance vs Sprint

• Endurance work (>30 min) typical of most canine activity (hunting, sledding, pet jogging).
• Sprint (≤90 sec, e.g., greyhound racing) relies heavily on 1 & 2 above; greyhounds uniquely adapted.
• Canine muscle fibre spectrum
  • Predominantly oxidative (slow & fast); virtually no glycolytic fast-twitch B fibres found in cats/humans.

Dietary Fat & Performance

• Downey & Kronfeld beagle study (steep-treadmill endurance) – diets expressed per 1000\,kcal:
  • Low-fat (≤33 g; ≤30 % kcal fat) → exhaustion at ~100 min / 24 km.
  • High-fat (≥59 g; ≥50 % kcal fat) → exhaustion at ~150 min / 32 km.
• Carbohydrate-loading (human concept) less relevant; canine stamina derives largely from fat oxidation, though muscle glycogen still needed for short bursts.

Starvation Physiology

• Humans/rats: prolonged fast → ↓blood glucose, ↑ketones; insulin:glucagon ratio plummets.
• Dogs: maintain euglycaemia, minimal ketosis; insulin:glucagon ratio remains stable.
  • Exception: very small breeds & neonates have limited glycogen → risk of hypoglycaemia.

Clinical & Practical Implications

• Don’t judge diet quality solely by ingredient list; evaluate nutrient composition & balance.
• Raw-meat diets must include bone + organs or be carefully supplemented.
• Firm vs soft stools can be manipulated by:
  • ↑insoluble fibre (firms)
  • ↓soluble fibre or ↑dietary fat (firms)
  • Excess soluble fibre/non-absorbed sugars (induces soft stool/diarrhoea).
• Fish-oil supplements: verify EPA/DHA amounts; ALA alone largely ineffective in dogs.
• Working/exercising dogs require
  • High-fat (>50 % kcal) ration for endurance
  • Plenty of water; avoid salt-laden electrolyte drinks.
• Cats cannot safely eat dog food (arginine deficiency risk); dogs can eat cat food (though calorically dense).

Key Numerical Reference Values & Equations

• REE_{dog} \approx 70 BW^{0.75}\;\text{kcal·day}^{-1}
• High-fat performance diet: >59\,\text{g fat} per 1000\,\text{kcal}\; (\ge 50\,\% \text{of calories}).
• Insoluble fibre often added at ~20\,\text{g per 1000 kcal} to firm stools (values vary by brand).
• Endurance beagle study: high-fat increased run distance from 24 \rightarrow 32\,\text{km} (+33 %).

Take-Home Points

• Dogs share omnivore digestive capabilities but possess distinct fat-centric metabolism.
• Dietary carbohydrates are optional if protein is ample, but moderate fermentable fibre is essential for colonic health.
• Lipoprotein pattern (high HDL) explains rarity of atherosclerosis.
• Vitamin D and essential fatty acids (AA, EPA, DHA) must be supplied; Vitamin C and high sodium are unnecessary.
• High-fat diets enhance endurance; inadequate protein in zero-carb scenarios causes reproductive & metabolic failure.
• During starvation, adult dogs remain normoglycaemic & non-ketotic—unlike humans.
• Always translate human data cautiously; canine physiology has key divergences in lipid handling, fibre fermentation, and hormonal responses.