13 Energy and growth

A surgeon’s view of the gastrointestinal tract

Requirements for growth

Many things have to happen for the nutrients we eat to actually turn into energy and components that we can use for growth.

We have to eat the right things and in the right proportions
  • Veterinary diets are formulated to solve this problem. The name brand foods (with a nutritionist involved) include essential vitamins and minerals as well as the right balance of proteins, carbohydrates and fats. If pets or livestock eat diets developed by nutritionists, they are usually eating the right things in the right proportions. When we change that diet, we can have issues.
  • Diet fads can impact animals. Raw diets are not healthier than dog chow. Cat are obligate meat eaters; dogs are not. Dogs do not need to eat “low carb”. Cats cannot survive as vegetarians.
We have to eat enough food to meet metabolic demands
  • We need to be ingesting enough calories to meet the body’s demand or we have a “negative” energy balance. Higher energy needs are required for growth, lactation, extra work, illness, neoplasia and the last trimester of pregnancy.
  • Nutrients may be lost through various disorders. In particular, proteins can be lost through the skin (burn wounds), the GI tract, and the kidneys. If proteins are being lost, more are required in the diet.
The things we eat have to be turned into digestible particles- chewing and swallowing
  • Chewing helps break food material into smaller pieces with more surface area; this allows it to be digested more effectively. Older animals may not be able to chew as well. Certain dietary components are also harder to chew or to break down (straw, later cuttings of hay, corn)
The material has to move through the GI tract
  • Most digestion typically occurs in the small intestine. Digestion can’t happen if food isn’t eaten, is regurgitated or vomited, or if it gets stuck proximal to the SI.
Digestive enzymes must break down food into its basic components for absorption
  • We don’t have transport mechanisms for larger proteins, fats or sugars. We can absorb disaccharides, monosaccharides, monoglycerides, fatty acids, amino acids and some peptides. We use salivary (in some species) and pancreatic enzymes to break larger food molecules into smaller absorbable molecules.
  • Many enzymes are stored in an inactive form to prevent self-digestion; these must be activated to work. Bile is required for fat digestion. Multistep processes mean more things can go wrong.
  • Segmentation (mixing) activity also helps mix the enzymes with the food. If the intestine isn’t contracting properly, digestion as well as absorption can be impaired.
  • Different species have different enzymes and hence different dietary requirements.
Nutrients must be absorbed
  • Absorption of nutrients mostly occurs in the villi of the small intestine (villi are folds of tissue that increase surface area)
  • The villi are designed so that the digestion products can easily reach the vessels and lymphatics by passive movement
  • If the villi are too thick, damaged or otherwise dysfunctional, absorption may not occur even with proper digestion.
  • When motility is sped up, absorption may not have time to occur.
Nutrients must be produced
  • Gut microflora play in role in digestion, particularly in ruminants. In ruminants, diets are designed to primarily feed the microflora. The microflora feed the cow.
  • If the microflora are killed off or are abnormal, digestion of hays and grasses will be impaired. In addition, microflora produce certain nutrients that are not otherwise¬† available in diets.
Nutrients must be distributed
  • Most nutrients pass out of the GI tract and go into the portal vein system and to the liver before being disseminated through the body. The liver filters out toxins and may further metabolize dietary components. If the portal system or liver function is abnormal, the nutrients may not make it to the rest of the body. Fats must also be disseminated by the lacteals.

Causes of weight loss- Malone lecture

Challenge Questions

Pick one to think through or research:

  • When do we(or other animals) gain weight vs lose weight?
  • What are the current recommendations for endurance diets (dogs, horses, humans)?
  • What vitamins and minerals are required(any species)? What happens with deficiencies?



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