Feeding and Fasting Flashcards
Why do carbohydrates provide for diets? Why is this?
While carbs are not an essential nutrient, they provide the majority of energy in most diets and are the least expensive source of calories. This is because of photosynthesis.
6CO2 + 6H2O + 673 KCal (solar E) —> C6H12O6 + 6O2
Animals would not exist without this capture of energy and production of oxygen.
Starch and cellulose are major energy storage forms in plants.
Which tissues preferentially oxidize glucose for energy?
- Brain (will also use ketones in long term in necessary if the body is fasting)
- red blood cells (no mitochondria, cannot burn fat)
- leucocytes (immune cells)
- gonads
- gravid uterus
What type of regulation is glucose under?
homeostatic regulation
- moderate acute hypoglycemia can lead to a decrease in brain function
- severe hypoglycemia can lead to coma or death
- high blood glucose can cause tissue damage
Why is blood glucose so important?
Blood glucose is the most important vehicle for moving energy in the body in absorptive and non-absorptive states (fed and fasted).
Sources of blood glucose vary with:
- type of GI tract
- diet
- time after eating
What do sources of blood glucose vary with?
- type of GI tract
- diet
- time after eating
Compare and contrast glucose absorption between a ruminant and non-ruminant. (ex. sheep vs. pig)
Ruminants like sheep will not absorb much glucose because the rumen microbes convert most of it to VFAs that are absorbed in the rumen. Therefore, little glucose is absorbed in the small intestine of ruminants.
In contrast, non-ruminants are able to absorb much more of the glucose they consume.
What happens to plasma glucose after a meal?
It increases from absorption (absorptive phase, see chart pg. 8)
How is starch broken down?
Salivary amylase breaks starch down to maltose and glucose.
Intestinal alpha amylase breaks down starch and intestinal maltase breaks down maltose into more glucose.
Breakdown of starch is a two step process.
How is sucrose broken down?
The enzyme sucrase breaks sucrose down to glucose and fructose.
How is lactose broken down?
The enzyme lactase breaks down lactose into glucose and galactose.
What is lactose intolerance and why do many animals have it?
Lactose intolerance is when the body cannot break down lactose since they do not have the lactase enzyme. It can still be fermented to a degree, but too much can cause issues like abdominal pain.
All mammals are lactose intolerant because they secrete lactose when they are young in order to take advantage of milk and colostrum, but stop producing lactase later in their development.
Describe general carbohydrate digestion in young animals.
Except for in newborn animals (24-48 hours), no di-, tri-, or polysaccharides are absorbed.
- immunoglobins are absorbed for passive immunity in the first 24 hours
- “leaky gut” allows immunoglobulins to be absorbed rather than digested after the young animal drinks colostrum within the first 24 hours of life, which establishes immunity
Describe carb digestion.
Carbs are digested to simple sugars, then absorbed into the enterocyte.
Except for in newborn animals (24-48 hours), no di-, tri-, or polysaccharides are absorbed.
Glucose transporters are required to move glucose, a polar molecule, across the cell membrane.
Active transport of glucose across the small intestine via Na-dependent glucose transporer SGLT1. It uses Na/K ATPase pump to create a gradient, allowing glucose to be transported into the cell.
Passive transport of glucose by GLUT2 proteins out of the epithelial cell into blood vessels (facultative transport that does not require ATP)
Describe the function of SGLT1
SGLT1 is a Na-dependent glucose transporter. It uses a Na/K ATPase pump, which creates a concentration gradient allowing for the active transport of glucose into the cell.
Describe the function of GLUT2
GLUT2 facilitates the passive transport of glucose out of the epithelial cell and into the blood vessels via facultative transport (no ATP) when glucose is in a higher concentration in the cell than in the blood
What is Vmax?
“How fast the enzyme can go”
Varies by different enzymes.
What is Km and what relationship does it have with the enzyme and substrates?
Km is the substrate concentration at half the maximal rate. It is a measure of the affinity of the enzyme for the substrate.
Low Km correlates with high affinity.
High Km correlates with low affinity.
Km varies by different enzymes.
What is nutrient partitioning?
The directing of nutrients to different tissues, i.e. where the nutrients is going.
Regulation of cellular glucose uptake is an example of nutrient partitioning.
How does GLUT vary?
GLUT isoforms have tissue specific expressions.
They are slightly different variations of the same ancestral gene.
They differ in regulation (ex. responsiveness to insulin) and biochemical properties (Km and Vmax)
Describe the brain’s relationship with glucose and its glucose transporter.
GLUT3 works with the brain. It has a low Km value, and therefore a high affinity for glucose. This is a good relationship since glucose is necessary for the brain to have and it can work well with little glucose present.
Describe the liver and its relationship with glucose and its transporter.
The liver utilizes GLUT2, which has a higher Km and therefore a lower affinity. Therefore, it works better when there is a higher concentration of blood glucose.
