Chapter 50

1. When gray whales are migrating and on their wintering grounds, their energy source is their stored fat (blubber). Fat has a high energy content and low water content and therefore is ideal for long-term storage of energy with minimal mass. It also contributes to the buoyancy of the whale body.

2. Vitamin A is fat-soluble and can accumulate in the body. Vitamin C is water-soluble and rapidly excreted in the urine, so a toxic level of it cannot accumulate.

3. The micronutrient iron is largely recycled in the body, but due to menstrual blood loss, premenopausal women do lose iron each month.

4. If proteins were added to a solution delivered into the blood, it would stimulate an immune response in the patient to foreign proteins. Therefore it is essential to supply all amino acids in such a mix.

1. Herbivores must spend much time feeding because their food has low energy content and requires considerable processing both mechanically (chewing) and chemically (digestion).

2. All of the digestive enzymes act through hydrolysis of their substrates.

3. Antibiotic therapy can greatly diminish or alter the gut microbiota and therefore result in altered digestion.

4. The symptoms are due to damage to the villi. Damaged villi are less effective at absorbing the products of digestion and therefore contribute to undernutrition and even lack of micronutrients such as iron. Unabsorbed digestive products pull water into the gut, resulting in diarrhea, and also support microbial metabolism, leading to bloating. Damaged villi compromise the surface area available for digestion, affecting fat absorption as well as absorption of nutrients that require transport. The undigested fat results in fatty stools.

1. The tongue pushes the chewed food to the back of the mouth stimulating the swallowing reflex that through activation of many muscles pushes the food into the esophagus. Stretching of the smooth muscle in the esophagus stimulates contraction of that smooth muscle pushing the food toward the stomach. The directionality of this movement is facilitated by the enteric nervous system causing the smooth muscle ahead of the bolus of food to relax. When the bolus of food reaches the esophageal sphincter, that anticipatory wave of relaxation opens the sphincter allowing the food to enter the stomach. The coordination of smooth muscle contraction and relaxation creates the waves of peristalsis that generally moves the food in the esophagus towards the stomach even in the absence of gravity.

2. Stomach acid is produced by chief cells in the gastric pits of the stomach. In these cells, carbonic anhydrase catalyzes the hydration of CO₂ to produce H₂CO₃, which dissociates into HCO₃^– and H⁺. The H⁺ is transported across the pit lumen side of the cell in exchange for K⁺. The HCO₃^– is exchanged across the opposite end of the cell into the interstitial fluid in exchange for Cl^– ions. The excess K⁺ in the cell leaks out the luminal end and is pumped back in via the H⁺/K⁺ exchanger. Thus the concentration of H⁺ in the stomach lumen and the separation of HCO₃^– in the interstitial fluid depend on the anatomical integrity of the stomach wall. If that integrity is destroyed, the H⁺ cannot be separated from the HCO₃^– and there will be no pH change.

3. Bile emulsifies fats in the diet, creating tiny micelles that present a large surface area for the action of water-soluble lipases. Bile prevents the fatty micelles from coalescing into larger fat globules with a smaller surface area. Equivalent molecules are not required in the lymph and blood because the fats in lymph and blood are incorporated into lipoproteins that are coated with water-soluble proteins.

4. The gut microbiota is a significant source of nutrition for ruminants because their microbiota grow on the ingested food in the rumen and reticulum before the semi-digested food is exposed to the HCl and digestive enzymes of the stomach and small intestine. In humans, abundant microbiota are present in the small and large intestine and therefore are not killed by the stomach acid and digested in the upper region of the small intestine, where they could otherwise serve as a major source of nutrition.

5. A-52

   The ruminant does not produce the cellulose hydrolyzing enzymes necessary to digest the plant materials it eats. The rumen and reticulum have cultures of microorganisms that produce cellulases and break down the plant matter. The resulting fermenting mixture of plant matter and microorganisms moves into the omasum, where water is reabsorbed. From the omasum, the mass of semi-digested plant matter and the associated microorganisms move to the abomasum (true stomach) that secretes HCl, which aids digestion and kills the microorganisms that are an important component of the animal’s nutrition.

6. If even a small amount of pepsinogen is activated by hydrolytic cleavage to produce the active enzyme pepsin, that pepsin will act on additional pepsinogen to release more pepsin molecules creating an autocatalytic cascade. This is an example of positive feedback in which a product of a reaction (pepsin) stimulates still more reaction (pepsinogen → pepsin), which amplifies formation of the product of that reaction (pepsin).

1. Introducing nutrient solutions into the jejunum would not stimulate release of secretin or CCK, therefore there would not be bile secretion to emulsify large complex lipids and there would not be pancreatic enzymes to digest complex carbohydrates and proteins. Thus, the jejunal formula would have to consist of medium to short chain fatty acids and partially hydrolyzed carbohydrates and proteins.

2. The three classes of lipoproteins differ in their relative compositions of protein, triglycerides, and cholesterol. The high-density lipoproteins have the lowest percentage of triglycerides and deliver cholesterol from tissues to the liver for excretion as bile. Low-density lipoproteins have less protein and triglyceride and more cholesterol. These particles have transferred most of their triglycerides to adipose and other cells and are left with a lot of cholesterol that can be deposited into the walls of arteries. Very low-density lipoproteins consist mostly of triglycerides, which they transfer to adipose cells, resulting in the production of low-density lipoproteins.

3. In both muscle and liver, insulin promotes the uptake of glucose and its incorporation into glycogen. When insulin levels fall, the effects are different in liver and muscle, in that in liver the reduced levels activate glucose phosphatase. This makes it possible for the glucose produced by the breakdown of glycogen to be released into the interstitial fluid. This process does not occur in muscle, so the glucose is trapped in the muscle.

4. In the experiments in which the lateral hypothalamus or the ventromedial hypothalamus was lesioned and subsequent changes in body mass measured, the animals either gained or lost a large amount of mass, but they eventually plateaued at a new level, indicating the continued ability to regulate, but at a different level.

5. Pyruvate and lactate produced by muscles working anaerobically enter the circulation and are taken up by the liver, where they are converted to glucose by processes of gluconeogenesis. This glucose can then return to the blood and support further glycolysis by the muscles.

6. Rats that have a mutation that eliminates their production of leptin eat more and become obese. If these rats are joined parabiotically with normal rats that produce leptin, the obese rats eat less and lose mass.

1. The average fasting blood glucose level was 11.00 mM for the lard-fed mice and 10.3 mM for the fish-oil-fed mice. These values are significantly different at the P < 0.05 level according to a two-tailed, non-paired t-test, (these calculations can be done in Excel).

2. The average fasting blood insulin level was 3.96 ng/mL for the lard-fed mice and 1.09 ng/mL for the fish-oil-fed mice. A t-test, two-tailed, non-paired showed that P < 0.0001 which is highly significant.

3. The time course of the response to an insulin challenge can be plotted. T-tests can then be used to see which points are significantly different.

   The 30-minute values are not significantly different, but the rest of the time points are: 60 minutes, P < 0.005; 90 and 120 minutes, P < 0.001.

   

4. The lard-fed mice were less responsive to insulin and had a slightly higher fasting blood glucose level. This suggests an impairment of their carbohydrate metabolism. The fish-oil-fed mice had a more robust response to the insulin challenge.

1. The higher triglyceride levels in winter indicate a greater dependence on fat metabolism during winter, and this is supported by the decrease in winter of lactate, a product of glycolysis (carbohydrate metabolism), and the increase in succinate, which is a citric acid cycle intermediate. Since both carbohydrate and triglyceride metabolites enter the citric acid cycle as acetyl CoA, these data support the idea of an increase in dependence on fat metabolism during winter.

2. The data show significant changes in three highly abundant phyla and no change in one highly abundant phylum. Thus we can conclude that there are major seasonal changes in the gut microbiome. These changes could be due simply to the presence or absence of food in the gut over a substantial period of time, or the changes could play a role in the different metabolic states of the bears in summer and winter.

3. The data indicate that the summer microbiota promote greater mass and fat gain than winter microbiota. Two hypotheses that could explain this difference are that (1) some signal from the summer microbiota stimulates greater food intake, and (2) the summer microbiota is more efficient at facilitating digestion when there is high nutrient flow through the gut.

4. The data suggest that a seasonal change in the gut microbiome of the bears supports and may promote increased food intake and digestive efficiency in the summer months. The change from the summer to the winter gut microbiome may result from some microbial species being able to survive a long duration of fasting, or the change in microbiome may have a functional significance in altering energy metabolism to promote storage in the summer and conservation in the winter.