PCP Shivering and Glycogen Depletion

Question 1

What is the hypothesis for the point and the counterpoint?

Answer 1

Point: shivering heat production is limited by glycogen depletion

Counter Point: shivering heat production is not limited by glycogen

Question 2

Evidence that glycogen is involved in shivering heat production AT ALL?

Answer 2

Shivering induced heat production is primarily fueled by carbohydrates, lipids, and proteins. To shivering heat production is limited by glycogen depletion, studies often perform cold water immersion. One study on cold-water immersion revealed that glycogen levels in vastus lateralis diminished from substantially as a result of shivering. A similar pattern of glycogen reduction was also seen in the deltoid muscle by the same group. Thus, glycogen’s involvement in shivering heat production clearly demonstrated.

Exp that first look at limit protein, and does limit in protein effect shivering? No not really any effect.
Looked at lipids, and blocked oxidation of lipid, and found not

Question 3

Evidence that lipids and proteins are not the rate limiting step in glycogen depletion? EVIDENCE FOR POINT

Answer 3

PROTEIN: One study showed that during the cold-exposure experiments, contribution of proteins as a fuel to total heat production decreased significantly, from ~21 at 28°C to ~10% at 10°C. These findings help to eliminate proteins as a key fuel to shivering heat production, leaving carbohydrates and lipids to compete for the “rate-limiting” title

LIPIDS: One study looked at cold-immersion involving the ingestion of nicotinic acid (NIC). NIC normally fxns to inhibit the breakdown of TGs to FFA and glycerol. A significant dec in muscle glycogen levels were seen during both control and NIC trials. These findings indicated that reduced availability of plasma FFA does not disrupt shivering heat production during cold water immersion, eliminating lipids as the most important or “rate-limiting” substrate. In fact, the fall in muscle glycogen in both control and NIC trials in that study clearly demonstrated that shivering heat production is limited by glycogen levels.

Question 4

Evidence for the counterpoint (shivering heat production is not limited by glycogen depletion)

Answer 4

1. Studies by Haman conclude that glycogen loaded men tended to use more CHO as fuel, while glycogen depleted men tended to have a higher portion of lipid oxidation to total heat production
2. The respiratory exchange ratio (RER) is the ratio between the amt of CO2 produced in metabolism and O2 used. An RER of 0.70 indicates that fat is the predominant fuel source, RER of 0.85 suggests a mix of fat and carbohydrates, and a value of 1.00 or above is indicative of carb being the predominant fuel source. RER and CHO oxidation has been shown to dec as a function of time, for all levels of glycogen, suggesting that the prolonged duration of shivering utilizes more lipids as glycogen reserves begin to diminish. This means that even glycogen loaded men will eventually begin oxidizing more lipids as the duration inc
3. Studies on relative contributions of nrg system during performance, show that, the contribution of glycolysis to nrg demand, decreases as a fxn of time. This is cause although glycolysis is anaerobic and can produce ATP rapidly, it produces lactic acid as a by-product. Once the lactic acid threshold is reached, glycolysis decreases and lipid oxidation increases in the total heat production. This also demonstrates that shivering heat production is not limited by glycogen depletion because the body can produce energy through lipid oxidation.
4. Haman et al found that the body can sustain the same thermogenic rate by varying muscle fiber recruitment during shivering thermogenesis, in order to conserve energy. Particularly, type 2 muscle fibers, which utilize mostly CHO and produce high intensity bursts. While type 1 are slow oxidative and utilize mostly lipids and produce slow continuous bursts of shivering