IRWIN TAUB: It is clear that, in any issue involving endurance when fatigue comes on and in other issues regarding the ability of people to function under stressful conditions, several of the techniques we have heard today certainly are applicable. In particular, what Gerald Shulman pointed out is very relevant.
In fact, the issue, as Jim Vogel brought up, is how performance is influenced by the intake of food, what you eat, in what form that food is, how quickly or how often you eat, and so on, which raises, then, the question that I have for Dr. Shulman. You demonstrated previously, and it is in some of your papers, that for an individual functioning at 30 percent ˙V O2max, you can get a steady state between the depletion of the glycogen and the synthesis of the glycogen, by the slow infusion of glucose.
What do you expect would happen if one were to work harder and to counteract it by more rapid ingestion and digestion of the food? Do you still think there should be this equilibrium or, I should say, this steady state, which means greater endurance, or do you think there are some other issues, like insulin and so on, that might compromise the situation?
GERALD SHULMAN: I believe you are referring to our work on glycogen turnover in muscle, which I did not discuss today. We are now in the process of examining other factors that might impact on muscle glycogen turnover.
GILBERT LEVEILLE: In measuring glycogen, can you use the technique to look at glycogen disappearance during exercise?
GERALD SHULMAN: Yes.
GILBERT LEVEILLE: How long is your measurement period?
GERALD SHULMAN: We can get down to about 8 minutes per measurement, with reasonable signal to noise.
G. RICHARD JANSEN: Have you separated out the effects of glucose and insulin by giving a glucose load and, say, giving diazoxide, which inhibits insulin secretion?
GERALD SHULMAN: We just finished a study in humans, in which we looked at the role of glucose and insulin in the regulation of hepatic glycogen synthesis.1 In the absence of insulin, hyperglycemia per se will not cause any 13C label incorporation into glycogen. Only after portal vein insulin concentration rises to greater than 150 pµ does significant hepatic glycogen synthesis occur.
REED HOYT: I was curious about the 13C-glucose costs for a given field strength and decoupling power, and how to balance it.
GERALD SHULMAN: When we first started these studies 10 years ago, it was $450.00 a gram, and we use about 20 grams per study. But now, when you buy in large quantities, you can get the cost down to about $45.00 to $50.00 a gram.
DENNIS BIER: I have probably been buying isotope longer than anybody in this room. Twenty-five years ago, it cost several thousand dollars for a few hun-
dred milligrams for anything we tried to do. The first experiments we did cost several thousand dollars apiece.
Today you can buy the same material, like leucine, glucose, and various amino acids, in the order of $60.00 or $70.00 a gram. In fact, they often are cheaper than the corresponding radiotracers. So there is a whole host of things you can do at rates that are realistically $50.00 to $100.00 per subject, at the most, and some of them are $20.00 to $30.00.
I wanted to make a comment about the cost because this was something that used to be a problem with the first stable isotope experiments. You can do one $100,000 experiment and get the right answer or ten $10,000 experiments and get the wrong answer. Now, which would you rather do (which is what we are addressing here)? I mean, is it worth putting the money up front to get the answers that are correct and move the field ahead or is it worth continuing to do the wrong ones because they are cheaper? I know which route I would take.
DAVID SCHNAKENBERG: Perhaps you said it, and forgive me if I missed it, but your methodology would allow you to follow glycogen repletion in muscle as well as depletion, right?
GERALD SHULMAN: That is right.
DAVID SCHNAKENBERG: Because I think in many instances, for military application, in terms of guidance to a commander or to a soldier it could very well be written on the label which foods inside that ration pack should be consumed to replete muscle stores most rapidly, when the soldier has just been through an exhausting, physically enduring event.
GERALD SHULMAN: We are now looking at optimal feeding and exercise regimens to achieve and maintain glycogen supercompensation.
DAVID SCHNAKENBERG: In some instances in the military situation, elements of a relatively small increase in the rate of recovery or the rate of (speed of) a response time (for example, if one guy happens to shoot one second before the other person) can have more than a 10-percent impact.
So even a 10-percent increase in the rate of recovery may be militarily important.
DOUGLAS WILMORE: Can you see changes in muscle swelling?
GERALD SHULMAN: We are now combining these measurements with MRI to assess muscle volume and swelling.
HAL GOFORTH: Along the lines of the discussion earlier, with the optimum repletion you would have the opportunity to look at the effect of fructose versus glucose as a recommended food source for liver glycogen, which may be more important for cognitive performance than for, say, just leg muscles.
GERALD SHULMAN: Yes, it also is possible to use the same methods to assess optimal regimens for liver glycogen repletion.
REED HOYT: For Dr. Wolfe, what you presented was remarkable. I wanted to have you clarify whether we could apply your techniques concerning 13C-12C ratios in urea, for example, to some of these field studies where we do have reasonable estimates of energy expenditure and food intakes and so forth, to get at just what they are combusting for fuel or whether you would need other tracers in addition?
ROBERT WOLFE: The accuracy of that method depends on the different ratios (of 13C to 12C) in carbohydrate, fat, and protein. We performed the quantitation of substrate validation by the ratio technique in two ways. In one case, no tracer was given. In the other, carbohydrate containing some 13C was given to stabilize the glycogen stores at a higher enrichment. The precision is improved with the additional 13C-glucose, but the method nonetheless can be used without any modification of the diet at all. It actually just requires a breath sample. A urine sample could be used, however, and the carbon enrichment of urea analyzed since that is derived from the bicarbonate pool.
With a breath sample, if you have any independent means of measuring total oxygen consumption, then you can quantify oxidation in absolute terms. So I think it is optimal if you have some extra 13C added to carbohydrate, but it is certainly not necessary.
HARRIS LIEBERMAN: To follow up on that, what sort of accuracies are you talking about in either case?
ROBERT WOLFE: It depends on exactly how you define accuracy in terms of the coefficient of variation. The determination in one person from day to day…
HARRIS LIEBERMAN: In a population sample of, say, eight people rather than an individual.
ROBERT WOLFE: I am just trying to recall the data. It depends on how you express it, because you are quantifying carbohydrate and fat oxidation, and if you have very low carbohydrate oxidation, then you will have a high percent error in that and a low percent error in the fat oxidation. Roughly, the error is about 5 or 10 percent, but the percent error depends on whether you are predominantly oxidizing fat or carbohydrate. The percentage error will be greater on the substrate that is the minimally oxidized substrate.
HARRIS LIEBERMAN: So 5 to 10 percent for individual variation?
ROBERT WOLFE: Right.