Exercise and the need for carbohydrates

[Wolf]

I exercise regularly, about every day, and most of the time it's pretty vigorous exercise. I'll typically go for at least 40 minutes to one hour and at the end my shirt is drenched with sweat. I'm mostly using an elliptical trainer or a bicycle and recently I'm working on getting back into running shape after being injured. So I'm not talking about a leisurely walk in the park.

Therefore, because I'm exercising hard, there's a need for carbohydrates, and maybe a fairly large amount. Of course, with my body's inability to handle large amounts, that presents a problem. Right now I'm thinking I should spread out the ingestion of carbs throughout the day to avoid creeping into the bad glucose level zone. Or should I stay with a low carb diet and not worry about not having enough carbs?

Since I'm relatively new to this type 2 diabetes thing, I'm still searching for the right way (for me) to do things.

[BlueSky]

.... Right now I'm thinking I should spread out the ingestion of carbs throughout the day to avoid creeping into the bad glucose level zone. ....
How many carbs do you have in mind? With that kind of exercise, most of the calories used will come from fat. And most of the carbs you consume throughout the day will be stored as fat, in preparation for the next workout, anyway. You can shortcut the process by just consuming fat/protein instead. It avoids the problem of processing glucose altogether.

The most useful time to eat carbs is immediately before and just after a workout. They provide calories in the initial high glucose demand period, and they replenish glycogen after you stop exercising. You don't need a lot of carbs to do this - try out some scenarios and see what feels best.

[REDLAN]

some questions.

With the 40 minute to 1 hour workouts are you exercising close to exhaustion? If so it is likely that you are running close to glycogen depletion.

there is (I believe) some misconceptions about glucose/fat metabolism during exercise.

your muscles will predominantly burn fat whenever they can. purely aerobic exercise will primarily utilise fats for energy. As exercise intensity rises and you move into the lactate zone (typically above 65% MHR), your glucose utilisation increases. Lactate is the end product of the pyruvate pathway, and pyruvate comes from glucose. High intensity exercise uses fast twitch muscles fibres to provide power, and unlike slow twitch muscle fibres which rely on fat, these rely heavily on glucose for energy. Physiologically I believe the primary reason is that the strong contraction of muscle fibres impedes blood flow, and the body becomes unable to deliver enough oxygen to completely burn fat.

The body's total glycogen supply is around 45 minutes running at 85% MHR (or thereabouts) - after this the individual becomes exhausted and is no longer able to maintain the intensity.

However got to agree with Bluesky - you only actually need to take enough glucose to get through the 45 minute workout, and ensure that you replenish your muscle glycogen.

Post exercise snacks are thought to be beneficial in this regard.

The other question concerns regime - are you on insulin? If you are diet controlled only then low-carbing will have no other effect than cause you to become tired if you fail to replenish your body's glycogen. If you are on insulin failing to replenish your glycogen reserves can have fatal consequences.

I found this rather small study somewhat amusing...

[Swimming for 12 hours leads to no reduction of ad...[Schweiz Rundsch Med Prax. 2007] - PubMed Result (http://www.ncbi.nlm.nih.gov/pubmed/18065048?ordinalpos=6&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsP anel.Pubmed_RVDocSum)

The athlete in question manged to lose 1.1 kg of muscle mass after swimming for 12 hours, yet his fat mass was unchanged - hmm I wonder which fuel his body was using?

[BlueSky]

.... The body's total glycogen supply is around 45 minutes running at 85% MHR (or thereabouts) - after this the individual becomes exhausted and is no longer able to maintain the intensity. ..... If you are on insulin failing to replenish your glycogen reserves can have fatal consequences. ....
I have had difficulty finding useful information on glycogen. So correct me if I am wrong, and I would be interested in looking at other sources. But from what I can make out, the liver stores 100-120 grams of glycogen, which it keeps topped up all the time. If you don't eat carbs, glucose from gluconeogenesis is used for this purpose. So failure to replenish these reserves by eating carbs is of no real consequence. One might expect to see a loss of muscle in the long term, but this is not borne out by actual experience.

Twice as much glycogen is stored in the muscles as is stored in the liver (200-240 grams of glycogen). By cycling glycogen depletion/replenishment, bodybuilders increase their glycogen reserves by another 50+%. Muscle glycogen is used mainly during intense cardio work (85+% of VOX2max) and resistance exercise. Apparently, because of the lack of an enzyme, muscle glycogen can only be used in the muscles it is stored in. So this glycogen can't be can't be used to deal with a hypo anyway. And failure to replenish it will only compromise physical performance.

So a well conditioned athlete should be carrying 360+grams of glycogen, or 1,440+ calories worth. On its own, this would fuel 2 hours of fairly intense activity. You also have about 80 grams of glucose in the bloodstream that. But even when working close to maximum aerobic capacity, a third of calories will still come from fat. And you would only expect to exhaust glycogen reserves after, say, 3 hours?

[Ronin]

Wolf, et al.,

One thing that is known, and was recently reported in the health section of the NY Times, is that most people over-estimate their caloric expenditure during exercise.

While I am sure that you are buring calories, you may be over-estimating your actual caloric need -- I know that I was for a very long time. A generally fit person doesn't need an unusual amount of calories for expenditures up to 90 minutes, a truly fit person goes even longer because that person is more efficient in caloric use. As a cyclist I always hear about the 6000 calorie days that is touted as what professional riders consume -- true, but that means six plus hours of racing with an average speed of 25+ MPH, including some rather nasty climbs. Most of us don't come close to that, and even if my effort was close to theirs, my daily routine would probably only consume 1000 calories at most and probably less.

That being said, some carbs prior to exercise is a good idea as this is quick energy. Generally speaking you will have sufficient stored glycogen in your muscles and liver to keep you going for the times you mentioned. And, your post exercise BG levels can actually go up if you get a liver-dump as a result of exercise.

[labob]

How to keep glucose levels in line while exercising is definitely one of those "your mileage may vary" types of things. If you are taking insulin, you have a whole range of concerns to deal with that most likely will not be an issue if you are not taking insulin. (There are a number of threads on this forum that address insulin and exercise.)

I also exercise pretty vigorously most days and I have never had to carb load -- nor have I ever gone low from exercise. Today I jogged for just over 30 minutes followed by strength training exercises (the dreaded weights) for about 45 minutes. The end result: a reading of 82 mg/dl, which is perfectly fine. I generally get the same results on pure cardio days, too, when I run for longer periods than today. But then again, I take 15 mg of Actos daily, not insulin or a sulfonylurea, so that's about as low as I seem to be able to get. But even when I took glyburide (a sulfonylurea), exercise didn't cause me to go dangerously low -- sometimes I would end up in the 70s, but it's not like readings in the 70s are dangerous. Plus, glyburide would sometimes send me in the 60s and 70s for no reason at all, which is the main reason I switched to another drug.

So I don't carb load before or after exercising and I don't seem to have problems keeping my glucose levels from going too low when I work out. Maybe I'm lucky. As I said, your mileage may vary, and the only way to figure this stuff out is to try different things and see what happens. You might want to keep some glucose tabs close by those first few times you exercise just in case, but you may also be surprised to learn that you don't need to do anything special.

[REDLAN]

I think talking about the energy capacity and calorie loadings of athletes, is not a useful guide for ordinary reasonably fit mortals such as ourselves.

I suspect that Bluesky is perfectly correct in maintaining that an athlete has enough glycogen to supply the body with enough energy for 2+ hours at 85% MHR - and this is borne out by the endurance capacity for professional athetes, and the timing of when non-professional but fit marathon runners "hit the wall."

XC cyclists will complete a course in around 2 hours with a heart rate averaging 170-180. A colleague of mine who ran for Hampshire, and finished regularly in the top 10 vets, and does triathlons. Has a heart rate of around 175 when he runs, and he is able to maintain this level for around 2 and a half hours.

The 45 min figure was not mine, but was pulled from a sports physiology page. And for normal reasonably fit mortals seems to be about right - certainly seems to be about the endurance level that me and mates can achieve whilst mountain biking.

following strenuous exercise where, you have nearly depleted your body's glycogen, and this can occur in a normal reasonably fit individual in around 45min to 1 hour running at 85% MHR. The muscles response is to become much less insulin resistant. They act like a sponge soaking up any spare glucose including whatever glycogen is left in the liver.

The point of a post exercise snack is to maintain glycogen levels in the liver - and as I said, if you are diet only, then this will not matter, you will just feel tired. However, if you are on insulin, failing to replenish glycogen after exercise is risky.

A woman I met on a course experienced exactly this phenomenon. During the day she had exercised hard in a longer than normal gym session. She had then eaten low carb that night - during the night she went into a hypo, and her husband couldn't wake her. The ambulance crew gave her glucagon (standard procedure in these cases), and she failed to respond. When she did not respond to the second dose, they had to put her on IV glucose. Apparently it took an hour for her to regain consciousness.

[JJM335]

[QUOTE=REDLAN;294185]
your muscles will predominantly burn fat whenever they can. purely aerobic exercise will primarily utilise fats for energy. As exercise intensity rises and you move into the lactate zone (typically above 65% MHR), your glucose utilisation increases. Lactate is the end product of the pyruvate pathway, and pyruvate comes from glucose. High intensity exercise uses fast twitch muscles fibres to provide power, and unlike slow twitch muscle fibres which rely on fat, these rely heavily on glucose for energy. Physiologically I believe the primary reason is that the strong contraction of muscle fibres impedes blood flow, and the body becomes unable to deliver enough oxygen to completely burn fat.
[QUOTE]

During aerobic exercise your muscles will preferentially utilize glucose as a primary fuel.This will be converted to pyruvate in an anaerobic process called the Emden Meyerhof pathway. Provided the oxygen supply is sufficient, pyruvate is then further metabolized to carbon dioxide and water in the TCA (or Krebs) cycle in an aerobic process. This is more efficient than burning fat because less oxygen is required. When the supply of oxygen is inadequate to allow full utilization of all the pyruvate via the TCA cycle, pyruvate is shuttled into lactate (which can build up in the muscles). Anaerobic metabolism of glucose through the Emden Meyerhof pathway allows glucose to continue to be metabolized to produce energy under conditions of oxygen limitation.

Slow twitch muscle fibres are rich in mitochondria and are therefore adapted for metabolism through the TCA cycle. They are rich in myoglobin an oxygen carrying protein. In contrast slow twitch fibres have few mitochondria and little myoglobin. They are primarily adapted for anaerobic metabolism of glucose via the Emden Meyerehof pathway.

[Wolf]

Thanks for the responses. A couple of times the question was asked if I'm on insulin. No I'm not. Just 500mg of metformin twice per day.

From reading the responses, it's becoming apparent that longer exercise (45 min. to over an hour) makes a much bigger difference than shorter exercise (30 min. or less). I realize there are variables involved, like intensity or anaerobic vs. aerobic, etc. But it seems to me, and maybe I'm wrong, that a 30 minute walk or a gentle run is going to mostly burn just fat and not have a big effect on glucose levels. Am I wrong about that?

[JJM335]

Wolf:


I'm T1 (a whole different ball game), but in my experience even 20-30 mins exercise a day has a HUGE effect on my insulin requirements. If I exercise every day (or at least 4-5 days a week) I can cut my basal insulin requirements by at least 25% and I also see an inprovement in my I:C ratio - this tells me that I am more insulin sensitive (which may be very relevant to T2's). This effect seems to be independent of the drops in BG induced immediately after exercise and in the succeeding few hours. At periods where I have increased the intensity or duration of the exercise, I haven't seen a big further improvement - I can get nearly all the benefits from as little as a 2.5 mile run or half mile swim.

This is an ongoing effect, I can afford to miss a day or two, but any longer and I have to start to take more insulin.

[REDLAN]

JJM335, when I first read your reply, I assumed that you were disagreeing with my post. But I re-read it, and I see that mostly we disagree on whether fat or glucose is used as a primary fuel in aerobic exercise.

and..

Slow twitch muscle fibres are rich in mitochondria and are therefore adapted for metabolism through the TCA cycle. They are rich in myoglobin an oxygen carrying protein. In contrast slow twitch fibres have few mitochondria and little myoglobin. They are primarily adapted for anaerobic metabolism of glucose via the Emden Meyerehof pathway.

I assume you meant fast twitch for the few mitochondria group - <phew> I thought for a minute I had muddled my fast and slow twitches up :D

I checked up and you may be interested in this...



and basically we are both wrong, and the picture about what fuel muscles will depend upon becomes much more complicated.

In the fasting state - low insulin levels mean that muscles will primarily use fat as fuel. In the non-fasting state with high insulin levels- i.e. a snack before exercise, and muscles will prefer glucose.

something interesting happens to athletes, in particular endurance athletes. Their muscles become much much better at oxidising fat. Which explains how they can make their glycogen supplies last so long. Also their muscle cells contain very high levels of triglycerides - despite their leanness.

so the actual balance of fat versus glucose, depends on when you last ate and how fit you are.

the article goes on to discuss how people with type 2 and insulin resistance in contrast have metabolically inflexible. They don't switch as strongly from fat to glucose in the non-fasting state, and they continue to utilise glucose in the fasting state.

in answer to your question Wolf, JJM335 is right, any exercise improves insulin resistance. Exercise that leaves you feeling like you've had a workout is the best kind. :D

The actual cause is down to an enzyme called AMP activated kinase. It is switched on by AMP i.e. the cell being in a low energy state. In muscle cells AMPK switches the genes on that produce GLUT4 (GLUT4 transport glucose into the cell), and so the cell becomes more sensitive to insulin. Depending on the intensity of the exercise the effects of AMPK can last up to a couple of days.

[REDLAN]

for some reason Diabetes forum refused to show the link here it is without the hotlinking...

[REDLAN]

for some reason Diabetes forum refused to show the link here it is without the hotlinking...
sorry can't link to a pdf for some reason.... DUH!

[Evermont]

for some reason Diabetes forum refused to show the link here it is without the hotlinking...
sorry can't link to a pdf for some reason.... DUH!

I ran into this problem too. The trick is in the "Additional Options" section just below the [Submit Reply]/[Preview Post] buttons.

Un-check the options labeled:

"[ ] Automatically parse links in text"
and
"[ ] Automatically retrieve titles from external links"

that worked for me.

[REDLAN]

hopefully this should work <fingers crossed>

http://www.jci.org/cgi/reprint/115/7/1699.pdf

cheers Evermont :)

[JJM335]

Redlan:

Really interesting article - thanks for posting. This resolves the Q of utilization of glc vs fat!

Joel

Thanks - I did mean to write fast twitch have low numbers of mitochondria.