Carb restriction diets and Exercise

Not sure if this the right forum, but I was wondering if there was anybody who followed a low carb or moderate carb diet that also did a lot of exercise… something along the lines of regularly training for long distance races (think 1/2 marathon to marathon).

Does restricting carbs negatively impact how well you train? or what kind of training you do?

While I haven’t switched over to a low carb diet…I have found that moderate amounts and spread out works tons better on maintaining good numbers…
On the other hand, I have recently (1-2 months) started back up with training (took an extended break) and find that I want to eat more carbs and feel really lethargic the rest of the day especially after longer runs unless I up my carb intake. I’m not sure if this is just because of the change in exercise habits and should just ride the period out…or if training in general (as opposed to being more sedentary) demands more carbs…
I also realize that there is an element of “it varies” but just wanted to see what other’s experience was… like if there are a lot of carb restricters that also do a lot of exercise… it’s seems more doable…

Type 2 chiming in. I’ve done 3 marathons and a bunch of tri’s. when you feel lethargic what do your #'s look like? I adjust according to the intensity of workouts and what my meter is telling me. hard/interval stuff can give me lows but that is me for some folks hard workouts cause high sugars as well.

Your lethargy could be because you’re just getting back into shape. I think it’s more likely caused by high blood sugar. Do you test after running? What are your results?

I find that I need carbs to keep my energy up when training for a half-marathon. I run two a year. That doesn’t mean you can’t eat low carb on a rest day. But I think a strict low carb diet is incompatible with endurance training. Just my opinion.

I eat carbs before a run and more carbs before a long run. I reduce my bolus by one unit for each thirty minutes I plan to run. On runs over one hour I reduce my basal rate, too. I keep a lot of carbs with me during a run, too (and ALWAYS my meter). I eat a moderat amount of carbs, say 15g, within 30 minutes after a run.

There is lots of excellent advice in the “Athletic Diabetics” group and in the book “The Diabetic Athlete” by Sheri Colberg. I highly recommend it.

Good running,


Thanks for responding!
I think the lethargy is more to do with increasing the intensity because before I only ran once a week and did some weight lifting. I don’t think it’s a high numbers thing…I’ve definitely felt the impact of highs on performance before…and wanting to run better is one of my big motivators to test often…so that when I go into a run, especially if it’s longer it will be around 100 +40-20 points… and I always test after too. I have seen it go up when it’s a short period of intense exercise and if I started out higher

I was wondering about the low carb thing… because a lot of the advice out there for people who do long distance running/endurance events (triatholons) is to eat more carbs and not just before big events but in general…though I think that a lot of recreational runners eat more carbs than needed…

I think highly of Sheri Colberg, but she is a carb advocate. There is little advice in the diabetes arena on endurance training on low carb. I think it can be done, but you are going to have to pretty much find your own way. The difficulty as a type 1 is that your body should normally shift to fat burning as your glucose/glycogen stores deplete, but that depends on carefully tuned signaling involving insulin and glucagon (among other things).

Your lethargy following your runs may simply be glycogen depletion. You may have burned through 200-300 g of glucose and it just takes a while to restore things on a low carb diet. Most non-diabetic low carb runner operate in ketosis. That is very difficult to do as a t1. If you are really interested in pursuing this, I would suggest that you just start small and work through it. Don’t run with too much insulin on board. Test frequently and use small corrections possible. Sip a gatorade or eat a smarty or two. You want to keep your blood sugar at lower levels in order to encourage ketosis, but you need to monitor carefully against lows.

I think carb loading is overrate and probably unnecessary unless the event last more than a 2-3 hours (just my opinion based on my limited experience)

I cannot advise you on this, but I do know that heavy exercise and Type 1 can be a tricky situation. So hope you are testing before, during and after to keep yourself safe. Take care.

Hi, this is Sheri Colberg. Actually, I’m not a carb advocate per se, but I realize the effect that carbs in moderation can have on performance. Despite what you say about increasing ketones and using fat, fat will NEVER be your body’s first choice of fuels during moderate to intense physical activity. It’s because using carbs is more fuel efficient–you get more energy out of carbs for a given quanitity of oxygen (5.05 vs. 4.7 calories per gram for carbs and fats, respectively). If you want to exercise intensely and you eat a low-carb diet, you will not be able to perform at your highest level. I do agree that most people overdo the carbs. If you’re eating enough calories to cover your basal needs and your exercise use, you can get by with 40% or less of your calories coming from carbs. Eating more than that will not necessarily benefit exercise (it’s not a case of some is good, so more is better). The fact of the matter is, though, that it takes 24-48 hours to fully restore muscle glycogen if you deplete it with exercise, assuming you’re eating some carbs. If you’re not, it will inevitably take longer, and you may be trying to do your next workout with less glycogen available. Being depleted also does not improve your fat use because, as we say in the exercise physiology world, “fat burns in a carbohydrate flame.” Hope this helps you understand my position better. Sheri Colberg, PhD

OMG. It’s Dr. Sheri Colberg!! I am really pleased that you responded to my post. I’ve really enjoyed your books and articles. While I understand your position, I do feel that diabetics can exercise effectively on a low carb diet . This is an area that has not been studied very much and I would hope that you could take a look at it. There are advocates such as zero carb Charles ( who have done endurance running on very low carb diets. While you make a clear case that endurance exercise works with moderate diets, and certainly this question was in the context of type 1, many type 2 diabetics depend on low carb diets for managing their diabetes. There can be significant benefits from low carb diets for both type 1 and type 2 diabetics and I don’t think that a low carb diet by necessity has to compromise exercise. I do hope that you will consider looking at exercise for diabetics on low carb diets in the future.

Thanks for taking the time to post and I do hope that we will hear from you again.

You’re right that it has not been studied in diabetic individuals. Let me give you one recent published example of its effects in a non-diabetic population on exercise to exhaustion done at a high intensity. Keep in mind that my comments are related to optimal performance at a higher level, not just doing mild or moderate activity in an average person. From a quick PubMed online search:

Braz J Med Biol Res. 2009 May;42(5):404-12.

Effect of carbohydrate availability on time to exhaustion in exercise performed at two different intensities.
Lima-Silva AE, De-Oliveira FR, Nakamura FY, Gevaerd MS.

Laboratório de Aptidão Física, Desempenho e Saúde, Universidade Federal de Alagoas, Maceió, AL, Brasil.

This study examined the effects of pre-exercise carbohydrate availability on the time to exhaustion for moderate and heavy exercise. Seven men participated in a randomized order in two diet and exercise regimens each lasting 3 days with a 1-week interval for washout. The tests were performed at 50% of the difference between the first (LT1) and second (LT2) lactate breakpoint for moderate exercise (below LT2) and at 25% of the difference between the maximal load and LT2 for heavy exercise (above LT2) until exhaustion. Forty-eight hours before each experimental session, subjects performed a 90-min cycling exercise followed by 5-min rest periods and a subsequent 1-min cycling bout at 125% VO2max/1-min rest periods until exhaustion to deplete muscle glycogen. A diet providing 10% (CHO(low)) or 65% (CHO(mod)) energy as carbohydrates was consumed for 2 days until the day of the experimental test. In the exercise below LT2, time to exhaustion did not differ between the CHO(mod) and the CHO(low) diets (57.22 +/- 24.24 vs 57.16 +/- 25.24 min). In the exercise above LT2, time to exhaustion decreased significantly from 23.16 +/- 8.76 min on the CHO(mod) diet to 18.30 +/- 5.86 min on the CHO(low) diet (P < 0.05). The rate of carbohydrate oxidation, respiratory exchange ratio and blood lactate concentration were reduced for CHO(low) only during exercise above LT2. These results suggest that muscle glycogen depletion followed by a period of a low carbohydrate diet impairs high-intensity exercise performance.

If all of your exercise training and competing is going to be done at a submaximal level (just to finish and not to compete), then this may not apply. However, one last point is that even people with type 2 diabetes will be able to handle some carbs better when doing regular physical activity that depletes some muscle glycogen (the main storage depot for excess carbohydrate consumption), and they may feel less tired and more energetic when eating some carbs during and/or after exercise in particular to speed up muscle glycogen repletion. On rest days, a lower carb diet is certainly better. Sheri Colberg, PhD

That was an interesting article, I was able to look up the actual article here ( What often concerns me about these studies is that it has long been understood that there is an adaptation period for low carb diets. Atkins formally uses an induction period of a couple weeks, and many people experience fatigue and difficulty during that time. It has been argued that the body is adapting from using carbs as a primary fuel to a state where fat primary fuel. That is why ketones are observed and are used to measure whether the body has entered the desired state of ketosis. Many long term adherents to low carb diets report that their energy levels are restored after a few weeks.

This study took recruited subjects off-the street and they were put on a low carb diet for 48 hours, far to short to enable adaptation. I could have predicted sub optimal performance without seeing the results of the study.

I do heartily agree that exercise is of great benefit for type 2 diabetics follow low carb diets, and I do exercise aggressively myself. I have just struggled with there not being good advice for type 2 athletes. I did find the book by John Ivy, Nutrion Timing helpful. I was able to adapt his advice and eat some measured carbs post workout when I am more insulin sensitive (and as you note can uptake the glucose). This seems to help exercise recovery. Any thoughts you have would certainly be valued.

Taking in carbs post-exercise is probably the most important time–during that “window of opportunity” from 30 minutes to 2 hours afterwards when glycogen repletion rates are highest, as is insulin action. I have to maintain, though, that your body cannot process fat as quickly or as efficiently as carbs during exercise (the caloric equivalent per gram is a non-disputable fact), so you will never reach your peak performance on fats alone. If you can still exercise as well as you’d like to using more fats than carbs and optimal performance is not your concern, then go for it! Sheri

Hi Xanthasun and others,

I haven’t read it (I keep meaning to), but you might want to check out The Paleo Diet for Athletes, by Loren Cordain and Joel Friel. Loren authored the original Paleo Diet books. Joel is the author of The Training Bible series for cyclists, mountain bikers, triathletes, etc.

And I find Sheri Colberg’s 2001 version of The Diabetic Athlete more useful than the most recent 2009 version. IMHO, it has better guidelines, in tabular format, to help guide your calorie intake and insulin adjustments relative to activity type. The 2009 version has more colloquial stories from diabetics along the lines of “this is what works for me.”

Cheers, Mike

I’m not a big-time athlete, just a normal person who tries to do some exercise every day. I don’t have this completely tweaked yet but I find that really low-carb works great for me for kickboxing, light aerobics, short distance jogging, spinning and moderate weight training. I have plenty of energy, even with about 30 g/carbs/day. For higher-intensity stuff like my HIIT training class, I do better having a little bit of carbs.

When I do low-carb and exercise in the evening I wake up with GREAT blood sugar - presumably because my liver doesn’t have as much glucagon to convert into glucose overnight.

I personally get much better control with a low-carb diet and I gain weight like crazy when I eat carbs and have to take more insulin.


Almost everyone gains weight if he/she has to take a lot of insulin to cover carbs or even just too many calories in any form (protein requires some insulin as well, and fat can make you insulin resistant). On a low-carb diet, if you exercise in the evening, you’re retaining most/all of your heightened insulin action overnight, both because your glycogen synthase enzymes remain almost fully activated since you haven’t restored much glycogen and also because the liver will let glucose go to refill the liver and replenish its stores last. My point before was that low-carb and high-intensity don’t work together well for prolonged events. Sheri Colberg

by the way I meant “glycogen” above, not glucagon. I’m a PhD biochemist too. And I teach and publish in metabolism, no less! Dumb mistake. However, I’m new to this diabetes stuff having been a type 1 for only 2.5 years. It’s SO confusing hearing the different camps about diet - Bernstein, ADA, etc.,

I respect your advice on these issues, as I know you collect data and anecdotes from people about exercise. I do get great #s on low-carb and it’s how I lost weight initially. I’d LOVE to eat more carbs but I tried that. I needed more insulin and I gained weight.

On the other hand, having recently gotten back on the low-carb bandwagon, I feel very little energy for intense workouts. I don’t know if I need to re-adapt or what. You say low-carb diets aren’t good for sustained exercise. I have only noticed a difference with my high-intensity exercise. (Is there any other kind of exercise? It seems like we’ve covered all exercise). Do you recommend that I increase carbs to feel more energetic? Past experience has said to keep good numbers, more carbs = more insulin = weight gain.

But anyway, biochemistry question: I don’t know what you mean by if I exercise in the evening I’m “retaining my heightened insulin action overnight”. Am I not always retaining my insulin action? How would I lose my insulin action? When did my insulin action get heightened? Isn’t it a good thing if my glycogen synthase enzymes are activated?

Also, are you saying I shouldn’t work out at night? Or that I should work out and then eat carbs to replenish glycogen. Bernstein says diabetics shouldn’t work out in the morning. (sigh). I personally think I should work out whenever the mood strikes because the mood doesn’t strike often. :slight_smile: :slight_smile: And when I find the time because time is scarce!

I’d so welcome and appreciate your experience and advice.
Best wishes,

I’m not a biochemist, but this has been my experience with low carb & exercise in case it’s of any use to you.

Exercising in the morning sends my BG high, really high. A continuation or intensification of the dawn phenonmenon.

Intense, prolonged exercise sends me high followed by a sharp drop later. This appears to happen to Type 1s. Adrenaline. Since I don’t have a pump to adjust basal rates & hate the roller coaster of high-low, I do less strenuous exercise. Yoga, weights, resistance, walking, dancing. This keeps me more level & builds muscle to help with insulin sensitivity. It’s not super heart pumping cardio, but it’s a good workout. I tried slightly more carbs, then less insulin, snacking half-way through exercising, doing shorter work-outs. None of these worked for me.

I say, work out whenever it works into your schedule. You are right, though, that it’s harder to control your BG in AM when cortisol levels and insulin resistance are higher. What I have found is that before breakfast exercise requires me to take a small amount of insulin, with or without food. After breakfast exercise will lower BG levels, and if you take insulin for breakfast and then exercise, you’ll have to cut back on the insulin a bit. Exercising any other time of day is less likely to raise your BG when you do moderate to intense workouts. If you do an intense workout at any time of day, follow it up with at least 10 minute of less intense cardio work to help use up some of the excess glucose released by adrenaline and the other glucose-raising hormones. Their release is intensity-dependent, and it goes up exponentially with intense work.
As for the insulin action question, your insulin action changed all the time, but it tends to be highest right after a workout. One of the main contributors to a heightened action is low glycogen levels; as your glycogen becomes replenished, the insulin action diminishes. If you’re glycogen is full up, you’ll be very insulin resistant in your muscles, but fat cells are still reactive to insulin then and are happy to store excess carbs as fat. Keeping those enzymes activated overnight is okay unless you’re prone to lows overnight, and then you’ll either have to lower your insulin overnight or snack extra, which can cause weight gain. Hope this explanation helps. Sheri

Gerri and Sheri,
These are so helpful. Thank you!!!


Second that! The discussion here has been really helpful because as Maria pointed out there are so many opinions about diet… so it’s good to hear about people’s personal experiences but also learn about what’s happening and why… all very helpful in trying to figure out what to eat to maintain good numbers but also to be able to do endurance training and hopefully to compete