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Ask Stacy: Fasted Training, Avoiding GI Distress, and Managing Carb Intake

Dr. Stacy Sims gives her advice on fasted training, the best ways to avoid GI distress during an Ironman, and ideal carb intake in the lead-up to a race.

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When it comes to sports nutrition, you’ll be hard pressed to find an expert with as much academic and in-the-field experience as Dr. Stacy Sims. A leading nutritionist and exercise physiologist who has worked with hundreds of professional athletes and age-groupers, Sims is a go-to resource on hydration, nutrition, and sex differences. In 2017, she was named as one of the top four people changing the landscape in triathlon nutrition—and today we have her third column here at Each month Dr. Sims will be answering readers’ questions, which you can submit to us at In this installment, Sims gives her advice on fasted training, the best ways to avoid GI distress during an Ironman, and ideal carb intake in the lead-up to a race.

Is it better to train fasted or after eating?

There are many trends that have led to people thinking that fasted training is the way to go, primarily due to the buzz of “training low” increasing “metabolic efficiency” for improved performance, and, of course, intermittent fasting. It is important to highlight that there are some key differences based on gender, so fasted training for men can lead to different results than for women. For men who are looking to manage their weight, there may be a slight benefit to fasted training. Research in the past couple of years has demonstrated that in young, healthy men, fasting before morning exercise decreased their 24-hour energy intake and increased fat oxidation during exercise. For performance, however, it is a different story. Fasted training does enhance fat metabolism (oxidation) in well-trained male endurance athletes, yet this might not translate to improved performance when exercising at moderate to high intensities (as is required in triathlon racing). Women, however, do not respond to fasted training in the same manner as men. Women do not have an increase in oxidative activity (fat utilization) of the muscle as a result of fasted training, but do incur a greater post-exercise immune stress and greater post-exercise inflammation as compared to men. They also show an increase in muscle tissue damage and breakdown (leading to potential losses in lean mass). The theory is that since estrogen promotes fatty acid utilization, women are already at a maximum capacity for fat oxidation.

Both men and women need to consider the implications of fasted training on their actual training session, as low glycogen induces fatigue earlier (this could result in having to shorten a training session) and hitting high intensity intervals is compromised. The risk of low energy availability is also increased, especially if two sessions a day are in order, which is fairly commonplace for many triathletes.

The short answer? Fuel for your training and recover well, then look to modify your diet elsewhere to garner greater health outcomes.

RELATED: The Pros and Cons of Fasted Workouts

What’s the best nutrition to use for Ironman if I struggle with GI distress when I take gels?

Ah, GI distress. Most people think GI issues are nutritional, but the nutrition aspect is minimal compared to the two main contributors, which are ischemia and mechanical/positional changes. Firstly, let’s tackle ischemia, which is a restriction in blood supply to tissues, causing a shortage of oxygen. This happens with the reduced blood flow that naturally occurs during exercise and it can cause mild injury to the intestinal lining, changing GI permeability (increasing the ability for things to go through the gut lining/cells). This can trigger nausea, bowel movements, and gas.

The second main contributor to GI distress is mechanical/positional changes in the body. Running is high impact and places repetitive stress on the intestines, which can cause lower GI symptoms—gas, diarrhea, and urgency.

When we look at Ironman, there is a very high incidence of GI distress, most likely due to a combination of the two issues above—plus the fact we tend to add in a concentrated carbohydrate (e.g. a gel) when the gut is already under extreme stress. This can often be the tipping point for the porta-potty shuffle. Viable alternatives to gels are largely dependent on the individual, the environmental conditions (heat increases the stress and thus gut motility), hydration status (how dehydrated are you?), and if you are a male or female. For men, it is slightly easier to use dried fruit and/or traditional sports foods with a maltodextrin and fructose base, but women absorb less fructose than men, thus fructose can also perpetuate GI distress.

It is better to approach Ironman nutrition in stages because what the gut can handle will change as the day progresses. What you are able to eat on the bike can be quite different to what you can stomach on the run. It is typically easier to ingest more solid food on the bike that your body can then draw from on the run. If and when you hit a low point on the bike (we all have them!) try popping a glucose tablet or two, just to boost your blood sugar, without affecting digestion. As your stomach settles a bit, you can then start eating again. On the run, for the first third of the marathon you can use on-course nutrition, such as bites of pretzels or potato chips (for salt and carb) and watermelon. It’s also worth experimenting (in your training beforehand) with your own energy bites, diluted salted maple syrup, or salted sweet potato bites—and if any of these work for you then carry these in your pockets and aim to eat them during the first third of the marathon. Hold off on chews/gummy candies until the second half of the run, if possible, and in the last six miles or so, look to keep blood sugar elevated with glucose tablets (and cola, if you’re using it). It sounds more complicated than it actually is, but your gut will thank you!

How much should you increase carb intake before a 70.3 or Ironman race to see benefits?

In the days before the race you should make a concentrated effort to fuel well (before and after) each training session, even if it is short. The goal here is to maximize liver and muscle glycogen. In the old days, extreme carbo-loading regimes were followed with days of no carbohydrate, days of extreme carbohydrate, a depletion run a week before, then trying to overcompensate by overeating carbohydrates. This practice is not necessary. Very high muscle glycogen levels can be achieved by just eating more carbohydrates throughout your taper week. Eating more carbohydrate does not mean overeating or eating as much as possible. It just means making sure more of your daily calories are coming from carbohydrate at the cost of some fat. It is a good idea to have the last large meal at lunch time the day before your race and to have a lighter meal on race eve. Note that your normal dietary practices can influence glycogen storage, so make sure you are getting adequate carbohydrates to fuel for your training, recovery, and health across the build-up—then add in a bit more (as per above) during your taper week.

You can find out more about Dr. Stacy Sims here, including details of her new seven-week online course, Women Are Not Small Men. 

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