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Ask Stacy: Dealing with Dietary Restrictions, Avoiding Under-Eating, and Analyzing Sweat Output

Leading nutritionist and exercise physiologist Dr. Stacy Sims answers your questions each month on

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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 second 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 for those training and racing with dietary restrictions, she reveals how you can tell if you’re tired due to lack of sleep versus lack of adequate food, and she debunks a few myths about sweat rate…

1. What is your best nutrition advice for those with dietary restrictions, such as gluten or dairy?

There are many nuances to dietary restrictions and it’s important to establish why you are following a certain diet before you can truly determine the “how.” For example, dietary restrictions could be medical (celiac disease or allergies), cultural (such as plant based) or part of a trend that you’re interested in trying (such as keto or Paleo). Whatever the cause, you want to ensure you are still taking in adequate nutrients to help fuel your training, racing, and recovery. If, for example, you are truly celiac and cannot have gluten, do not just blindly reach for gluten-free products. Gluten-free foods are often high in sugar, refined carbohydrates, and other fillers to bulk up the products to assume the texture and mouth-feel of the foods they are replacing. It is always better to have a real food alternative wherever possible. Do you love toast? Try sweet potato toast slices. Simply slice a parboiled sweet potato length-wise and pop it in the toaster! Seriously, it works!

Athletes with celiac disease should try to minimize inflammation and poor absorption of minerals and vitamins. Make sure you have a wide variety of fruit, vegetables, lean protein, and naturally gluten-free whole grains to dampen inflammation, improve the nutrient density of your diet, and provide the much needed prebiotics for your gut bacteria.

When it comes to dietary restrictions that are culturally driven—such as a plant-based diet—similar guidelines apply. Are you getting enough carbohydrates to fuel your training and recovery? Are you eating adequate protein, particularly quality leucine, for muscle protein synthesis and post-exercise repair? Healthy fats like avocados, nuts, and seeds contribute to the micro-nutrient density as well as providing essential fatty acids.

As for trends that lead to dietary restrictions, there is a plethora of data to refute and rebut the benefits of each one, but I won’t get into that here. What I will say is that if you are consciously restricting whole food groups to gain a competitive edge, ask yourself if it’s really working—or if it’s more to do with the fact you’ve cleaned up your diet. Is that the real reason that you are now leaner, faster, fitter, sleeping better and/or recovering better? Often just taking out the processed foods from your diet can improve overall health, reduce inflammation, improve gut health, sleep, and ultimately your training and performance. Always go back to the basics: Are you eating enough carbohydrate, fat, and protein to fulfill your health and training needs? Additionally, are you hitting the nutrient density mark of whole foods for gut health and getting enough micro nutrients for the increased demands of being an endurance athlete?

2. How do you tell if you’re tired from under-eating versus under-sleeping?

Poor sleep and inadequate fueling/nutrition can both contribute to that “dead to the core” feeling, but there are ways of discerning if it is due to one rather than the other. We know that under-fueling is a form of stress that can negatively impact training and health. Food provides the nutrients the body needs to rebuild and repair muscle tissue, replace glycogen (carbohydrate) stores, maintain immune health, and keep vitamin and mineral stores within a healthy range. Fueling adequately becomes challenging for athletes with a sudden uptick in training loads (volume and/or intensity) because appetite becomes muted and nutritional needs alter with the change in training as well. Often endurance athletes underestimate how many calories they need, and often sacrifice carbohydrate in our typically carb-phobic sports environment. When we underfuel, we increase overall stress, reducing the body’s ability to recover and adapt (basically all that effort you put into your 6 a.m. swim squad won’t be absorbed). Signs of underfueling include:


Fatigue, low energy levels throughout the day with a lack of mojo for training
Intense cravings or constant focus on food
Trouble sleeping
Hunger pangs during workouts
Gastrointestinal (GI) distress
Muscle cramps or weakness


Hypothermia (cold intolerance)
Stress fractures
Changes in, or complete loss of, a regular menstrual cycle (for women)
Low testosterone (for men)
Significant weight change (gain or loss; usually abdominal fat gain)

Many of us have lost the connection of what “sleepiness” feels like. Sure, we feel tired and fatigued, and often fall asleep in front of the TV at night, but chronic sleep loss can lead to a wrath of issues, both for health and performance. Fatigue from lack of sleep is marked by extreme drowsiness and an extreme desire to fall asleep. Feelings of sleepiness increase the longer a person stays awake. This has to do with the build-up of a chemical in the brain called adenosine. It’s a signal that we need sleep. Fatigue from low sleep is remedied by (you guessed it!) getting sleep.

The flip side is that fatigue from low sleep, under-fueling, and hard training is often not relieved by getting a few good nights of sleep. That “dead to the core” feeling stays with you. If this happens, you need to revisit your fueling in and around your training, as well as your training itself. To adapt you need to fuel and recover, with most of the full recovery occurring while you sleep.

3. How important is it to know your sweat output and composition?

It isn’t. Let me quantify this. There has been an incredible upsurge of companies offering sweat composition analyses, sweat rate analyses, and solutions for the endurance athlete. Unless you are tested in a real-time race situation with proper air flow, environmental conditions, stress, and sensors that actually work (and don’t just measure the composition of sweat of the area under the sensor!), then it is better to put your time and effort into nailing your nutrition and hydration strategies based on your own individual profile (male or female, and if female, what phase of the menstrual cycle you are in or if you are peri/post menopausal); where you are in your training cycle and what season it is. For example, have you naturally adapted to the heat because it is summer, or is it the cusp of the season and heat is still foreign to you?

There is no direct relationship between sweat-electrolyte composition and what you need, especially with sodium. If you are eating foods and drinking functional hydration (one of the low carbohydrate, higher electrolyte drinks, not liquid calories), then you will be consuming what you need.

Sodium is used for fluid uptake, and the mistake I see so many triathletes making is using salt tablets and/or trying to drink to match sweat rates. First, you can’t stop dehydration, you can only slow it down. Gastric emptying rates and intestinal osmolality have a lot to do with the amount of fluid you can handle and absorb per hour of activity, which is why you want to have a drink that works with your physiology, not against it.

Salt tablets have become endemic on race courses due to the myth that sodium is needed in long distance events. Yes, it is, but not as concentrated salt tablets. Firstly, there is a sex difference in blood sodium levels during exercise: men become hypernatremic (high sodium levels), while women either stay eunatremic (no change) or become hyponatremic (low blood sodium levels). If you become or are on the verge of hyponatremia, then yes, concentrated sodium is needed; but for everyone else, your food and fluid will keep you on track. The key for moderating hydration levels is to keep sipping on a functional hydration drink, and know that you will need more on hot days and for early season races.

dietary restrictions
Submit your questions for Dr. Stacy Sims at