Runners: To Carb Load or Not to Carb Load

Dr. Sears on Carb Loading Before a Race

By Mary Perry, Director of Clinical Trials: 

Eating a giant bowl of spaghetti, three bagels and a couple of baked potatoes right before a big event is actually hurting your athletic performance. According to Dr. Sears, the idea that you should load up on carbohydrates the night before a race is a myth.


Mary Perry talks about what inspired her to train for three marathons and conveys training nutrition best practices, while sharing Dr. Sears’ expertise on the concept of carb loading for athletes.

My first job in Boston was located at about the 24-mile mark on the Boston Marathon route, not far from the Citgo sign at Fenway. In April, during the big race, my coworkers and I would take some time to go and stand on the sidelines looking for names on t-shirts, or something to identify individuals, so we could cheer them on as they went on to complete the last 2.2 miles.

Watching all of these runners with various levels of athleticism, spanning the ages of 20-70+, running either for charity or because they had qualifying times was absolutely inspiring. I saw that with the appropriate training, fueling, and determination that they didn’t have to be born runners to do this. This became my motivation to train and complete three marathons myself.

What I learned about performance nutrition during my training

It can be an individual journey involving a lot of trial and error to figure out what your body needs, when it needs it, and how to stay adequately hydrated, especially when Mother Nature is involved. I remember during my first marathon I was shocked to see that my weight was creeping up during my training instead of going down despite my activity level.

Looking back, it was a combination of not being as active on the days I trained due to soreness and fatigue, relying heavily on carbohydrates, and thinking I needed more calories than I actually did for all this activity. I began training for my second marathon shortly after starting my job at Zone Labs. Although at that point, I didn’t know what I know today, that even simple changes like decreasing carbohydrates and bumping up protein helped me to train smarter.

Despite having an injury and having to spend the last two months doing non-running activities, I was able to get through my second marathon without gaining weight and optimizing how I fueled. Although I’ll never see a qualifying time for the Boston Marathon, I was able to shave 27 minutes off my first marathon, which was my personal best.

With more and more individuals taking part in endurance events, whether they participate in marathons, triathlons, or the Ironman, there is a lot of advice out there from coaches, the internet, and apps on how people should train and fuel their bodies in order to successfully complete these races. Yet, an internet search may not be the best way to learn about good training nutrition.

Carbs are good, but must be balanced

According to Dr. Barry Sears,  carbohydrates play an important role in athletic performance, but their over-consumption limits our body’s ability to use fat as an energy source. Fat should be considered “high-octane” fuel as you can generate far more energy (i.e. ATP production) from a gram of fat compared to a gram of carbohydrate. To maximize energy production, you want the enzymes in the mitochondria (the site in the cell that produces ATP) to have the maximum ability to burn both fat and carbohydrates. Being able to switch between carbohydrates and fats as an energy source is called “metabolic flexibility.” You can increase or decrease your metabolic flexibility with the composition of your diet.

Depending on the balance of carbohydrates and fat in the diet, the mitochondria will adapt their enzyme composition to maximize energy production based on which fuel it is able to access. If you are eating a high-carbohydrate diet, the mitochondria adapt by making more of the carbohydrate-metabolizing enzymes and less of the fat-metabolizing enzymes. The end result is that you are stuck using glucose, which is, for an athlete, the equivalent of putting low octane fuel into a Maserati. The results are much better using fat or high-octane fuel. So if you want the greatest metabolic flexibility your diet should consist of relatively equal intakes of both carbohydrates and fat.

Balancing your hormones play a role in sports nutrition

Dr. Sears further believes the balance of the hormones insulin and glucagon play a role in good sports nutrition. If you are eating a high-carbohydrate, low-protein diet, your blood sugar levels will constantly rise and fall. Carbohydrates stimulate insulin secretion which drives down blood sugar levels, whereas protein stimulates glucagon secretion which increases your blood sugar levels. You need a balance of the two so that blood sugar levels remain stable. This is why as an endurance athlete you have to work to balance your blood sugar levels at each and every meal as well as maintain metabolic flexibility. It may sound difficult, but it’s pretty easy.

How to get into the performance Zone

To maximize performance, a good diet leading up to race day is one that contains about equal levels of carbohydrate, protein, and fat on a caloric basis. The ideal diet is the Zone Diet™ because it is based upon your protein requirements, and from that knowledge you can automatically generate the amount of carbohydrate and fat you need for metabolic flexibility as well as maintain stable blood glucose levels.

Your diet should be comprised of colorful vegetables and fruits, adequate amounts of lean protein to maintain muscle mass, and enough good fats like those found in olive oil, nuts and seeds to displace omega-6 and saturated fats known to cause inflammation. Eating like this will give your body the metabolic flexibility it needs to convert whatever fuel you use on race day into the energy you need to compete at your highest level.

By following The Zone Diet, and including Zone PastaRx in your diet instead of your old carb loading, you will develop a powerful advantage over your competitors.

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About Mary Perry

Mary is a registered dietitian and researcher who serves as the clinical trials director for both Zone Labs and the Inflammation Research Foundation (IRF). Mary oversees all clinical trials, investigating the positive effects of anti-inflammatory diets in conjunction with leading universities and institutions worldwide. Mary served as a research dietitian for Brigham and Women's Hospital in Boston, MA. In that role, she served as one of the nutrition and behavior counselors for patients on a major two-year dietary study focused on varying carbohydrate, fat and protein contents to determine the most effective diet for weight loss and maintenance. Mary completed her undergraduate and master's degrees at the University of Connecticut.


  1. Bob Armstrong

    I have often wondered if Dr Sears has ever compiled in one book, chapter or article his history, stories, outcomes and benefits of athletes who converted to a “zonal diet “. I believe such a source would be very motivational in encouraging athletes to get in the zone,compete at a higher level, and be much healthier.

    • Dr. Sears

      Thanks so much for your comment. Over the course of my career, I have been fortunate to work with a spectrum of athletes from Olympians to weekend warriors. In my first book, Enter the Zone, I dedicated a chapter to work with Elite athletes chronicling the various teams and their successes on the Zone, but also the trial and error learned along the way in order to tailor their diets for success. In Mastering the Zone, I wrote on the work with the 1996 Olympic athletes and included menus for both male and female athletes. Over the years, I have continued to build on earlier work with the Zone Diet to include research on the use of omega-3 fatty acids and polyphenols to further enhance athletic performance by reducing inflammation and improving recovery.

      I agree it could be fun to have all our athlete success stories in one place!

  2. Patsy Ward

    My Daughter obrained a couple boxes of MRE’S (meals ready to eat). Canadian army fuel. Each breakfast includes a good supply of protein and a “soldier fuel” bar that we’d take on our hikes for ” lunch ” . The bar is carbs, fat & protien – not a heavy lunch but satisfying & keeps you walking.

    • Edward Kashuba

      We appreciate Dr Sears input on quality fish oils! (1) I am wondering about the fish oil :”Omega 3 Acid Ethyl Esters”the Generic prescription for Lavaza.” The cost is much less than the cost of Lavaza. I have been having much trouble in keeping the prescription accurate through the CVS Drugstore chain! A quality experience should be the main thing.
      I am experiencing much stress because my wife and I do not agree about diet. Even though we both studied The Zone, she still believes in a high fat and high Carb eating plan…with frys and gravy. Her type 2 Diabetes is now Diabetes…with insulin and pills as her choice. She would not read her doctors book (pamphlet) “The Metabolic Syndrome” with a good section on “glycmic load. The book on foods is much like Zone thinking; though Dr Baily who is quite eccentric…does not like Dr. Sears. Also Carol does not think highly of Dr. Sears or Dr. Bailey. Who needs the Shrink me after age 80, Carol, or Dr. Bailey?
      Carol at at 79 is five feet three 197 pounds and unconcerned.

      • Barry Sears

        likable is now a generic product. The standards for selling a prescription drug form of Lovaza are up to 10 times less rigid that the minimum standards we set for OmegaRx. Furthermore, Zone Labs is the only company in the world that posts the levels of PCBs in every omega-3 fatty acid product we sell. You can check out the purity and cost of any fish oil product (including Lovaza) using our fish oil calculator at will be very surprised by the results.

        Even though your wife might not like Dr. Bailey or myself (and I consider myself a very likible person), she would definitely enjoy using Zone PastaRx, which was developed and clinically tested to reduce insulin resistance. How does she knows it is working? So will not be hungry for 5 hours after eating one of our Zone Pasta recipes. Who knows, she might even start liking me.


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