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For runners, is Metabolic Efficiency Training the new carb-loading?

Nutrition for endurance running has long held onto the idea that “topping off” your body’s carbohydrate stores (called glycogen) the night before the event is the best way to maximize performance on race day. But a new school of thought called “Metabolic Efficiency Training” (MET) has been gaining popularity in recent years.

If I was a betting woman, I would put my money down that on the night before the Broad Street Run, there is more pasta consumed in Philadelphia than on any other day of the year. But does the old pasta party and carb-loading idea still stand true?

Nutrition for endurance running has long held onto the idea that "topping off" your body's carbohydrate stores (called glycogen) the night before the event is the best way to maximize performance on race day.  But a new school of thought called "Metabolic Efficiency Training" (MET) has been gaining popularity in recent years.  MET is the idea of training your body to use fat as an efficient fuel source during an endurance event instead of carbohydrate energy. This new theory has been made famous by Bob Seebohar, a certified sports dietitian, and an official dietitian for U.S. Olympic team.  Seebohar's philosophy promotes a more modest amount of carbohydrates, with higher levels of protein and fats with nutritional value.

So what's a Broad Street runner to do?  Let me explain a little bit more…

During periods of lower intensity training, the body is able to convert fat to use as energy.  When exercise intensity increases, the body prefers to use carbohydrates for fuel. The human body typically holds about 80-110 mmol/kg of glycogen (the stored form of carbohydrate) for energy use.  A runner who has carb-loaded can pack up to 200 mmol/kg of glycogen, potentially offering a longer period of sustained energy before your body hits a "wall" in an endurance race (often noted as over 90 minutes).  Other than an extra source of energy, carbohydrates also help the body hold on to additional water, necessary for hydration during a race.  Inadequately fueled and hydrated muscles can lead to early fatigue.

The MET model hinges on training the body to use fat as an energy source for a longer period of time before the body has to switch over to the use of your carbohydrate energy stores.  What does a MET nutrition plate look like? The priority on your plate is a lean protein and an omega-3 rich fat source (think nuts, fatty-fish, and flaxseed, to name a few).  The next largest section of your plate is for vegetables and fruits, and to save a small amount of room for whole grains.  It is by no means a "low-carb" diet, but still more modest than a typical diet that recommends you get 50-60 percent of your energy from carbs.

The potential benefits of the Metabolic Efficiency Training model are many — a more sustained energy source without constant carbohydrate replenishment, a stated potential for decreasing body fat, etc. However, if you are looking to convert to MET, the final week of your Broad Street Run training is not the time to try it. A drastic change in your diet the week before a race is a set up for an unhappy GI tract and poor performance. Seebohar recommends starting during a period of less intense training, and transitioning over a period of a few weeks.  Start with making small changes to each meal, and choose snacks rich in protein and omega-3s.

Since metabolic efficiency is not a healthy transition to make the week before a race, then it's a green light to go crazy with the carbs, right?  Not so fast.  Adequate carbohydrates have been shown to help maintain performance, but current research indicates that there is a limit: Too few or too many carbs can actually decrease physical function.

Sports nutrition specialist Nancy Clark recommends eating some extra carbohydrates within two days before the race, but not overstuffing yourself.  A good goal is about 60 percent of your meal from a mix of healthy carbohydrates - grains, dairy, fruits, and vegetables.

Instead of a big meal the night before the race, consider making breakfast or lunch the biggest meal of the day.  This will allow your body enough time to process the carbs, instead of doing all the digesting and absorbing in the12 hours before the race.

The morning of the race, have a small, easy-to-digest breakfast that contains some carbohydrates and protein.  Your body will use these carbs first before tapping into your glycogen stores.  My fail-proof race day breakfast?  Toast with a small amount of peanut butter and half of a banana, coffee, and water.  For more pre-race meal ideas, see our comprehensive 1 Week to Broad Street Menu Plan.

See you at the race, fellow runners. Good luck!

Beth Wallace Smith, a registered dietitian at the Children's Hospital of Philadelphia, has more than eight years of experience in providing nutrition care for adolescents and children. She earned her degree in dietetics from the University of Connecticut, and is completing a masters degree in communications from Johns Hopkins University.