Hydration in Marathon & Half-Marathon Training

Pittsburgh Dietitian Blog Posts
March 25, 2015

Over the last 4 weeks we have investigated fueling for sport in terms of energy. While energy is mandatory for performance to occur, there is another equally important area of nutrition that dictates your ability to perform well. The general term hydration refers our body’s need to maintain proper fluid balance. Fluid balance involves proper intake of water, as well as management of the main electrolytes sodium, chloride, potassium, calcium and magnesium. While I do not plan on going into detail about what each of these do in our body, I do hope to leave you feeling confident about consuming adequate amounts of each from food daily, as well as replenishing them during and after a physical demand such as a half-marathon or marathon.Daily fluid needs is widely underestimated in the general population. I find most of my clients still cite 8 cups per day as adequate intake of fluid daily. The truth is, this 64 fl. oz. recommendation is an estimate of needs to prevent dehydration in an “average-sized”, sedentary adult. While a large majority of our population now meets the term sedentary, average-sized is less applicable. Not only have we grown as a population since the 1950’s and 60’s (inches taller and pounds heavier), we also have to factor in gender, age and muscle mass before we make recommendations. Since this blog is devoted to those of us training for a half-marathon or marathon, the 8 cups per day rule falls drastically short of fulfilling our needs. The Institute of Medicine (IOM) recommends 95 fl. oz. of fluid per day for women and 130 fl. oz. of fluid per day for men, before sweat loss calculations. These numbers will vary slightly depending on age, height, weight and body composition, but serves as a better threshold for hydration.The recommendations above do not account for fluid needs during and after activity. Fluid needs during exercise will vary based on the conditions (temperature, humidity), as well as the sweat rate of the individual. Generally, 7-10 fl. oz. per 30 min of activity will keep an individual hydrated during moderate-intensity activity. For those who sweat more than the average person, or those training in hot and humid environments, fluids needs can easily exceed 16 fl. oz. per 30 minutes. Additional fluid needed is based on the sweat rate, or amount of water lost during exercise. This can be calculated fairly easily by simply weighing yourself before and after activity to determine the pounds (or kg) lost during activity. For every pound lost, rehydrate with 24 fl. oz. of water (1.5x the weight lost). For more accurate assessment of weight lost, sweat rate considers water consumed during training, and deducts water lost through urine output. This equation is typically reserved for professional athletes or extreme sweaters. For the rest of us, simply rehydrating using 1.5x pounds lost during the run will ensure adequate fluid replenishment. Without actually performing all the calculations, it is pretty clear that most women will need at least 1 gallon of fluid (128 fl. oz.) on training days, and men will likely exceed 160 fl. oz. to maintain optimum levels of hydration!With all this water going in and coming out, what else do we need to consider? First is carbohydrate intake (for those of us fueling with carbohydrates). For the half-marathoners in need of a small energy jolt, or the full marathoners in need of calories that are convenient and easy to consume, sports beverage is a popular choice for obvious reasons. It contains a balanced amount of sugar (~6-8% of product), has appealing taste, and provides the fluid needed. Sports drinks are a choice that cover all of the needs of an athlete in one palatable form. For those running the half-marathon, taking in sports drink during the race will satisfy your body’s needs for fluid and carbohydrates. In addition to carbohydrate, these beverages include the other necessary component of hydration, in the form of a pre-balanced electrolyte profile based on the needs of an active and sweating individual.Electrolyte balance is a largely misunderstood component of proper hydration. While it is true most people in the United States under consume potassium and overconsume sodium, this is not always the case for endurance athletes. Truthfully, potassium deficiency is a result of a lack of potassium in the diet. Contrary to popular belief, we lose very little potassium when we exercise. The same goes for calcium and magnesium. These three electrolytes present as a problem when there is not enough consumed daily. The needs of these nutrients in athletes is not significantly different compared to the general population. Sodium and chloride on the other hand, vary greatly. Both sodium and chloride (the elements that make up table salt) are lost in our sweat in large quantity. In hot training conditions, the amount of sodium and chloride lost can exceed our recommended daily intake of 1500mg and 2300mg (respectively), effectively doubling the need of both for an athlete in a given training day. Many of the symptoms of low sodium and chloride levels in the blood are commonly recognized as symptoms of low potassium, so it is important to consume adequate amounts of all three, and be in tune with what your body truly needs. The next time you feel fatigued, nauseous, have a headache, or get muscle spasms, recall your intake of sodium, potassium and chloride for the day. If you are recovering from a workout during which you lost a lot of sweat, chances are you’re low in sodium and/or chloride. I often say the salt shaker can be a runner’s best friend, and for good reason!Electrolyte loss during exercise can be detrimental to performance and is dangerous at all levels. For extended activities, electrolyte replenishment is mandatory. While most utilize sport beverages, which are premade and formulated with the balance considered ideal, many athletes find other combinations that meet the same criteria. The components of a sports beverage are:1. Fluid (water)2. Sugar (usually 60-80g of carbohydrate per 32 fl. oz.)3. Electrolytes (500-700mg of Sodium and Chloride, 300mg Potassium per 32 fl. oz.)For those interested in replicating the balance of a sports drink without the refined sugar, the above can be accomplished with 32 fl. oz. of water, ¼ tsp of salt, and a variety of carb containing snacks to be consumed throughout the run including items such as dried apricots or banana for potassium, portability and palatability. The priority is to make sure you are consuming water AND electrolytes during the activity. Water by itself is not adequate for activities lasting longer than 1 hour. We need both to function and perform at our best. Before and after the training or event, make sure your meal has a source of sodium, chloride and potassium in it. Sodium and chloride are easily attained in our diet using salt either naturally occurring or added. Potassium is abundant, but often under consumed. Here are your best sources:Best Potassium Sources: (DRI 4700mg per Day)• Banana (450mg in 1 med banana)• Orange/Orange Juice (350mg in 6oz)• Milk (380mg in 8oz) and Yogurt (550-600mg in 8oz)• Avocados (700mg in 1 cup)• Prunes (500mg in ¾ cup)• Sweet Potatoes (700mg in 1 med-sized potato)• Carrots (500mg in ¾ cup carrot juice)• Squash (450mg per ½ cup)• Tomatoes and Tomato Sauces (300-650mg depending on concentration)• Beans (lima, kidney, soy) (300-500mg per ½ cup)• Beet Greens and other Dark Greens (400-650mg per ½ cup)Pick your fuel source and create a strategy for hydrating before, during and after your event. Attention to fluid intake and electrolyte balance will help ensure your success come race day!

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