ATHLETE x SCIENCE
ATHLETE x SCIENCE
The debate between manipulating carbohydrate and fat metabolism for various weight loss and performance outcomes has gone back and forth by researchers for several decades, with no conclusive evidence supporting the extreme elimination diets we see so heavily marketed today. In the 1990s, high-carbohydrate nutrition was favored by the sports nutrition guidelines, recommending that at least 55% of energy come from carbohydrates in a given day (Burke, 2010). For endurance athletes, this number was recommended at greater than 60%; however, the research failed to support ‘why’ athletes needed this sort of macronutrient ratio for training (Burke, 2010). The evidence came after the millennium when it was found that higher carbohydrate intake could reduce (though not completely prevent) overreaching stress symptoms such as fatigue, sleeplessness, hormone disruption, and sub-par performance (Burke, 2010). In fact, withholding carbohydrates during the first few hours of recovery may hinder the functionality of the immune system and accentuate the immunosuppression occurring post-exercise (Burke, 2010). In addition, it was not found that moderate carbohydrate intake provided performance enhancement over a high-carbohydrate intake, so the guidelines remain in favor of carbohydrate availability for training purposes (Burke, 2010). High-fat nutrition has renewed interest once again, but is there enough to support a case for today’s endurance athlete?
The availability of a given substrate in the body largely determines our body’s fuel of choice at rest (Spriet, 2014). Exercise increases the metabolic demand on the body several-fold upon beginning a training session from rest, after which the body strives to achieve a steady state of aerobic intensity where the proportion of carbohydrates and fats finds an equilibrium in relation to an individual’s preference of fuel source (Spriet, 2014). When the power output of exercise exceeds 60% of maximal oxygen uptake (VO2max), studies have shown a decreased reliance on fat oxidation as a fuel source (Spriet, 2014). This decrease in free fatty acid release at higher intensities is likely due to a diversion of blood flow from adipose tissue to contracting muscles (Spriet, 2014). Above 75% of the VO2max, the majority of energy is derived from carbohydrate use, specifically muscle glycogen, in moderately trained individuals (Spiret, 2014). This is an important concept to consider when an endurance athlete is aiming to compete at 70-75% of maximum for extended periods of competition. The question remains whether or not it is possible to “teach” the body how to metabolically prefer fat as a fuel source at these competition intensities, which would go against the “default,” so-to-speak, of our innate preference for carbohydrates at these speeds.
Carbohydrates have gotten a bad rap for supposedly contributing to an ever-growing trend of obesity and metabolic syndrome in our country. On a physiologic level, carbohydrate intake results in a release of insulin by the body to help shunt glucose into depleted cells, or in the case of an inactive population, into the fat cells for conversion and storage, resulting in excess weight gain. Upregulated insulin inadvertently inhibits the transfer of fat across membranes, blocking fat oxidation (also known as lipolysis) during exercise and even at rest (Spriet, 2014). The reverse is true also: in the presence of high-fat, carbohydrate metabolism is down-regulated (Hawley & Leckey, 2015). The proposed theories of high-fat, low-carb exploit this physiologic mechanism as a way to increase fatty acid oxidation at the expense of restricting carbohydrate intake. The attraction to high-fat, low-carb diets for athletes has recently caught the attention of many through the media highlights of any given elite who has successfully clinched a podium spot in a championship, purely by running on a ketogenic diet or the like. While these performances are being attributed to the nutritional habits of these athletes, the research says there is no correlation between increased fat oxidation and performance (Hawley & Leckey, 2015). Carbohydrate-, not fat-based fuels, are the rate limiting factor in performance in trained endurance athletes (Hawley & Leckey, 2015). Fat-rich diets directly impair rates of muscle glycogenolysis, limiting high-intensity ATP-production necessary for energy at these paces (Hawley & Leckey, 2015).
Continue reading the full article and other sport science research at https://www.freelapusa.com/keto-or-not-to-keto-that-is-the-question/