A Poor Diet Will Affect Performance—Even If You Look "Fit"

How does a poor diet affect performance if it doesn’t make me gain weight?

A: A daily diet of pizza, cookies and Ben & Jerry’s (read: processed foods high in sugar and trans fat and low in fiber) affects more than your waistline—your lung capacity and immune system may suffer even if your skinny jeans still fit.

That’s because your nutritional status determines how effective your mitochondria, your cells’ powerhouses, are at producing energy. Key micronutrients like iron (found in beef, lentils, kidney beans and pumpkin seeds) help your blood transport oxygen to your working muscles. So if your diet is lacking in iron and other essential elements including the B vitamins, your muscles will not have the strength and endurance to perform optimally. And if you’ve always eaten poorly, you might not even know your true athletic potential; you may find you’ll sustain a quicker pace longer with better fueling.

You can also expect slower recovery time and increased soreness after intense workouts and races because a poor diet may not supply sufficient antioxidants, particularly vitamin C, which is found in high amounts in broccoli, sweet peppers and citrus, and vitamin E, which is found in olive oil, nuts and seeds. Your body needs antioxidants to prevent exercise-induced damage to cells.

Furthermore, the fluctuations in blood sugars and insulin from simple sugars like sweets and soda can result in an energy crash and cravings for more. A day including a blueberry muffin, a 20-ounce soda and a 32-ounce sweet tea adds up to 1 cup of sugar. The only time you may need that amount of simple sugar for quick energy is to sustain workouts or races lasting 6 hours or more. Daily meals of fast food, pizza and chips leave little room for the health-promoting, energizing meals that supply a balance of macronutrients and micronutrients required to feel your best and achieve true health inside and out.

Fill your engine with quality fuel, and your athletic performance will noticeably improve. Look for nutrient-rich carbohydrates (a variety of vegetables and fruit, sweet potatoes, winter squash and legumes), lean protein like chicken and fish and healthy fats like avocado and nuts.

Of course, there is room for small indulgences occasionally if you feel a need. Enjoy a square of dark chocolate or share that piece of pie once a week. When in doubt—or if you just need someone to tell you to slow your roll on the chocolate cake—seek the advice of a registered dietitian for a personalized plan for proper fueling.

RELATED: Triathlete’s Complete Guide to Nutrition and Fueling

Shelby Killion, a graduate of University of Texas at Austin, has been a registered dietitian for more than 28 years. She is an integrative and functional nutrition-certified practitioner, the owner of Food & Fit Solutions and co-author of the cookbook Healthy Tex-Mex: The South Texas Slim Down. She’s also a triathlete who’s competed in several half-iron-distance races.