Sep 30, 2015
Diet by DNA
Nanci S. Guest

As the emerging field of nutrigenomics grows, more and more athletes are looking to personalize their diets for performance gains. The first step is understanding the genetic testing process.

The following article appears in the October 2015 issue of Training & Conditioning.

Suppose two female collegiate endurance athletes with low ferritin start taking iron supplements to prevent anemia. They follow similar diets and adhere to the same training schedules. After three months, one still suffers from low iron and struggles with training, while the other responds well to training and has fully-stocked iron stores. Why would two similar athletes have such different outcomes with iron supplementation? The answer may lie in their genes.

While it has long been suspected that genetics play a critical role in determining how an athlete responds to foods and nutrients, only recently has research in the emerging field of “nutrigenomics” been able to demonstrate this scientifically. As a result, there has been an interest in using genetic testing to gain a precise understanding of how an athlete’s DNA could impact his or her response to micro- and macronutrients.

But what exactly goes into the genetic testing process? And how do you apply the findings? As nutrigenomics continues to grow, the tests have become more affordable, and athletes are increasingly wanting to personalize their nutrition plan. Here’s how to guide them down the right path.


Nutrigenomics uses genomic information and advanced technologies to uncover the relationship between genes, nutrition, and human health and performance. Research in this field has determined how an athlete’s genetic makeup affects the way they absorb, metabolize, and utilize nutrients, and how the interaction between genes and diet can influence their health and performance.

One of the advances originating from nutrigenomics is the use of genetic testing to assess genetic variations known as single nucleotide polymorphisms (SNPs). Each SNP represents a difference in a single DNA building block, called a nucleotide. All nucleotides have either a purine or pyrimidine base. In DNA, the purine bases are adenine (A) and guanine (G), while the pyrimidine bases are cytosine (C) and thymine (T). So an SNP may replace C with T in a certain stretch of DNA.

Genetic testing can identify areas in a gene where an athlete carries either a typical or elevated risk for a specific trait based on genetic variations that are related to their response and metabolism of certain macro- and micronutrients or other performance-related variables. For example, lactose intolerance and caffeine sensitivity are caused by SNPs.

In addition to health outcomes, genetic variations can be used to help athletes achieve an optimal body composition and gain the most from their nutrition and training and conditioning programs. For example, vitamin C can affect an athlete’s health and endurance fitness goals depending on how well they process it, which is determined by their genes. Vitamin C is an antioxidant that aids in reducing exercise-induced free radical damage, which translates into better recovery from training and greater resistance to fatigue. However, too much vitamin C can negatively impact how an athlete responds to training aimed at improving aerobic capacity, as some free radicals are necessary to stimulate adaptations. Some athletes have impaired utilization of vitamin C, so they are at a greater risk of depleted levels in their blood. Others may have very efficient processing and should be cautious with supplements. (See “Genetics of Training” below.)

Although much of the research is still in its early stages, obtaining an athlete’s genetic profile has already proven to be an effective tool in helping athletes make the best dietary choices. Testing will also enable those of us training and advising athletes to use nutrition to its fullest potential, tailoring balanced diets for optimal health and fitness.


If an athlete wants to undergo genetic testing to determine their performance-related needs, the first step is to find the right test. Many consumer genetic tests assess a wide variety of health factors beyond nutrition, so hone in on those that focus on diet and exercise. Most genetic tests can be purchased online directly by athletes, but others can only be obtained by a coach, athletic trainer, dietitian, or other sports professional who has become an authorized provider.

It is also important to get a genetic test from a reputable company. I have seen some companies offering tests that misinterpret the science, so a little research can go a long way here. Choose a company that can provide you with links to published, peer-reviewed journal articles on the specific genes being tested. In addition, look for companies that have a scientific advisory board consisting of credentialed professionals in nutrition and genetics.

A final tip is to be careful not to choose quantity over quality. Companies may offer testing for a large number of SNPs, but further investigation can reveal poor evidence supporting their science. Choose tests that guarantee all SNPs have the highest level of evidence to ensure accurate results. This might not be obvious to someone with little scientific training, which is why it’s often best to go with a company that uses trained or certified providers.

Once you’ve found a reputable company, you can order the test. Most kits will come with a vial and a liquid preservative. The athlete will need to spit about a quarter of a teaspoon of saliva into the tube and then shake it up to mix the sample with the preservative. The whole process takes 30 seconds. Once it’s complete, seal the vial and send it back to the company’s lab for analysis.

One test that I recommend to clients is the Nutrigenomix-Sport. Launched this fall by Nutrigenomix, Inc., a University of Toronto biotechnology start-up that I do research with, it’s one of the most comprehensive sport performance panels to date. The test evaluates approximately 45 SNPs related to sports nutrition, food intolerances, body composition, exercise physiology, exercise tolerance and pain, and sport psychology. Nutrigenomix-Sport was designed to guide athletes in choosing foods, supplements, and training strategies that enhance skill development, minimize risk of injury, and determine when there may be increased need for psychological support.


The athlete should receive their personalized “Athletic Performance Profile” a few weeks after they return their saliva sample to the testing company. This report will identify what genotype they are for a number of SNPs. It will also include information on what foods the athlete needs more of, what foods they should limit, and how taste preferences and intolerances should guide their nutrition plan. (See “The Results Are In” below for a sample report.)

Included in the report will be a variety of genetic information. Here’s a breakdown of what each piece means:

Dietary Component: This is the nutrient area being examined. In the sample case looking at body composition, the test determined how the athlete would respond to saturated and unsaturated fats and protein.

Gene: A gene is always written in italics and is often an abbreviation of the protein or enzyme it encodes. For instance, the FTO gene shown in the body composition sample encodes an enzyme that has been associated with obesity in humans.

Risk Variant: This identifies the version or “variant” of the gene that an athlete has. It’s written in one of three possible combinations with two letters, which represent the nucleotides in a DNA strand. Athletes inherit one letter from each biological parent, so the possible combinations can be, for example, CC, CG, and GG.

Your Variant: Genotypes are categorized as variants with “typical risk” or “elevated risk.” Usually, an elevated-risk genotype requires action, while a typical-risk genotype means the athlete can follow standard nutritional guidelines for that particular dietary component.

Recommendations for Elevated Risk: This provides specific dietary recommendations that aim to improve the athlete’s health and performance.

Outcome in Elevated Risk Group: Athletes with an elevated risk-variant of a gene can expect this outcome if they don’t follow the recommendations. Risk variants are often very common and may apply to half the population, not just a rare few. These variants require action from the athlete in order to achieve better health and performance.


An athlete I recently worked with who requested nutrigenomic testing was “Kate,” an 18-year-old middle-distance runner. She just started training with the Canadian national team and hopes to compete in the 2020 Olympics, and she turned to genetic testing to help optimize her performance.

A university student, Kate was living away from home for the first time. She followed a vegetarian diet that included some dairy, but she lacked the skills to prepare proper meals. Plus, with her busy schedule of classes and training six days a week, she didn’t have much time for food prep, anyway. Going into the testing, I knew Kate had suffered from anemia in the past and often complained of low energy.

Her genetic testing report revealed that she was at an elevated risk for iron and vitamin C deficiency. Kate also had an increased risk for Achilles tendon rupture and was a slow metabolizer of caffeine. We took these results and immediately got to work changing Kate’s diet.

To increase Kate’s iron status, I recommended incorporating a variety of iron-rich foods into her diet, such as lentils, beans, dried fruit, and trail mix for an easy, healthy snack to eat on the go. After four weeks, Kate’s iron levels saw no improvement, so I started her on an iron supplement.

Kate also added more vitamin C-rich foods to her meal plan, including bell peppers, citrus, and strawberries, to make up for her elevated risk of deficiency in this nutrient. Since vitamin C consumption also helps increase iron absorption, this offered a double boost.

Minimizing Kate’s risk for Achilles tendon injury meant altering her training. She stopped hill running and doing plyometrics, which are two high-risk exercises that add strain to the Achilles tendon.

Finally, I limited Kate’s caffeine intake to no more than 200 mg a day, as slow metabolizers have an elevated risk for heart issues when more than this amount is consumed. She was permitted to use caffeine prior to training sessions when needed.

As Kate’s example shows, genetic testing can help greatly in constructing an effective diet plan for an individual athlete. Once personalized nutrition is integrated into routine practice, sports performance professionals will be better able to predict the efficacy of various training diets and ergogenic aids, determine risks for nutrient deficiencies, and help athletes choose the right fueling strategies to optimize health and performance. When it comes to sports nutrition, nutrigenomics and genetic testing will provide the competitive edge of the future.


Strength and conditioning coaches have long tailored training to each athlete without knowing the genetics involved. However, along with identifying how athletes absorb and metabolize certain nutrients, genetic testing can also determine an individual’s injury risk and response to specific training modalities and mental stimuli based on their genetic profile. In the near future, strength coaches will be able to design and implement personalized training programs based on this research.

For example, the SNP for mental stamina affects athletes with the risk variants AA or AG in the BDNF gene. Athletes with either of these risk variants are more likely to have a negative response to or poor coping mechanisms for mental stress. They may have low motivation to exercise and experience negative thoughts during practice and competition. The recommendation for these athletes is to employ strategies that improve mental stamina. Some methods may include getting at least eight hours of sleep a night, practicing yoga and meditation, incorporating positive self-talk, and creating a positive personal mantra.

Another performance-based SNP relevant to athletes is the COL5A1 gene, which pertains to injury risk. This gene provides instructions for making a component of collagen, and collagens form a family of proteins that strengthen and support many tissues in the body, including skin, ligaments, bones, tendons, and muscles. Research shows a strong correlation between an elevated-risk variation for this gene and Achilles injury. Based on these findings, athletes who have the elevated-risk variant are recommended to thoroughly warm up and cool down for workouts, minimize uphill running, and be cautious with plyometric exercises that may produce surges of force or overstretch the tendon.


Increased interest in nutrigenomics and personalized nutrition among athletes has been accompanied by the emergence of customized supplements and vitamins. These new products claim to be designed to meet an athlete’s individual nutritional and performance needs, often by gathering information via a detailed questionnaire or survey.

If athletes express a desire to try these products, I advise taking caution for a number of reasons. First, the “customization” moniker can be a bit misleading. No supplements or vitamins that I have seen take into account an athlete’s unique genetic makeup, and the results of the surveys may not be all that specialized. For instance, I filled out the same survey three times to determine my vitamin and mineral needs for one product, using different diet information and health conditions for each entry. My recommended formula was virtually the same each time.

Second, sometimes the suggested supplementation plan is composed of dozens of different vitamins and minerals. Athletes typically aren’t deficient in this many areas, so the extra vitamins and minerals are unnecessary. It’s essential to only add what athletes need, and for most, that will only be one or two nutrients.

Customized vitamins and supplements may be more beneficial in the future as we become more efficient at assessing macro- and micronutrient needs based on genetics. This will ensure supplements are truly personalized to the athlete. But until then, my recommendation is to meet any vitamin and nutrient deficiencies with whole foods before turning to supplements. Remember, taking a supplement won’t do anything to change a poor diet.

Nanci S. Guest, MSc, RD, CSCS, is a PhD candidate (grad 2016) in nutritional sciences at the University of Toronto, studying nutrition, genetics, and athletic performance. A sports dietitian and strength and conditioning coach, she offers international consulting and nutrigenomics testing through her Toronto-based company PowerPlay: Nutrition, Training, Genetics. She works with amateur and professional athletes and served as the Lead Dietitian for the 2010 Winter Olympics and 2015 Pan Am Games. Guest can be reached at: [email protected].

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