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Playing the Numbers Game with InsideTracker

InsideTracker is a service that analyzes an athlete’s biomarkers to offer personalized guidance on nutrition, supplementation, and tweaks to training that is backed by science. 

Whether you’re a competitive athlete, weekend warrior, or fitness fanatic, chances are you’ve played the numbers game. With such a wide array of apps and devices dedicated to logging heart rate variability, power to mass ratio, miles per week, minutes per mile, or calories per day, the numbers game is a fun (albeit addictive) way to monitor and improve athletic performance.

We get it. That’s why earlier this year GU and InsideTracker partnered up to take a look under the hood of some of our top competitive athletes. We had 12 athletes pick an important event or race they were training for and then provided four testing opportunities for them (two before, two after) to see what effect nutrition and exercise had on over 40 health and performance related biomarkers.

The results were surprising, but they gave us an opportunity to implement science-based recommendations for training, nutrition, and supplementation to optimize overall health and performance. Here were some of the most common issues we found and how to fix them:

Vitamin D

Measured as serum 25(OH)D

Findings: Vitamin D was one of the most common micronutrient deficiencies.

Why It Matters: Vitamin D is important for bone metabolism and calcium handling, muscle health and recovery, cognitive and immune function.

What Causes It: Vitamin D is known as the “sunshine vitamin” because it can be produced in the body with adequate sun exposure. Outdoor athletes may think they are getting plenty of vitamin D because they spend so much time outdoors, but they often still fall short. Sunscreen, protective clothing, darker pigmented skin, and living in northern latitudes (where the sun is less intense) all impair vitamin D production, which can lead to deficiency despite ample sun exposure.

Deficiency Symptoms:

  • Fatigue
  • Muscle pain/weakness
  • Frequent stress fractures

The Fix: Increase sun exposure, 15-30 minutes daily in the midday sun (less if more fair-skinned). Increase intake of vitamin D containing foods, which include:

  • Fatty fish (salmon, sardines, mackerel, tuna)
  • Eggs (with the yolk)
  • Mushrooms

In some cases, athletes might still need to supplement with vitamin D for several weeks or more, particularly during winter. Doses typically range from 1,000-5,000 IU depending on the severity of the deficiency. D3 (cholecalciferol) is the preferred supplemental form (as opposed to D2, or ergocalciferol), since this form is more effective at raising blood levels. Taking supplements daily with fat-containing meals enables optimal absorption.


Measured as hemoglobin, hematocrit, serum iron, ferritin, transferrin saturation, total iron binding capacity

Findings: Serum ferritin (the body’s iron storage form) tended to be less than optimal for many of our athletes. Serum ferritin levels are often overlooked with standard blood tests that may only look at hemoglobin or blood iron levels. These levels can show up as normal even though serum ferritin levels are low.

Why It Matters: High training volumes—and running in particular—can deplete stored iron in order to facilitate red blood cell turnover, as well as aerobic energy production. It can also be diet related, particularly in vegetarian and vegan athletes who aren’t consuming iron-rich foods, such as red meat, that provide better absorption rates than plant based sources of iron.


  • Tiredness
  • Paleness
  • Dizziness
  • Shortness of breath
  • Headache

The Fix: Steak and spinach salad, anyone? Athletes should first increase dietary sources of heme (animal derived) and non-heme (plant derived) iron taken alongside vitamin C rich foods, as vitamin C helps boost absorption. Supplementation should only be considered in cases of a diagnosed deficiency and under the guidance of a qualified physician or nutrition professional. The body has a hard time eliminating excess amounts of iron, and toxicity is possible even at moderate supplemental doses. This can ultimately damage vital organs.

Here’s a list of foods that are a good source of iron:

  • Lean red meat
  • Poultry
  • Shellfish
  • Lean pork
  • Beans
  • Dark chocolate >70% cacao
  • Dark leafy greens

Cooking in a cast iron skillet is another way to incorporate more iron into meals. If taking supplemental iron, it is best taken on an empty stomach, preferably not in close proximity to exercise, which can interfere with absorption.


Measured as creatine kinase (CK), high-sensitivity C-reactive protein (hsCRP) and white blood cell (WBC) count

Findings: Several athletes had sub-optimal levels of inflammation, even before their target race.

Why It Matters: High levels of these inflammatory markers are indicative of an elevated, whole body stress response. Left unchecked, chronic inflammation can lead to injury, illness, and overtraining, which ultimately impairs performance.

What Causes It: Training may be too intense, volume/load progression may be inappropriate, or recovery insufficient to allow the acute inflammatory response to subside after exercise. It’s important to know the difference between normal (acute) and prolonged (chronic) inflammation, because some inflammation is natural. Often, the only way to tell is from blood work.


  • Chronic fatigue and training staleness
  • Frequent colds and upper respiratory infections
  • Prolonged decrements in performance and inability to recover

The Fix: Monitor training load and progress gradually to allow sufficient recovery between training sessions. Get at least 6 hours of sleep per night, ideally more. Certain foods can decrease inflammation, including blueberries, grapes, olive oil, black and green teas, and spices such as turmeric, ginger, and garlic. Eat a variety of fruits and vegetables daily and decrease highly processed foods in the diet, which can have an inflammatory effect. Consider adding 1-3 g of a EPA/DHA Omega-3 supplement (fish or algae source) during strenuous training blocks.

High Sensitivity C-Reactive Protein (hsCRP): higher levels (dots in yellow and red) indicate inflammation in response to a stressor such as intense, damaging or unaccustomed exercise (e.g., downhill running).

Other Common Issues

  • Insufficient Energy Intake: intense exercise can sometimes blunt hunger and athletes have elevated caloric needs due to training. It can be a full-time job just to prepare (and eat) meals!
  • Insufficient Protein Intake: the goal for endurance athletes is 1.2-1.6 g per kilogram of bodyweight per day, and this need can be even higher if training demands warrant it.

The Fix: Increase meal and snack frequency. Aim for at least 20 g protein with each meal. Post workout recovery time is another opportunity to get protein in: aim for 20-30 g.

What about nighttime snacking? Research shows that a small protein-rich snack before bed does not inhibit overnight fat breakdown and is a great way to boost muscle recovery and repair. Our favorite: Plain Greek yogurt with fresh berries and slivered almonds.

Interested in taking a look under the hood? Not sure if your nutrition and training regimen is producing optimal results? InsideTracker works with athletes of all levels and tests can be performed at your local Quest Diagnostics laboratory, or you can even have someone visit your home or office! It’s the best way to take the guessing game out of your training and nutrition.