CrossFit is defined as constantly varied, functional movement, executed at high intensity. This exercise prescription gets us fitness, defined as work capacity across broad time and modal domains. But more than just exercise, CrossFit is also a lifestyle, characterized by safe, effective exercise and sound nutrition. In his original 2002 article, “What is Fitness,” CrossFit’s founder Greg Glassman laid out World Class Fitness in 100 Words, which opens with the line: “Eat meat and vegetables, nuts and seeds, some fruit, little starch, and no sugar.”
Consider for a moment that the first line of a prescription for achieving world-class fitness has nothing to do with exercise or training at all. It tells us what to eat.
Our combination of training and sound nutrition creates health — a long-lasting form of fitness that we define as work capacity across broad time and modal domains throughout one’s life. In short, health is fitness that sticks.
We contend that anything you care to measure about health will have values that sit well ordered on a continuum, with measurements associated with overt disease (“sick”), those in the normal range (“well”), and those that are exceptionally good (a state of super-wellness, aka “fitness”). Health markers in the best third of the range hedge against sickness, bestowing the strongest shield against the chronic disease epidemic that plagues modern society. This state of super health can only be achieved through the type of lifestyle characterized by CrossFit. No pills or procedures even come close to delivering the same results.
In the nutrition prescription, the first two words — eat meat — are used as a simple placeholder for animal foods in general (beef, pork, chicken, fish, eggs, dairy, etc.) in which the primary macronutrient is protein. In this six-part series, we have been examining all the weird and wonderful ways the body uses the amino acids derived from our dietary protein to create health. So far, we’ve looked at muscle mass (Part 1), bone density (Part 2), mental health (Part 3), connective tissue (Part 4), and the antioxidant system (Part 5). We will be wrapping the series here today by looking at how protein supports the immune system.
Not discussed often enough is that, in addition to providing the world’s greatest hedge against chronic disease, the CrossFit lifestyle protects against acute infectious disease.
The Body’s Defense Department
Your immune system is a complex network of cells, tissues, and organs that work together to protect you from harmful invaders like bacteria, viruses, and other pathogens. It’s always on high alert, identifying and neutralizing threats before they can make you sick. And just like your muscles need protein to grow and recover, your immune system needs protein to stay strong and ready for battle.
The production of immune cells relies heavily on protein. These cells, including lymphocytes, macrophages, and neutrophils, are the soldiers of your immune system. They’re on the front lines, identifying, attacking, and destroying pathogens. They work in partnership with antibodies, which act like the body’s internal GPS, marking harmful invaders so immune cells can lock in and destroy them. But without a steady supply of high-quality protein, these immune cells can’t multiply, function, or carry out their duties effectively.
Not all amino acids are created equal; certain ones play a specialized role in supporting your immune system’s response. Let’s highlight a few of the MVPs:
- Glutamine: This amino acid is the fuel of choice for rapidly dividing immune cells, especially during illness or physical stress. During intense workouts, like Fran or Murph, your glutamine levels can deplete rapidly, so your body needs to draw from dietary sources to replenish them.
- Arginine: Known for its role in producing nitric oxide, arginine is crucial for wound healing and immune response. It enhances the function of T-cells (a type of lymphocyte), which are key to recognizing and attacking pathogens.
- Cysteine: A building block of glutathione, the master antioxidant in the body (which we learned in Part 5). Glutathione helps protect immune cells from damage caused by free radicals and oxidative stress. Without cysteine, your body struggles to maintain adequate glutathione levels, compromising immune resilience.
Complete Proteins Matter
Animal-based proteins are superior for immune support because they deliver all amino acids in the right proportions. In addition to Glutamine, Arginine, and Cysteine, these include the branched-chain amino acids (BCAAs) — leucine, isoleucine, and valine — which are used not just for muscle repair but also for supporting immune cell metabolism. Animal proteins are also more bioavailable, meaning your body can digest, absorb, and utilize them more efficiently. This is critical for athletes who need a quick turnaround in recovery and a fortified immune response after intense physical exertion.
Examples of top protein sources:
- Ruminants (beef, bison, elk, venison)
- Poultry (chicken, turkey, duck)
- Fish and seafood (salmon, tuna, shrimp)
- Dairy products (Greek yogurt, cheese, milk)
- Eggs (nature’s multivitamin)
Protein Deficiency and Immune Weakness
Without sufficient dietary protein, your immune system weakens, leaving you more susceptible to infections and illnesses. This is especially important for populations at risk of protein deficiency, such as those on restrictive or inadequate diets, older adults, or individuals with increased protein needs due to intense physical training. Even a mild protein deficiency can lead to a reduced production of immune cells and antibodies, impairing your body’s ability to fight off infections. In CrossFit, we recommend taking a rest day after every second or third training day — to choose rest before rest chooses you. One of the first signs of overtraining (i.e., under recovering) is getting sick more often or having a nagging illness you just can’t shake.
During the COVID-19 pandemic, an analysis of global data found that a diet higher in protein and fat positively affected recovery from the virus. This is important because more time spent being sick means less time spent in the gym getting fitter, faster, and stronger.
The Bottom Line
A diet rich in high-quality, complete proteins is essential for building muscle, boosting endurance, and enhancing performance, but also for keeping your immune system strong and ready to defend against chronic and acute illnesses. By following CrossFit’s nutrition prescription — “Eat meat and vegetables, nuts and seeds, some fruit, little starch, and no sugar” — you’re fueling your workouts and investing in your long-term health. There is no substitute for consistency in training and nutrition and no pill or shortcut can replace the benefits of a quality diet that prioritizes nutritious whole foods. By embracing this lifestyle, you’re building more than just physical strength. You’re fortifying your body’s defense mechanisms, making you fitter and more resilient to whatever life throws your way.
About the Author
Jocelyn Rylee (CF-L4) and her husband David founded CrossFit BRIO in 2008, starting in a modest 1500 sq ft space and focusing on personal training. Her dedication to excellence has also earned her a position on CrossFit LLC’s Level 1 Seminar Staff, a role that allows her to share her passion and expertise with aspiring coaches. Jocelyn holds specialties in Endurance, Gymnastics, Competition, and Weightlifting and is also a certified Strength and Conditioning Specialist through the NSCA. As a Level 2 Olympic Weightlifting Coach and a Level 3 referee, she has been deeply involved in the sport, even serving as a board member of the Saskatchewan Weightlifting Association for five years. Her achievements include being Saskatchewan’s top-ranked female Olympic Weightlifter from 2012 to 2015, during which she held provincial records in the Snatch, Clean & Jerk, and Total in her weight class. With an MS in Human Nutrition, Jocelyn loves sharing her knowledge on nutrition and performance through her blog and Instagram as “The Keto Athlete,” where she delves into the science of nutrition and its impact on athletic performance.
The Primer on Protein: Part 6 - The Immune System