When Sickness Strikes, Strike Back!

What to do when you’re sick

 No one wants to be sick, and despite their fitness, athletes are not immune to illness. Like many other aspects of health, nutrition can influence both an athlete’s susceptibility and response to infection. Both macronutrients and micronutrients are involved in many immune processes.

Athletic success often depends on maintaining a consistent training schedule. For this reason, sickness poses a significant challenge in sport. Evidence shows that Olympic and World Championship medalists experience fewer and shorter upper respiratory infections (URIs) than less successful athletes, and illness rates are inversely related to training volume. Put simply, the less often athletes are sick, the more they can train.

Consequently, many athletes choose to train or compete while ill, driven by fear of missing opportunities and a belief that success requires pushing through discomfort. However, exercising during illness can impair performance and prolong recovery. More concerning are the serious medical risks associated with heavy exertion during or soon after infection, including muscle damage, cardiac inflammation, arrhythmias, or exertional heat illness. Research and case reports suggest that viral infections combined with intense exercise markedly increase these risks, even among highly trained athletes. Respiratory and gastrointestinal infections not only limit training and competition availability but can also have severe health consequences, underscoring the importance of resolving symptoms before returning to exercise.

Prevention is the best strategy: good hand hygiene, good nutritional status, stress management, adequate sleep, and recovery. When looking specifically at nutrition, both macronutrients and micronutrients play a role. Macronutrients are involved in immune cell metabolism and protein synthesis, and micronutrients in antioxidant defenses. It’s important to have these PRIOR to the onset of illness. Here are examples that may affect immune function:

Probiotics:

‍Probiotics may benefit athletes’ health by manipulating the gut microbiota and immune function. The precise mechanism of action is not yet fully understood. However, it’s thought that probiotics may strengthen the intestinal barrier by competing with pathogens for attachment sites and nutrients and by producing metabolic by-products, such as lactic acid, that inhibit pathogen growth. Foods that contain probiotics include yogurt, kefir, miso, sauerkraut, kimchi, tempeh and other fermented foods.  

Vitamin C ‍

Vitamin C is found in high concentrations in leucocytes (immune cells), but these level falls dramatically during a common cold, when oxidative stress increases. Good sources of vitamin C include fruits and vegetables like citrus and bell peppers.

Vitamin D

Immune cells like T cells, and B cells express the vitamin D receptor. Vitamin D is well recognized for its role in supporting immune defense through the stimulation of antimicrobial proteins. For many, Vitamin D insufficiency is common, particularly in the winter months. Sufficiency can often be achieved by sunlight exposure in the sunny months. When a blood test indicates insufficiency, 1000 IU/day vitamin D₃ supplementation may be warranted.

Omega-3

‍Omega-3 fatty acids, specifically eicosapentanoic acid (EPA) and docosahexanoic acid (DHA) can modulate low-grade inflammation caused by exercise and stress. Found primarily in fatty fish, such as mackerel and herring (or algae oil for plant based athletes), Omega 3’s are essential for brain development and linked to cardiovascular and neurological disease reductions. Strong conclusions and clear recommendations about the use of EPA/DHA supplementation for illness remain undetermined. However, optimal sports performance requires optimal health, and it’s well established that higher omega-3 intakes are linked to improved health outcomes.  Although the evidence isn’t definitive, there is little risk to low-dose omega-3 supplementation, and may result in improved recovery and immune function. Long term supplementation of 1.5-2.0 g/day (6+ weeks) has more often shown positive benefit than short term.

After the onset of sickness:

Parents with kids in daycare understand this all. With best intentions to stay healthy, sometimes becoming ill is unavoidable. Once sickness has taken hold, what’s the best way to move forward?

Rest

If you have an inclination to ride the trainer while experiencing a fever, lung congestion, coughing, chills, or vomiting, please don’t. While athletes can still complete light exercise with lesser symptoms, it’s important to know when enough is enough. Athletes don't get better by training hard through sickness. It delays recovery and forces athletes to take more time away from productive training. If symptoms are mild and limited, light activity may acceptable. Reduce or avoid high-intensity sessions, long endurance work, and training in hot environments until feeling better.

Sleep

Sleep as much as possible. Sleep is when the body upregulates its focus on recovery. Immune activity is enhanced, inflammatory responses are regulated, and tissue repair is prioritized. Illness often disrupts sleep quality, yet this is precisely when athletes need more rest, not less. Athletes who are unwell should aim to extend sleep duration where possible. Include daytime naps if needed. Short-term reductions in training, combined with increased sleep, are far more effective for recovery than attempting to “train through” illness with inadequate rest.

Hydrate

During illness, fluid needs can increase due to fever, increased respiratory rate, sweating, vomiting, or diarrhea. Even mild dehydration can worsen fatigue, impair thermoregulation, and place additional strain on the cardiovascular system, making training more difficult or dangerous.

Sick athletes should prioritize regular fluid intake throughout the day rather than relying on thirst alone, as illness can blunt thirst cues. If GI symptoms exist, like vomiting or diarrhea, fluids with electrolytes can be particularly useful. Additionally, soups or herbal teas may help relieve congestion and provide additional nutrients. Athletes returning to training after illness should continue to monitor hydration status closely, as residual dehydration can continue on for a few days even when feeling better.

Starve a fever?

The old adage, “feed a cold, starve a fever,” isn’t a good idea. It’s important to feed both, especially when a fever is present.  Your body’s metabolic rate increases as the immune system ramps up. This raises energy and fluid needs, not lowering them. In fact, it’s estimated that basal metabolism increases roughly 7% for each degree Fahrenheit above normal. Eating when sick can be challenging, so small, frequent meals or snacks are better tolerated than large portions.

Zinc for the common cold

Zinc is one of the only micronutrients that may have an effect on cold symptoms after their onset. A meta-analysis showed that zinc lozenges (75 mg/day of elemental zinc) reduced upper respiratory infections duration by ~ 3 days (33%) when taken within 24 hours after the onset of symptoms, and continues with for the duration of the illness.

Although the mechanisms remain unclear, zinc is thought to exert antiviral effects by reducing the ability of common cold viruses to bind to host cells. In addition, the therapeutic benefits of zinc lozenges in upper respiratory infections have been attributed to the antioxidant and anti-inflammatory actions of elemental zinc, which may also enhance immune responses.

Within the Fuelin App.

If you are sick, update your training to reflect the work that you’re currently doing (or not doing). If you're working with a Fuelin Coach, let them know what’s going on so they can guide you and keep them up to date on your progress. Additionally, it’s a good idea to move away from a calorie deficit (adjust your settings so your goal weight equals your current weight). Deliberately focusing on a caloric deficit during illness can lead to several problems. Immune cells require energy to function, and insufficient energy may prolong illness and delay a return to training. What’s worse, muscle will be catabolized for fuel, which is particularly detrimental for athletes trying to maintain strength and performance. Undereating can also worsen fatigue, dehydration, and thermoregulation, increasing the risk of serious issues such as exertional heat stress if training resumes too soon.

Summary

Illness can really knock athletes off their feet and out of regular training. It’s important to establish habits that support a strong immune system: adequate sleep, stress management, and a diet including sufficient macronutrients, vitamin C, vitamin D, probiotics, and omega-3s. If illness sets in, it’s important that athletes monitor their nutrition, hydration, and rest. Training through the flu or other serious illness is not a sign of toughness and can delay recovery, delay a return to sport, or potentially lead to long-term complications. By respecting your body’s recovery needs, you protect performance, preserve muscle and strength, and stay on track for consistent, long-term training success.

Thank you,

Megan

References:

Calder PC, Carr AC, Gombart AF, Eggersdorfer M. Optimal Nutritional Status for a Well-Functioning Immune System Is an Important Factor to Protect against Viral Infections. Nutrients. 2020 Apr 23;12(4):1181. doi: 10.3390/nu12041181. PMID: 32340216; PMCID: PMC7230749.

Hemilä H. Zinc lozenges and the common cold: a meta-analysis comparing zinc acetate and zinc gluconate, and the role of zinc dosage. JRSM Open. 2017 May 2;8(5):2054270417694291. doi: 10.1177/2054270417694291. PMID: 28515951; PMCID: PMC5418896.

Temperature and Basal. JAMA. 1925;85(22):1732. doi:10.1001/jama.1925.02670220050019

Thielecke F, Blannin A. Omega-3 Fatty Acids for Sport Performance-Are They Equally Beneficial for Athletes and Amateurs? A Narrative Review. Nutrients. 2020 Nov 30;12(12):3712. doi: 10.3390/nu12123712. PMID: 33266318; PMCID: PMC7760705

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