What causes increased susceptibility to illness?

What Causes Increased Susceptibility to Illness?

Multiple interconnected factors weaken your immune defenses and increase illness frequency, with nutritional deficiencies, chronic stress, inadequate sleep, poor hygiene practices, and excessive training loads being the primary culprits.

Nutritional Factors That Compromise Immunity

Inadequate protein-energy intake or deficiencies in essential micronutrients directly decrease immune defenses against pathogens. Specific deficiencies that make you more susceptible to infection include 1:

• Iron, zinc, magnesium, manganese, selenium, and copper deficiencies impair immune cell function and initial healing responses 1
• Vitamin deficiencies (A, C, D, E, B6, B12, and folic acid) weaken immune surveillance and pathogen defense 1
• Low energy availability is directly associated with increased illness risk, particularly when combined with carbohydrate restriction during training 1
• Inadequate protein intake below 1.2 g/kg body weight/day compromises optimal immune function 1

Vitamin D deficiency deserves special attention, as multiple studies demonstrate its critical role in defending against common colds and respiratory infections 1. Those deficient or insufficient in vitamin D benefit from 2000 IU/day supplementation, particularly during winter months 1.

Micronutrient deficiencies trigger inflammatory pathways through nuclear factor kappa B (NF-κB) activation, which dysregulates proinflammatory mediators and creates chronic inflammation that further impairs immune responses 2.

Stress and Psychological Factors

Chronic psychological stress suppresses protective immune responses while paradoxically exacerbating inflammatory conditions. The distinction between acute and chronic stress is critical 3, 4:

• Short-term stress (minutes to hours) can actually enhance immune responses by mobilizing immune cells and preparing the body for challenges 3, 4
• Chronic stress (weeks to months) suppresses immunity by decreasing immune cell numbers and function, promoting regulatory T cells that dampen responses, and dysregulating cytokine profiles 3, 4
• Stress and depression are specifically identified as risk factors for increased illness episodes 1

Chronic stress activates central stress axes continuously, pushing the immune system into constant activity that leads to unresolved inflammation and increased vulnerability to chronic disease 5. Modern lifestyle factors create never-ending stress that the immune system was not designed to handle 5.

Sleep Deprivation

Lack of sleep directly weakens immunity and increases susceptibility to infections. Sleep deprivation causes 6:

• Impaired mitogenic proliferation of lymphocytes (reduced ability of immune cells to multiply in response to threats) 6
• Decreased HLA-DR expression (reduced antigen presentation to activate immune responses) 6
• Variations in CD4+ and CD8+ T lymphocytes (disrupted balance of critical immune cells) 6
• Shorter sleep durations are associated with increased risk of common cold 6

Sleep is essential for maintaining homeostasis, and adequate recovery sleep is as important as managing training load in reducing infection risk 1.

Training Load and Physical Overexertion

High training loads, particularly during preseason, combined with low energy intake significantly increase illness risk 1. The mechanism involves 1:

• Immunosuppressive stress hormone responses from excessive training without adequate recovery 1
• Overreaching and overtraining that deplete immune reserves 1
• Fixture congestion periods when infection risk is highest 1

Exercise only when feeling physically well—wait until symptoms of cold or flu (including fever) have been absent for 2 days before resuming activity 1. Exercising while ill can worsen immune suppression and prolong recovery 1.

Environmental and Hygiene Factors

Poor personal, home, and training venue hygiene directly increase pathogen exposure and infection risk 1. Additional environmental factors include 1:

• Seasonal variations, with autumn and winter months presenting highest infection risk 1
• Heat stress and dehydration that compromise immune function, particularly in elderly persons, obese individuals, and those taking diuretics 1
• Alcohol consumption, especially binge drinking, which negatively impacts immune cell functioning 1

Limit alcohol to no more than 2 units per day and avoid binge drinking to maintain immune competence 1.

Underlying Health Conditions

Certain medical conditions create chronic immune dysfunction 1:

• Undernutrition increases frequency and severity of infections including tuberculosis, candidiasis, pneumonia, and persistent diarrhea 1
• Chronic inflammatory conditions (like inflammatory bowel disease) impair nutrient absorption and increase infection susceptibility 2
• HIV infection creates a vicious cycle where malnutrition and infection worsen each other 1
• Aging is associated with immunosenescence (decreased immune function) and increased viral and bacterial infections 1

Environmental Chemical Exposures

Exposure to immunotoxic chemicals like PFAS causes small shifts in immune function that have major population-level impacts on infection susceptibility 1. These exposures 1:

• Decrease antibody levels and vaccine responses 1
• Increase risk of infections from community-based pathogens including SARS-CoV-2, influenza, and other agents 1
• Affect vulnerable populations disproportionately, particularly children and elderly 1

Poor Oral Health

Poor oral health has been reported with pain and psychosocial impacts that affect eating, sleeping, training ability, performance, and recovery—all of which can compromise immune function 1.

Critical Pitfalls to Avoid

Do not restrict calories or carbohydrates during periods of high infection risk or when fighting illness. Low energy availability and carbohydrate restriction increase immunosuppressive stress hormones 1. During illness, the metabolic response differs by pathogen type—bacterial infections may benefit from fasting while viral infections require glucose availability 1.

Do not use chronic anti-inflammatory medication without medical necessity, as these suppress the natural inflammation resolution process (Resoleomics) and prevent proper immune recovery 5.

Do not exercise vigorously when experiencing any symptoms of illness—this compounds immune suppression and delays recovery 1.