Why is salivary midnight cortisol level low after consuming sugar and vigorous exercise?

Why Salivary Midnight Cortisol May Be Low After Sugar Consumption and Vigorous Exercise

Vigorous exercise and sugar consumption immediately before a midnight salivary cortisol test can artificially lower cortisol levels, leading to potentially false negative results in the diagnosis of Cushing's syndrome. 1

Physiological Effects of Exercise on Cortisol

• Exercise typically increases cortisol levels acutely, but can lead to subsequent cortisol depletion, especially when performed vigorously right before testing 1
• Exercise-induced increases in salivary cortisol can persist for up to 24-48 hours, potentially disrupting the normal diurnal rhythm of cortisol secretion 1
• The normal circadian nadir of cortisol secretion can be altered by vigorous physical activity, affecting the diagnostic utility of midnight salivary cortisol tests 2

Impact of Sugar Consumption on Salivary Testing

• Sugar consumption affects salivary biomarkers through multiple mechanisms, including:
  • Increasing secretion of digestive enzymes that can dilute cortisol concentration 1
  • Creating matrix effects that interfere with accurate measurement of cortisol 1
• Carbohydrate-rich meals significantly impact salivary composition, potentially diluting cortisol concentration in the sample 1

Mechanisms Behind Low Midnight Salivary Cortisol

1. Dilution Effect:

   • Sugar consumption stimulates saliva production, potentially diluting cortisol concentration in the sample 1

2. Cortisol Depletion:

   • Vigorous exercise can cause an initial spike in cortisol followed by depletion, especially if measured after the initial spike has subsided 1

3. Disruption of Normal Diurnal Rhythm:

   • The combination of exercise and sugar intake can disrupt the normal circadian rhythm of cortisol, which typically reaches its nadir at midnight 2

Importance in Clinical Testing

• Late-night salivary cortisol (LNSC) is a key diagnostic test for Cushing's syndrome, with sensitivity above 90% 2
• The diagnostic utility of LNSC is based on the assumption that patients with Cushing's syndrome lose the normal circadian nadir of cortisol secretion 2
• For accurate results, patients should:
  • Avoid strenuous physical activity for at least 24-48 hours before sample collection 1
  • Refrain from eating for several hours before sample collection, particularly sugar-rich foods 1
  • Collect samples at the appropriate time (midnight or bedtime) 2

Common Pitfalls in Salivary Cortisol Testing

• Failure to account for lifestyle factors (exercise, diet) can lead to misinterpretation of results 1
• At least two or three LNSC tests are recommended to account for variability 2
• Testing should not be performed in patients with disruption of the normal day/night cycle, such as night-shift workers 2
• Sample contamination (particularly by topical corticosteroids) can affect results 3

Understanding these factors is crucial for correctly interpreting midnight salivary cortisol results and avoiding false negatives in the diagnosis of Cushing's syndrome.