This study will test whether astaxanthin, a naturally occurring antioxidant supplement, can improve lung function, reduce airway inflammation, and improve asthma control in adults with mild-to-moderate asthma. Participants will receive astaxanthin and placebo in random order in a double-blind crossover design. Each treatment period lasts 4 weeks and is separated by a 3-week washout period. The study also measures indoor and personal air pollution exposure to examine whether pollution influences asthma symptoms, airway responsiveness, and response to treatment. Exploratory thoracic bioelectrical impedance spectroscopy will be assessed alongside standard respiratory tests.

This is a randomized, double-blind, placebo-controlled, 2-sequence, 2-period crossover trial in 25 non-smoking, non-vaping adults aged 18 to 59 years with mild-to-moderate asthma or asthma-like symptoms confirmed at screening. Participants will be allocated 1:1 to one of two treatment sequences: astaxanthin followed by placebo, or placebo followed by astaxanthin. Each intervention period lasts 4 weeks and is separated by a 3-week washout. Astaxanthin will be given orally at a total dose of 12 mg/day as three 4 mg capsules taken with the participant's largest meal; placebo capsules will be matched. Outcome assessments include FeNO, spirometry, impulse oscillometry, body plethysmography, hypertonic saline challenge with induced sputum collection, home peak expiratory flow monitoring, questionnaires, and exploratory thoracic bioelectrical impedance spectroscopy. Indoor and personal air pollution exposures will be measured throughout the study to evaluate whether pollution modifies respiratory outcomes and treatment response.

Inclusion Criteria:

• Adults 18+ y

• Confirmed clinical diagnosis of mild-to-moderate asthma, defined as:

  • A physician diagnosed asthma condition consistent with BTS/NICE/SIGN (2024)
  • Asthma managed at GINA (2024) step 1-3 therapy level, with stable maintenance treatment for ≥4 weeks prior to enrolment
  • No asthma exacerbation requiring systemic corticosteroids within the preceding 6-8 weeks

(those with suspected Asthma will be invited to a familiarisation and eligibility session where they will have clinical investigations to assess for asthma and thus be included or excluded)

• Ability to demonstrate acceptable and repeatable spirometry in accordance with ATS/ERS standards
• Willing to refrain from antioxidant and anti-inflammatory supplementation (e.g. Omega-3, turmeric, NSAID supplements) for the duration of the study
• Not consuming high dietary ASTX sources (e.g., frequent salmonid or crustacean intake), assessed at screening
• non-smoking and non-vaping
• Able and willing to take daily study capsules and attend all required visits
• Able and willing to complete home monitoring, including peak flow, air-quality monitoring, and questionnaires
• Able to provide written informed consent

Exclusion Criteria:

• Current smokers or vapers which is associated with chronic airway remodelling, reduce hyperresponsiveness to bronchodilators, and increased neutrophilic inflammation, which can obscure the true effects of asthma-target interventions
• Respiratory tract infection within the preceding 4 weeks
• Pregnant or lactating individuals
• Presence of significant co-morbidities, including CVD or autoimmune or systemic inflammatory diseases
• Renal or gastrointestinal disorders that may affect astaxanthin absorption and metabolism
• Known allergy or hypersensitivity to ASTX or any components of the study supplement
• History of current evidence of alcohol or substance abuse
• Participation in another interventional drug or supplement study within the preceding 3 months
• Use of medicines with a narrow therapeutic index where supplement interactions may pose risk (e.g., ciclosporin/tacrolimus, warfarin), assessed by investigator
• Other significant chronic respiratory disease (e.g., COPD, bronchiectasis)