This clinical trial aims to evaluate the safety and efficacy of topical application of Lactobacillus crispatuson the forearm for treating obesity-induced skin barrier impairment in individuals aged 18-45 with BMI ≥25. The study focuses on the following questions:

Can topical application of Lactobacillus crispatusreduce skin barrier damage, as measured by transepidermal water loss (TEWL), in obese participants?

Does modulation of the skin microbiota with Lactobacillus crispatusimprove skin barrier function, hydration, and reduce local inflammation?

Is the topical intervention safe and well-tolerated, with minimal adverse effects?

Preliminary data from obese volunteers and mouse models revealed significantly elevated TEWL and reduced Lactobacillusabundance in the skin microbiome of obese individuals, supporting the investigation of probiotic restoration.

Researchers will compare outcomes across two groups:

Intervention Group (Obese) : Daily topical application of active Lactobacillus crispatussolution (1×10⁷ CFU in 3 mL) to a 3-cm diameter area on both forearms.

Placebo Control Group (Obese) : Daily topical application of inactivated Lactobacillus crispatussolution (identical appearance and volume).

Participant Procedures:

Apply the assigned topical solution daily to the forearm for 4 weeks.

Undergo non-invasive skin testing, including TEWL measurements and skin hydration assessments, at baseline and study completion (Week 4).

Provide skin swab samples at baseline and Week 4 for microbiome analysis (16S rRNA sequencing) and inflammation marker detection (e.g., IL-1β, TNF-α via ELISA).

Complete weekly check-ins to report adverse effects (e.g., skin irritation, erythema) and adherence, with follow-ups at Weeks 1, 2, and 3.

Maintain a daily electronic or paper diary to record application time, dose, and any skin reactions.

Inclusion Criteria:

• 1. Meets 2020 WHO BMI classification:
• a. Normal weight (18.5-24.9 kg/m²)
• b. Overweight (25-29.9 kg/m²)
• c. Obesity (≥30 kg/m²)
• 2. Presence of skin barrier impairment (e.g., dryness, erythema, desquamation, or itching)
• 3. Age 18-40 years
• 4. Generally good health (no active systemic diseases)
• 5. Able and willing to provide written informed consent
• 6. No use of oral/topical medications or probiotics within 6 months prior
• 7. No active skin disease or traumatic skin lesions

Exclusion Criteria:

• 1. Known allergy or hypersensitivity to probiotics, placebo, or investigational product
• 2. Active skin disease (e.g., psoriasis, eczema, infection) requiring treatment
• 3. Severe medical conditions:
• a. Cardiopulmonary disease (NYHA class III/IV)
• b. Uncontrolled diabetes (HbA1c >9%)
• c. Autoimmune disorders
• 4. Pregnant or breastfeeding women
• 5. Any condition that may interfere with protocol compliance (per investigator judgement), including:
• a. Inability to understand study procedures
• b. History of poor clinical trial adherence
• 6. Concurrent participation in other interventional trials