Background:  The influence of nutrition on the physiological functions of man is well studied. Numerous diseases can be exacerbated by obesity. However, it has not yet been determined whether body weight and body mass index (BMI), as an indicator of a high body fat store, can influence skin sensitivity. Objective:  This study investigates the correlation between body mass index and the epidermal functions, evaluated by bioengineering methods, before and after an irritant patch test with sodium lauryl sulphate (SLS). Methods:  Epidermal functions were evaluated using an evaporimeter, chromameter and laser‐Doppler‐flowmeter. Patch testing was conducted for 48 h with two different concentrations of SLS (0.25% and 0.5%) on the forearms of healthy volunteers. Measurements were performed 24 h after patch removal. Results:  Obese individuals showed significantly increased transepidermal water loss (TEWL), skin blood flow and skin colour (red) as compared to a control group. However, the degree of skin sensitivity to SLS was not correlated with BMI. Conclusion:  Basal biophysical parameters of the skin are primarily correlated with the BMI. This may be caused by obesity‐induced physiological changes, e.g. increased sweat gland activity, high blood pressure and physiological temperature‐regulating system. The epidermal barrier function, as evaluated after SLS patch testing is, however, not correlated with a high BMI, indicating a normal skin barrier.

Japanese women may complain about stronger sensations reflecting a different cultural behaviour rather than measurable differences in skin physiology; however, a faster penetration of SLS in Japanese cannot be excluded.

We conclude that the concentration of SLS influences the test outcome to a larger degree than the exposure time. We present formulae by which the outcome of SLS patch testing at various SLS concentrations ranging from 0.125% to 2% and any exposure time between 3 and 24 h can be estimated.

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We conclude that on clinically healthy skin the impairment of skin barrier by frequent bathing cannot be completely compensated by subsequent UVA irradiation. When conducting a treatment with 8-MOP bathing and UVA irradiation a concomitant therapy supporting the recovery of skin barrier, e.g. with moisturizer, should be performed.