Inclusion Criteria:  women and men who are diagnosed at the introductory term with any of the following signs of brittle nails on at least one of the assessed nails (preferably at least two signs):  rough surface of the nail plate or presence of keratin granulation (trachyonychia) lamellar stratification - horizontal layering / peeling (onychoschizia) longitudinal stratification or splitting of the distal edge, rupture (onychorrhexis) generally healthy willingness to participate in research signed statement of conscious and free consent to research - informed consent willingness not to use any dietary supplements with the active substances contained in the test product (CoQ10, biotin, niacin, vitamin C, β-carotene, vitamin E, zinc, selenium, copper, MSM, L-cysteine, L- proline), other than those to be allocated by researchers willingness to follow all research procedures, including participation in all study visits and keeping a diary during the testing of preparations  Exclusion Criteria:  Allergy to or suspicion of any ingredient in the test products menopause pregnancy, lactation in the past, surgery on the index finger / middle finger / ring finger of the right hand Disorders and deformities of the index finger, middle finger and ring finger of the right hand due to a malformation of the underlying bone, which would interfere with the evaluation of photographs or changes in the nails the presence of raised nails from the nail bed (onycholysis) on the index finger, middle finger and ring finger of the right hand supplementing the diet with dietary supplements containing CoQ10, biotin and other ingredients of the test product in the last three months before inclusion special eating habits used by a small part of the population (veganism, LCHF, calorie-restricted diets) diets prescribed by the medical profession uncontrolled metabolic and endocrine diseases Chronic dermatoses e.g. psoriasis, ichthyosis, various types of dermatitis, lichen planus, alopecia areata, scleroderma systemic diseases such as SLE, DLE, diabetes, neuropathy, peripheral artery disease have a known or suspected genetic condition that affects the nails (eg Darier's disease, nail patella syndrome, tuberculous sclerosis) the presence of anemia mental eating disorders mental incapacity that prevents proper understanding or cooperation sawing the surface of the nail plate or. polishing it in the last 6 months before inclusion and during the study any type of manicure involving gelling, artificial nails in the last 6 months before inclusion and during the study any type of classic manicure in the last month before inclusion and during the study change in the nail and hand care routine in the last month before inclusion and during the study change of dietary habits or dietary supplementation in the last 3 months before inclusion and during the study infections of the hands and nails on the hands (fungal, bacterial, viral) biting fingernails or cuticles mechanical irritation and damage to the nails, repeated prolonged exposure of the hands to water or repeated use of hand sanitizers, use of antimetabolites, retinoids, statins, antiretroviral drugs, fungicides in the last 3 months before inclusion and during the study changes in the use of hormonal preparations in the last 6 months before inclusion and during the study