Joints-comprising bone, cartilage, ligaments, and synovial fluid-enable stable daily movement but may deteriorate with age, excess body weight, overuse, injury, or disease. Such degeneration, common from age 40 onward, underlies joint disorders like osteoarthritis, particularly in the knees, hips, hands, and spine, and is a major cause of chronic pain and disability. While antiinflammatory analgesics remain the standard approach to symptom control, they neither cure disease nor regenerate cartilage and can produce significant long-term adverse effects (gastrointestinal, hepatic, renal, and cardiovascular). These limitations motivate interest in natural or complementary strategies with safer profiles.

Within this context, marine algae, especially agar derived from red seaweed, have attracted attention for potential intestinal, metabolic, and joint benefits, aided by their fiber content. Another promising option is undenatured type II collagen (UC-II), the principal structural component of articular cartilage, whose endogenous production declines with age; clinical studies indicate that a daily intake of 40 mg UC-II can improve mobility, reduce pain, and enhance quality of life in individuals with osteoarthritis or exercise-related joint discomfort. Based on these considerations, an agar-agar supplement has been developed to relieve knee joint symptoms.

The primary objective of this study was to evaluate the effect of agar-agar (A-A), the combination of A-A with undenatured type II collagen (A+UC-II), and undenatured type II collagen (UC-II) on knee joint pain, assessed by Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), in subjects with mild to moderate joint pain without a diagnosis of osteoarthritis, compared with a control compound.

The secondary objectives were to evaluate the effects of A-A, A+UC-II, and UC-II consumption on:

• Knee joint stiffness and functional capacity (subjective assessment).
• Overall perception of knee health (subjective assessment).
• Pain, stiffness, and functional capacity of the hand (subjective assessment).
• Range of motion of the knee.
• Functional capacity of the knee (objective assessment).
• Functional capacity of the hand (objective assessment).
• Frequency of medication use for joint pain relief (anti-inflammatories, analgesics).
• Inflammatory blood markers.
• Blood markers of collagen and bone metabolism.
• Body composition.

The primary endpoint is patient-reported knee joint pain, assessed using the Pain subscale of the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) questionnaire.

This is a randomized, placebo-controlled, triple-blind, parallel-group nutritional intervention with an allocation ratio of 1:1:1:1. A total of 80 participants (men and women) will be enrolled. Following a pre-screening visit to verify inclusion and exclusion criteria, eligible participants will be randomly assigned to one of four groups (n=20 per group) to consume, once daily for 12 weeks, one of the following products: agar-agar (A-A), A-A+UC-II (A+UC-II), UC-II alone, or placebo.

Each participant will complete three in-person visits and one interim telephone follow-up: a pre-screening visit (eligibility assessment), a baseline visit to initiate the intervention, a mid-study telephone follow-up, and an end-of-study visit. The total duration per participant will be 13 weeks.

The primary endpoint is patient-reported knee joint pain, assessed using the Pain subscale of the WOMAC questionnaire.

Inclusion Criteria:

• Men and women between 40 and 70 years of age.
• Submit a score of 6 to 10 out of 20 on the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) knee pain questionnaire.
• Sign the informed consent form.
• Know how to read, write, and speak Spanish and/or Catalan.

Exclusion Criteria:

• Present body mass index (BMI) values <18.5 kg/m^2 or ≥30 kg/m^2.
• Present intolerances or allergies related to Agar-Agar, or any of the components of the products being evaluated, such as collagen or maltodextrin.
• Take or have taken medications that may affect the study results. This is especially true for those who have been treated with oral or intra-articular corticosteroids within the 4 and 12 weeks prior to screening, respectively, or medications for osteoarthritis at the time of screening.
• Patients who depend on prescription medications for pain control.
• Take multivitamin or mineral supplements, collagen derivatives, or supplements containing bioactive compounds that may exert an anti-inflammatory effect, such as turmeric, ginger, omega-3, or polyphenol supplements.

(Individuals who consume multivitamin, mineral, or collagen-derived supplements or supplements will be eligible to participate in the study, provided they are willing to discontinue their intake for at least two weeks prior to the start of the study and throughout the entire study period).

• Be an active smoker or have been one for less than 6 months.

• Alcohol consumption:

  • Men: Consume 4 or more Standard Drink Units (SDU) daily or SDUs weekly.
  • Women: Consume 2 or more Standard Drink Units (SDU)* daily or SDUs weekly.

• Have a diagnosis of osteoarthritis, active rheumatoid arthritis, and any inflammatory arthritic condition such as secondary inflammatory arthritis, gout, pseudogout, knee infection, marked angular deformities, or significant injury to the target joint within the 6 months prior to the start of the trial; arthroplasty and joint surgery of the target knee within the 2 years prior to the start of the study.

• Have any gastrointestinal disease that affects the absorption of the compound and/or the study results, such as celiac disease, Crohn's disease, active cancer of any organ of the digestive or renal system, or hepatitis.

• Be ineligible to participate in the study, according to the pre-screening evaluator's criteria.

• Have lost 3 kg or more in the last 3 months.

• Be pregnant or intend to become pregnant.

• Be breastfeeding.

• Be unable to follow the study guidelines.

• Be participating or have participated in a clinical trial involving drug intervention or nutritional intervention within the last 30 days prior to inclusion in the study.