There are many reasons. I think the following are also important.
- Laboratory animals have better essential nutrient nutrition, allowing the added agent, be it just a single vitamin or a small cocktail, to be more effective because all agents work within the context of the otherwise well nourished, well-functioning cell, tissue, organ, and whole body.
- The humans tend to be sicker than the animals: if they are past the point of disease/medical condition reversibility (I have posited that every disease and every medical condition has a period of reversibility; it is critical to treat during this period), of course a supplement or two will not help. In streptozotocin-induced diabetes in lab animals, the researchers start treatment as soon as the blood sugar is stably elevated. People with diabetes for 10-20 years are treated in clinical trials.
- The animals have no choice but to accept any lifestyle changes enforced by the researchers. The humans resist any lifestyle changes that increase pain and unhappiness but tell researchers that they are following their directions.
- The animals are not taking drugs, while the humans are taking one or more prescription medications that can interfere with overall nutrient sufficiency and seriously perturb not just the targeted pathway but other, even non-related pathways (statins seem to produce most of their benefit not from inhibiting synthesis of cholesterol, but from their beneficial effects on NOS metabolism, and a small positive effect from a small reduction in protein isoprenylation). All prescription medications disrupt metabolism, leading to shortfalls, and doctors usually give a prescription to address a single shortfall. Example: statins reduce cholesterol, isoprenylpyrophosphate, dolicols, CoQ10, and squalene, to name a few, and doctors merely recommend a CoQ10 supplement. Lame squared. Thiazide diuretics have diabetics urinating out zinc, thiamine, potassium, and so many other things, and doctors prescribe only potassium. Get serious! If the clinical trial was the effect of taurine on some complication of diabetes, how well will it work when the diabetics are urinating out thiamine (not to mention their taurine supplements)?