When you’re deciding on a supplement, you want to know how well it works. In Labdoor's rankings, we call this attribute “Efficacy”.
As opposed to subjective consumer reviews and marketing hype though, we rely on scientific research to support our Efficacy Scores. Using our most recent Zinc Rankings as an example, here’s a look at some of the points we consider:
A zinc product’s formulation will affect the percentage of zinc you get from the supplement and how well the zinc is absorbed. These two characteristics are factored into zinc’s quality rankings.
Labdoor uses “elemental zinc” in its calculations for accuracy, safety, and efficacy. The percentage of elemental zinc present in a supplement will depend on the other chemical elements coupled to the zinc itself. For example, zinc can be formed as zinc oxide, which is 80% elemental zinc by mass, or as zinc gluconate, which is only 14% elemental zinc by mass. Essentially, you would need close to 6 times more zinc gluconate than zinc oxide to consume an equivalent amount of elemental zinc.
Zinc can be made in a number of other formulations and each one will have a different percentage of elemental zinc by mass. These formulations include (but are not limited to) zinc acetate, zinc citrate, zinc chloride, zinc sulfate, zinc polyascorbate, and zinc monomethionine.
For each formulation, Labdoor also considers relative bioavailability, or the fraction of a supplement dose that actually reaches your bloodstream as compared to other formulations.
Bioavailability differs between formulations and does not necessarily correlate with elemental zinc content. Zinc oxide, for example, has an extremely high elemental zinc percentage at 80%, but clinical studies show that only a small portion of zinc in this form is readily absorbable. On the other hand, zinc monomethionine, in which zinc is chelated to an amino acid, is only 21% elemental zinc, but has one of the highest bioavailabilities of all the zinc formulations. Researchers hypothesize that this is due to a mechanism in our intestinal wall that specifically supports amino acid absorption.
Research shows that zinc is optimally absorbed at a specific dose and that increasing doses above this level produce progressively diminishing benefits in terms of the percent of zinc you absorb. After finding out how much of a supplement is available for absorption based on its formulation, Labdoor scales efficacy scores based on where this amount falls on a modeled curve of ingested versus absorbed quantities like the one below:
On this curve, you can see that increasing intake from about 5 to 10 mg of elemental zinc produces a subsequent increase of about 4 mg in absorption. At a higher dose of 20 mg of zinc, increasing the dose to 30 mg only increases absorption by 0.2 mg. This decreasing benefit with higher doses occurs until your body reaches a limit for how much zinc you can absorb. In Labdoor’s efficacy algorithm, as products approach the flatter part of the graph, they will similarly approach a limit in terms of the weight they receive in their efficacy score.
3) Recommended Dietary Allowances (RDAs) and Tolerable Upper Intake Limits (ULs)
In addition to the above components inherent to a supplement and its formulation, Labdoor also takes into account whether a product’s serving size meets established values for what you need to prevent nutrient deficiencies and if the product exceeds safe limits. In healthcare practice, these values are known as Recommended Dietary Allowances (RDAs) and Tolerable Upper Intake Limits (ULs), respectively, and are published by the Institute of Medicine (IOM), a division of the US National Academy of Sciences.
For zinc, RDAs for adults above the age of 18 are 11 mg for males, 8 mg for females, 11 mg for pregnant women, and 12 mg for lactating women. The UL for these groups is 40 mg. Any products with serving sizes below the RDAs or above the UL are penalized.
For more information regarding RDAs and ULs, please visit the IOM’s reference tables here.
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