Absurd: Will Supplemental Zinc Gluconate Deplete Your Zinc Levels?

Normally, I restrict myself to very recent studies, but the results of this 2007 (Tompkins. 2007) study on the bioavailability of zinc-enriched yeast and zinc gluconate in healthy volunteers that recently resurfaced on the Mind&Muscle boards, is too counter-intuitive and, if not scientifically flawed, important not to make it into the news.

In the study, which used a randomized, two-way crossover design, the volunteers were randomly assigned in groups of three to consume a single dose of either 20 mg of total Zn in the form of Zn gluconate or Zn yeast with 150 ml of water, after that urine, blood, and fecal samples were collected and analyzed over a 48-h period. The results were surprising, while "Zinc gluconate gave higher Zn concentrations in the blood in the first 6 h" it did not only show greater loss in the feces, but ...

the net Zn balance after 48 h for Zn yeast was 9.46 but for Zn gluconate it was -2.00, indicating that Zn gluconate supplementation contributed to a net loss of Zn.

As far as the reasons for this observations are concerned, the scientists speculate:

Presumably, this “extra” Zn lost was released from the body’s store of Zn metallothionein; however, we do not have any evidence of the source of the Zn material. This may suggest that the Zn salts stimulated a Zn detoxification pathway.

If this were true and the results are not biased by the background of the Institut Rosell, which is obviously financed by Lallemand Inc. who produce (pro-)biotic products (i.e. yeast based products), you might want to reconsider your own supplementation practices. The high doses of Zn Salts commonly used by bodybuilders and fitness enthusiasts (many suggest >50mg Zinc per day) may in fact lead your body to believe he becomes intoxicated with zinc. It is then a very natural reaction to excrete zinc and as the data from this study shows, this "excess" zinc (i.e. more zinc than you supplied) must - at least in parts - stem from zinc stores somewhere in your body.

Zinc, a BodyBuilder's Favorite Mineral: All That Glitters is not Gold

I know, you think you can hardly get enough Zinc and have to supplement 30mg+ of zinc in the form of highly bio-available (and expensive) supplements. Right? Have you ever thought about side effects besides reducing estrogen? No? Then you should have a look at a recent study by Brazilian scientists (Marques. 2010).

Figure 1: Zinc supplements
are a bodybuilder's favorites

By supplementing only 22mg zinc-gluconate per day over 30 days, the plasma copper levels of 7 trained cyclists decreased and remained low 30 days after cessation. While supplementation had no effect on thyroid metabolism, the scientists made another disquieting discovery:

Fasting serum insulin and HOMA2-IR increased (27% and 47%, respectively) on day 60 compared to baseline (p = 0.03), suggesting a delayed effect of zinc supplementation. Moreover, plasma zinc was positively associated with serum insulin (r = 0.87, p = 0.009) and HOMA2-IR (r = 0.81, p = 0.03) after zinc supplementation (day 30), indicating that supplemental zinc may impair glucose utilization in cyclists.

If this happens in endurance athletes, in bodybuilders, who are always scared they'd lose muscle if they do cardio, the increase in insulin could become problem. If, in addition, you follow a high carb diet and down a few scoops of a weight gainer every other day, you are probably in for trouble. So better keep your zinc supplementation within reasonable limits; plus, if in doubt leave it out ;-)