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The Hunt Is On
New Study Refutes Correlation between FZA and Zinc Status

"Even though we have known for 50 years that zinc deficiency occurs, and even though we suspect that at least half the pop-ulation is at a marginal zinc intake status, it's unfortun-ately extremely difficult to document."

In a new study in last month’s issue of the American Journal of Clinical Nutrition, nationally renowned CHORI scientist Janet King, PhD, and her colleagues examine for the first time the independent effects of dietary zinc intakes on overall zinc absorption in order to determine whether the proportion of zinc absorbed from a diet is higher when individuals are in poorer zinc status.

Researchers have traditionally used plasma zinc levels as a marker of zinc status - whether the body's tissues have high, adequate or low levels of zinc. Unfortunately, plasma zinc is influenced by a number of factors totally unrelated to zinc nutrition.

"For example, if you eat a meal, your plasma zinc goes down 15 to 20 percent - more than it would if you were on a low-zinc diet for over a month," explains Dr. King. "If you're on oral contraceptives, if you have an infection, or if you're under high stress levels - you're plasma zinc levels go down. Consequently, it's very challenging to determine if plasma zinc levels are low because of an individual is in poor zinc status, or if it's low because of other reasons."

Recent studies suggested that perhaps a way around this problem would be to use the proportion of zinc absorbed, or fractional zinc absorbed (FZA), from a meal as an alternative marker to indicate zinc deficiency.

"We wanted to sort out whether the increase in FZA that was observed on low zinc diets was because the intestine rapidly adjusted to the amount of zinc you're eating at that point in time, or because your tissues actually have lower levels of zinc, or poor zinc status. This is the first time anyone has tried to sort out the effects of the amount of zinc consumed versus tissue zinc status on zinc absorption," says Dr. King.

By measuring FZA of patients on 2 different diets, a low and an adequate zinc diet, during 3, longer-term dietary zinc periods, Dr. King and her colleagues were able to establish that the increased FZA during the low zinc diet period reflected a rapid adjustment in zinc absorption due to the low zinc intake, not a decline in tissue zinc status.

"The amount of zinc you absorb from your diet is dictated almost entirely by what you ate in the previous meal or two. Your tissue zinc levels have an insignificant effect on fractional zinc absorption," explains Dr. King.

Dr. King is the first to acknowledge that their study results are based on only 9 participants. Nevertheless, any subtle relationship that might exists between zinc status and zinc absorption that wasn't captured due to the small sample size would be insignificant compared with the overwhelming effect of current zinc intake.
"What this study makes clear is that we can't use fractional zinc absorption as a marker for determining an individuals' overall zinc status," says Dr. King.

While Dr. King's latest publication puts to rest the erroneous idea that FZA could be a marker for zinc status, it also confirms what Dr. King has long begun to suspect.

Having dedicated the last 35 years of her life to pioneering discoveries related to how zinc is regulated, what dietary zinc requirements are, and how those requirements increase with pregnancy, or change with aging, Dr. King believes that focusing on whole body regulation of zinc metabolism, i.e., looking at the body’s absorption, excretion or turnover rates of zinc, doesn’t hold the key to identifying an individual’s zinc status.

“We know that there are about 25 different transporters that regulate the amount of zinc going into the cell and coming out of cell. That’s where we’ve got to move on to: looking at zinc at the cellular level,” says Dr. King.

Dr. King suspects that either the balance of those transporters, or the amount of zinc found in cells, could prove to be a viable marker of zinc status.. In fact, a comprehensive study is already being planned to look at the cellular regulation of zinc in order to come up with a model for evaluating zinc status.

"I don't think it will be only one indicator - there may be several that you have to measure to tell you whether someone's zinc status is good or bad," says Dr. King. "But the hunt is on to find a better way to evaluate zinc status."

No doubt, Dr. King will continue to spearhead that hunt. Given her dedication and commitment to investigating zinc - one of the most essential nutrients for life - the future of determining zinc status is in no better hands than Dr. King's.

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Tuesday, May 17, 2011 8:19 AM

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