March 4, 2010 -- The kitchen or bathroom may be the worst place in the house to store your vitamins.
A new study shows high humidity and temperatures, such as those found in the bathroom and kitchen, can quickly degrade the potency of vitamin C and shorten the shelf life of vitamin supplements -- even if the bottle cap is on tightly.
Researchers found the most common types of vitamin C used in vitamin supplements and other fortified products are prone to a process called deliquescence, in which humidity causes a water-soluble substance to dissolve.
"Opening and closing a package will change the atmosphere in it. If you open and close a package in a bathroom, you add a little bit of humidity and moisture each time," researcher Lisa Mauer, associate professor of food science at Purdue University, says in a news release. "The humidity in your kitchen or bathroom can cycle up quite high, depending on how long of a shower you take, for example, and can get higher than 98%."
"If you get some moisture present or ingredients dissolve, they'll decrease the quality and shelf life of the product and decrease the nutrient delivery," Mauer says. "Within a very short time -- in a week -- you can get complete loss of vitamin C in some products that have deliquesced."
Humidity and Vitamin C Don't Mix
Powdered vitamin C is a popular ingredient for food fortification and is one of the most commonly added nutrients to vitamin supplements. Researchers say because vitamin C is very unstable and its content must be declared on nutrient labels, it is commonly used as an indication of the shelf life of foods and supplements.
For example, monitoring deterioration of vitamin C until it no longer meets its declared label value is one way to determine a product's shelf life.
Researchers say temperature and water are the two most frequently cited factors affecting shelf life. But information on deterioration and shelf life of vitamin C is based on models in which temperature and relative humidity were varied at the same time.
In contrast, this study looked at how various changes in relative humidity and temperature, such as those found in a bathroom or kitchen, affect the deterioration of two common forms of powdered vitamin C, ascorbic acid and sodium ascorbate.
The results, published in the Journal of Agricultural and Food Chemistry, showed relative humidity had the largest impact on vitamin C degradation, and this effect was magnified at elevated storage temperatures.
The study showed that at room temperature, sodium ascorbate and ascorbic acid deliquesce at 86% and 98% humidity, respectively. Once the humidity or temperature level was brought back down, the product will solidify again, but researchers say the damage has already been done.
"Any chemical changes or degradation that have occurred before resolidification don't reverse. You don't regain a vitamin C content after the product resolidifies or is moved to a lower humidity," Mauer says. "The chemical changes we've observed are not reversible."
They say keeping vitamin supplements away from warm, humid environments is the first step to maintaining their effectiveness.
The first signs of nutrient degradation are usually brown spots, especially on children's vitamins. Maurer recommends discarding any vitamin supplement that is showing signs of moisture in the container or browning.
"They're not necessarily unsafe, but why give a vitamin to a kid if it doesn't have the vitamin content you're hoping to give them?" Mauer says. "You're just giving them candy at that point with a high sugar content."