Bonnie Beezhold, a graduate student in the laboratory of Dr. Carol Johnston, presented the most recent study, on the impact of vitamin C depletion on a short-term diet, on April 3 at Experimental Biology 2006 in San Francisco. The presentation was part of the scientific program of the American Society for Nutrition, Inc. (ASN).
Before beginning a controlled four-week, low-fat diet, 20 obese men and women were randomized by gender and body weight into either a Vitamin C group, taking a 500 mg vitamin C capsule daily, or a control group, taking a capsule, identical in appearance to the vitamin, containing a placebo. Neither participants nor researchers knew who was receiving which capsule until the study was over. All participants consumed a low-fat diet that the researchers adjusted individually to promote slow weight loss (about two pounds per week). The diet contained 67 percent of the USRDA (recommended daily allowance) for vitamin C (40 mg/d).
At the beginning of the clinical trial, participants with the lowest concentrations of vitamin C in their blood had the highest body fat mass and tended not to oxidize fat well compared to their less obese counterparts. As the participants moved through the four week diet, with a steady amount of vitamin C being consumed, blood vitamin C concentrations increased 30 percent in those taking vitamins and fell 27 percent in the control group whose only vitamin C intake was the 67 percent of the USRDA contained in the food. As vitamin C blood concentrations fell, so did the participants' ability to oxidize fat (an 11 percent reduction).
The highly-controlled diet worked for all participants. Although body fat mass decreased slightly more in the vitamin C group, approaching but not reaching statistical significance, both groups lost an average of nine pounds, indicating that vitamin C depletion did not appear to affect the ability to lose weight in the short term. But because the study supported early findings in Dr. Johnston's laboratory of a decrease in fat oxidation, the researchers are now studying whether the impact of vitamin C status is associated with a gradual gain in body fat in non-dieting individuals.
It is important to understand the impact of vitamin C deficiency, says Dr. Johnston, because it affects about 15 percent of adults in the United States, up from only 3-5 percent 25 years ago. She believes the increased processing of the food supply is part of the problem, since vitamin C in foods is destroyed by exposure to light, oxygen, and/or heat.
How does vitamin C affect fat oxidation and thus the risk for weight gain and obesity? Vitamin C is an essential cofactor for the biosynthesis of a small protein-like molecule known as carnitine. Carnitine functions to shuttle fat molecules to the site of fax oxidation in tissue cells. When cells do not have access to fat molecules, feelings of fatigue ensue since energy metabolism is affected. Moreover, fat tends to accumulate in tissues when carnitine concentrations are reduced.
This research was supported by a grant from the General Mills, Bell Institute of Health and Nutrition.