Vegetable soup seems like a healthy meal, and a new study provides even more compelling evidence that it is. Scientists at Tufts University in Boston report that volunteers eating a type of vegetable soup twice a day had lower amounts of stress-related molecules in their blood after just 7 days. The researchers say the beneficial effect was due at least in part to an increase in vitamin C intake.
The volunteers consumed gazpacho, a Mediterranean-style cold soup of uncooked vegetables. It contained generous amounts of tomatoes, which are rich in vitamin C, along with peppers, cucumbers, onions, garlic, oil, and seasonings.
Many studies have shown that vitamin C is a potent antioxidant that counters damage to human cells and tissues caused by highly reactive molecules called free radicals. But this study indicates that "the protective effect of vegetables may extend beyond their antioxidant capacity," says lead investigator Antonio Martin.
The results, he says, point to other important functions of vitamin C at the molecular level. These functions include a major role in preventing the formation of compounds involved in abnormal inflammation and a biochemical process called oxidative stress, both of which can alter cells in ways that set the stage for chronic diseases.
People probably don't have to consume vegetables in the form of a soup to get the benefits seen in the study, Martin and his colleagues note in the November Journal of Nutrition. They don't rule out, however, that additional nutrients in the particular mix of vegetables used in the gazpacho may provide a synergistic effect.
They fed the volunteers the prepared soup, they explain, because "it's a simple way to ingest generous amounts of vegetables." Gazpacho is a popular food in Spain, Martin's native country.
His team recruited 12 volunteers, half men and half women. All were in their early 20s and none had a major health problem. The participants maintained their usual lifestyles and ate as they normally would, except for the bonus soup. None of them took vitamins and minerals or medications before or during the study.
Before the volunteers began the twice-daily soup regimen, the researchers collected blood samples from them and measured concentrations of vitamin C as well as several compounds that indicate inflammation and oxidative stress.
The gazpacho was prepared in a home-style blender, then vacuum packed in individual portions. The volunteers consumed a total of 17 ounces of the soup every day for 14 days. The researchers collected blood samples again on the 7th and 14th days of the study.
The results showed that as of the 7th day, the amount of vitamin C in the volunteers' blood increased by 26 percent for the men and 25 percent for the women. The concentrations remained elevated during the final week of the study.
The researchers also measured blood concentrations of several molecules that the body secretes in response to stress. High readings for these molecules can indicate increased vulnerability to various illnesses because stress molecules, such as various hormonelike prostaglandins, induce inflammation and oxidative stress. Inflammation normally aids the repair of injured tissue as part of the body's infection-fighting defenses, but when a person's internal regulatory mechanisms are faulty, systemic low-grade inflammation can cause progressive damage.
After a week of consuming the soup consistently, the volunteers had a significant decrease in blood concentrations of prostaglandin E2, which is produced during inflammation and influences immune responses.
The young men and women also had lowered their amounts of two other stress markers: a particular isoprostane molecule that increases with age and in chronic diseases, and a molecule called monocyte chemotactic protein-1, which has been found in high concentrations in artery-clogging plaques that pave the way to heart disease.
The blood concentrations of these three compounds dropped inversely in proportion to the increase of vitamin C concentrations in the volunteers' blood. This clear inverse correlation, the researchers say, suggests that the vitamin C gave strong protection against the damaging effects of inflammation and oxidative stress.
Moreover, the volunteers had decreased amounts of uric acid in their blood. A buildup of uric acid causes the joint inflammation called gout. Research suggests that high amounts of uric acid may also contribute to the development of vascular dysfunction and heart disease by impairing some cells that line blood vessels. In this study, uric acid concentrations were 18 percent lower in the male volunteers and 8 percent lower in the women on the 7th day of the soup experiment than on the first day.
Yes, make it soup
Current dietary recommendations say that everyone should eat at least five portions of fruits and vegetables a day to reduce the risk of major illness, including cancer and heart disease. The results of this study, Martin says, "strongly suggest that increasing vegetable consumption could improve human health."
Martin says he and his colleagues designed this and some earlier studies to examine the effects of ingesting vitamin C through diet rather than in the form of vitamin supplements. While vitamin C is a critical nutrient in a number of biological processes, the human body can't produce it naturally and must acquire it from dietary sources.
A year ago, Martin's team did a study that found similar benefits from regular consumption of orange juice. Blood concentrations of vitamin C rose significantly among a group of volunteers who drank two glasses or orange juice every day for 2 weeks. Citrus fruits are the chief source of vitamin C in Western diets, Martin points out, but tomato-based products are also an important source.
Martin says "hundreds of studies" have demonstrated that vitamin C is an important nutrient for human health, "although we don't know the exact mechanisms behind these healthy effects." Analyzing the effects of vitamin C in the bloodstream, he says, is a stand-in for studying various functions of vitamin C in human tissues.
"Vitamin C is one of the most amazing nutrients in the human diet," Martin says. Among other things, it's important in the production of collagen proteins, which form bone and cartilage in vertebrates. It's also a cofactor in the production of several signal transmitters in the brain.
Brain tissue, Martin notes, contains the highest concentrations of vitamin C in the body. In a study 2 years ago, he and his colleagues found that increased concentrations of vitamin C in cells of human brain tissue improved the function of lysosomes, which are cell parts containing enzymes that break down and eliminate waste products. Scientists now know that the function of lysosomes decreases as people age, Martin points out.
While the present study examined the effects of increased vitamin C intake in healthy young people, Martin says he's convinced that the results would hold in other populations, including the elderly.
Nutrition and Neurocognition Laboratory
Jean Mayer U.S. Department of Agriculture-Human Nutrition Research Center on Aging
711 Washington Street
Boston, MA 02111-1524
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