Research news from Tufts University, Friedman School of Nutrition Science and Policy: May 2004
A Low Glycemic Index Diet May Help Decrease Disease Risk Factors
Currently, an estimated 20 to 25 percent of the US population have a condition known as the metabolic syndrome, predisposing them to increased risk of type 2 diabetes mellitus and cardiovascular disease. Recently, Nicola McKeown, PhD and colleagues at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts found that people who ate a low glycemic index diet improved the symptoms of this syndrome, supporting their previous findings that a high dietary glycemic index was associated with a higher risk of metabolic syndrome in the same population.
The glycemic index characterizes the rate of carbohydrate absorption after a meal. A low glycemic index diet involves eating certain carbohydrate-containing foods that blunt the rise in blood sugar levels after eating. High glycemic index foods include foods such as potatoes, bananas, white breads and rice, french fries and refined breakfast cereals. These foods tend to raise blood sugar faster than low glycemic index foods that slow down absorption, such as high fiber breads, oats and milk.
The research team examined the relationship between the dietary glycemic index and several metabolic markers of disease risk in the Framingham Offspring Cohort and found certain health benefits associated with low glycemic index diets. McKeown and her colleagues found that a low glycemic diet improved several factors that affect diabetics including lowering blood pressure, fasting triglycerides, fasting insulin and other metabolic risk factors.
Additionally, the diet was found to raise HDL cholesterol (the good cholesterol.)
The researchers suggest that as metabolic syndrome is an identifiable and potentially modifiable risk state for both diabetes and heart disease, people may benefit from replacing foods that have a high glycemic index with low glycemic index carbohydrates to help reduce the risk of developing these diseases. Experimental Biology 2004 Washington D.C.
Abstract # 566.1 McKeown et al., Dietary glycemic index is related to metablic risk factors in the Framingham Offspring Cohort.
People Need to Eat More Beta Carotene to Get Enough Vitamin A
According to most nutritionists a ½ cup of carrots a day may help improve vision, lower the risk of cancer and other diseases but recently researchers at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University have shown that this may not be enough. But how much beta-carotene does one need to consume daily to maintain adequate vitamin A?
New research, lead by Guangwen Tang, PhD and colleagues, showed that men who ate a diet that contained 100 grams (about ½ cup) of pureed carrots absorbed less beta carotene (a precursor to vitamin A) than the researchers previously thought. Previously, the conversion factor for beta carotene was 12 micrograms for one retinol activity equivalent. In this study, Tang and colleagues found that this number increased to 15 micrograms of beta carotene for one retinol activity equivalent, meaning more food needs to be consumed to get adequate vitamin A.
Additionally, the researchers found this conversion factor became even more significant when they tested this theory with the beta carotene in spinach, giving 300 grams (about 1 ¼ cups) of pureed spinach to the same study volunteers five months later. They found that like the carrots, the beta carotene in spinach was also not as well absorbed, in fact, it was even less well absorbed than from carrots.
For spinach, like carrots, the old beta carotene to vitamin A conversion factor was 12 units of beta carotene for one unit of vitamin A, but the researchers now contend that the conversion factor for beta carotene in spinach is 22 units to one. These new findings may influence the next set of US dietary guidelines for beta carotene and vitamin A. Experimental Biology 2004, Washington D.C.
Abstract # 131.10 Guangwen Tang et al., Vitamin A value of spinach and carrots assessed using a stable isotope reference method in adults.
How to Figure Out if People are Eating More Than They Report?
Most people underestimate what they eat on any given day and people who are overweight tend to underestimate this amount to a greater extent. For researchers studying why people are overweight this is problematic. Megan McCrory, PhD an assistant professor at the Friedman School of Nutrition Science and Policy, found that in a US dietary survey, women on average underestimated their calories by 21 percent and men did so by 15 percent.
In order to objectively determine how many calories people are eating, McCrory and her colleagues at the Jean Mayer USDA Human Nutrition Research Center on Aging, used age, weight, height, and sex to predict how many calories people require and a calculated cutoff range (22% in this particular survey) to detect whether people either over or under estimated the amount they reported they ate in the survey.
Eating amounts under or over this 22% window is not physiologically possible over the long-term because the subjects would then either weigh a lot less or a lot more than they do at the beginning of the study. Those who fall outside of that range, the researchers recommend, should be taken out of studies as their results may skew the findings of the entire study.
An analysis of the complete study group (including those outside of the 22% "window") showed that adults who are overweight ate less frequently and fewer calories at meals and snacks than their normal weight counterparts. When McCrory's new method was applied to the same survey, the results were opposite: people who are overweight eat more frequently and more calories at meals and snacks than normal weight people.
According to McCrory, "Given that dietary intake is difficult to measure and dietary reporting is the most commonly used tool, it is important to future research to have this method to validate results." Experimental Biology 2004 Washington D.C.
Abstract # 566.1 McCrory et al. New Methodology for Determining Implausible Energy Intake Reports in Studies of Diet and BMI.
The Gerald J. and Dorothy R. Friedman School of Nutrition Science and Policy at Tufts University is the only independent school of nutrition in the United States. The school's eight centers, which focus on questions relating to famine, hunger, poverty, and communications, are renowned for the application of scientific research to national and international policy. For two decades, the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University has studied the relationship between good nutrition and good health in aging populations. Tufts research scientists work with federal agencies to establish the USDA Dietary Guidelines, the Dietary Reference Intakes, and other significant public policies.
Source: EurekAlert.org (this is a press release).