Scientific Update
A Review of Recent Scientific Papers Related to Vegetarianism
By Reed Mangels, PhD, RD, FADA
Nuts and Gallstones
Most people don’t think about their gallbladder until it bothers them. The gallbladder is a small sac that squirts bile, a yellowish liquid which helps to digest fat, into the intestines. Under certain conditions, bile can harden and form gallstones that can block tubes leading from the gallbladder to other parts of the body. This blockage produces symptoms like nausea, vomiting, and stomach pain and can even lead to death. More than 800,000 Americans are hospitalized each year because of gallstones; many have to have their gallbladder removed.
Factors like obesity, diabetes, fasting, and rapid weight loss can increase the risk of gallstones. A new study suggests that eating nuts frequently can reduce the risk of gallstone disease in women. Researchers from Harvard Medical School studied more than 80,000 women for 20 years to see who developed gallstones. Women who ate 5 or more ounces of nuts per week had a lower risk of having to have their gallbladder removed because of gallstones compared to women who rarely or never ate nuts. These results were for both peanuts and tree nuts. Factors in nuts that may reduce the risk of gallstones include monounsaturated and polyunsaturated fats, fiber, phytosterols, and magnesium.
- Tsai C-J, Leitzmann MF, Hu FB, et al. 2004.
- Frequent nut consumption and decreased risk of cholecystectomy in women. Am J Clin Nutr 80:76-81.
Insulin Sensitivity in Chinese Vegetarians
Insulin is a hormone that plays a role in keeping the level of sugar in our blood fairly constant. Since hormones act as messengers in our bodies, it is important that the cells to which the hormones are delivering a message are able to receive that message. In some conditions, like obesity, cells in our bodies are not sensitive to the insulin’s message. Despite insulin telling cells to take in sugar from the blood stream, cells don’t respond and blood sugar levels can go too high. Ideally, insulin sensitivity should be high to reduce the risk of diabetes, hardening of the arteries, high blood pressure, and other conditions. In a study from China, 19 lacto-ovo vegetarians were compared to 17 non-vegetarians. Despite having similar body weights and blood pressures, the vegetarians were more insulin sensitive than the non-vegetarians and had lower blood sugar levels. This increased insulin sensitivity may help to explain why vegetarians tend to have lower rates of diabetes and other chronic diseases.
- Kuo C-S, Lai N-S, Ho L-T, et al. 2004.
- Insulin sensitivity in Chinese ovo-lacto vegetarians compared with omnivores. Eur J Clin Nutr 58:312-16.
Fats and Prostate Cancer
Vegetarian diets are often low in a group of fats called omega-3 fatty acids. Fats in this group include alpha-linolenic acid (found in flax seeds and flax seed oil and, to a smaller extent, in canola oil and soy products), docosahexaenoic acid (DHA, found in fatty fish), and eicosapentaenoic acid (EPA, also found in fatty fish). Omega-3 fatty acids have been associated with a reduced risk of heart disease and may have other health benefits. Since vegetarians do not eat fatty fish, they have been encouraged to include sources of alpha-linolenic acid like flax seeds and flax seed oil in their diets regularly.
A new report raises questions about alpha-linolenic acid for some men. A study of more than 45,000 men in the U.S. found that those men whose diets were highest in alpha-linolenic acid did not have an increased risk of developing prostate cancer in general but did have an increased risk of developing advanced prostate cancer. This was true whether the alpha-linolenic acid came from meat and dairy products or from salad dressing and margarine (flax seed or flax seed oil intake was not reported). The other omega-3 fatty acids, EPA and DHA, were associated with lower prostate cancer risk. We cannot be sure that the same results would be seen in vegans because they frequently use sources of alpha-linolenic acid that were not examined in this study. However, until more information is available, the results of this study suggest that men who are at increased risk of prostate cancer should not use high amounts of alpha-linolenic acid. Some vegetarian men may opt to use a vegan DHA supplement derived from microalgae instead of other sources of omega-3 fatty acids.
- Leitzmann MF, Stampfer MJ, Michaud DS, et al. 2004.
- Dietary intake of n-3 and n-6 fatty acids and the risk of prostate cancer. Am J Clin Nutr 80:204-16.
Adequate Vitamin D Reduces Risk of Gum Disease
Vitamin D has been shown to play an important role in bone health and in reducing risk of certain cancers. This vitamin, which vegetarians get from fortified foods and through sun exposure, also appears to help reduce risk of gum disease, especially in older people. Gum disease can cause tooth loss and may even increase risk of heart disease. Certainly proper dental care, brushing, and flossing can reduce risk of developing gum disease. There is also evidence that dietary choices can affect gum health.
The National Health and Nutrition Examination Survey (NHANES III) was conducted between 1988 and 1994 to examine the health of people in the U.S. One part of this study was to look at people’s dental health, including examining their gums for bleeding and other problems. Levels of vitamin D in their blood were also measured. In people age 50 and older, those with the highest blood vitamin D levels had the lowest rate of gum disease. Older adults do not produce as much vitamin D in their skin as do younger adults, so they must rely on other sources of vitamin D. Good sources of vitamin D for vegans include vitamin D-fortified soymilk or rice milk, some fortified cereals, and vitamin D supplements.
- Dietrich T, Joshipura KJ, Dawson-Hughes B, et al. 2004.
- Association between serum concentrations of 25-hydroxyvitamin D3 and periodontal disease in the US population. Am J Clin Nutr 80:108-13.
Pass the Salad Dressing?
Beta-carotene and other related substances like lutein and lycopene (called carotenoids) are found in fruits and vegetables. They may be a part of the reason why people who eat lots of fruits and vegetables have lower rates of many kinds of cancer and other diseases. For carotenoids to be used, they must be absorbed. Carotenoids are best absorbed when fat is present in the intestines at the same time that the carotenoids are being absorbed.
Many people eat salads containing generous amounts of foods high in carotenoids like spinach, romaine lettuce, shredded carrots, red peppers, and tomatoes. A recent study found that the type of salad dressing used determines whether the carotenoids in the salad are absorbed. When subjects ate a salad with fat-free salad dressing, almost none of the carotenoids from the salad were absorbed. When the salad was eaten with a reduced-fat salad dressing, more carotenoids were absorbed. A full-fat salad dressing led to an even higher carotenoid absorption. The results of this study suggest that use of a salad dressing containing at least 3-5 grams of fat in a serving can increase the amount of carotenoids absorbed.
- Brown MJ, Ferruzzi MG, Nguyen ML, et al. 2004.
- Carotenoid bioavailability is higher from salads ingested with full-fat than with fat-reduced salad dressings as measured with electrochemical detection. Am J Clin Nutr 80:396-403.
Overweight Women Give Vegan Diet High Marks
What if overweight women who wanted to lose weight were placed on either a standard low-fat diet or a very low-fat vegan diet? Which diet would be easier for the women to follow? Which would be more effective? These were some of the questions that Dr. Neal Barnard and co-workers at Physicians’ Committee for Responsible Medicine set out to answer. They recruited 59 overweight and obese women and assigned them to a diet with no animal products and with 10 percent of calories from fat or to a non-vegetarian diet with 30 percent or fewer calories from fat. The women were taught about their assigned diets and were expected to follow them for 14 weeks. Both groups lost weight, but the vegan group lost an average of 4 pounds more. Women following the more standard diet reported they felt more constrained and restricted than did women on the vegan diet. Both groups found their diets were acceptable and that foods were easy to prepare. Both groups reported increased energy and improved sleep. More than 85 percent of the women following the vegan diet felt that they could continue with the diet in the future. This study suggests that vegan diets can be acceptable to people who have not followed this type of diet in the past and who are interested in weight loss.
- Barnard ND, Scialli AR, Turner-McGrievy G, et al. 2004.
- Acceptability of a low-fat vegan diet compares favorably to a Step II Diet in a randomized controlled trial. J Cardiopulm Rehab 24:229-35.
