Scientific Update

Researchers in the UK estimated greenhouse gas emissions from 61 different categories of foods. The emissions were based on food production, transportation, packaging, and storage. They then created six scenarios based on typical UK eating habits with modifications in the types of foods, but no change in the calorie level, and estimated the impact that each scenario would have on greenhouse gas emissions.

In Scenario 1, meat was directly replaced with dairy products. This change was estimated to reduce greenhouse gas emissions by 22%, but resulted in a higher fat diet.

Scenario 2 was developed to be similar to the diet of a typical vegetarian in the UK. A change to this scenario would result in an 18% reduction in greenhouse gas emissions.

Scenario 3 replaced meat with grains, fruits, vegetables, nuts, seeds, and dried beans. Dairy product consumption was unchanged. Greenhouse gas emissions would be 25% lower with this scenario compared to the typical UK diet.

Scenario 4 replaced meat and dairy products with a mixture of healthy and unhealthy plant-based foods (sweets, alcohol, soft drinks, etc). This scenario was estimated to reduce greenhouse gas emissions by 31%, but was also higher in sugar and was not believed to be a health-promoting diet.

Scenario 5 was similar to the diet of a typical vegan in the US. Greenhouse gas emissions would be about 23% lower.

Scenario 6 replaced meat and dairy products with grains, fruits, vegetables, nuts, seeds and dried beans. Greenhouse gas emissions would be about 25% lower. In addition, this scenario is lowest in fat and sugar.

On average, changing from a nonvegetarian to a vegetarian diet could reduce greenhouse gas emissions by 22% (average of scenarios 1-3). Changing from a nonvegetarian to a vegan diet was estimated to reduce greenhouse gas emissions by 26% (average of scenarios 4-6). Any of these changes would be less expensive than the average diet in the UK and would have adequate protein. Scenarios 3 and 6 would offer significant health benefits including a lower fat content and greater use of fruits, vegetables, beans, and grains. Further modifications to Scenario 6 including use of more local, in-season produce and reduction of unnecessary packaging might lead to reductions in greenhouse gas emissions in excess of 50% compared with a current “typical” UK diet, according to the study's authors. Dietary choices can make a significant impact on greenhouse gas emissions.

Stress fractures are a common overuse injury seen in both competitive and recreational athletes. They are painful and can limit an athlete's participation during a sports season. A recent study examined the role of dietary factors in stress fractures in teenage girls. Over 6500 girls, ages 9 to 15 at the start of the study, reported what they ate, what supplements they used, their physical activity, and any stress fractures over the study period of seven years. About 30% of the girls participated in at least an hour a day of high-impact activity (running, basketball, soccer, tennis, cheerleading, or volleyball). Overall, 4% of the girls had a stress fracture, with most stress fractures occurring in girls who regularly did high-impact activities. There was no protective effect of dairy product consumption — girls who ate 3 or more servings of dairy products per day were no less likely to have a stress fracture than girls who consumed no dairy. Higher calcium intakes also did not seem to be protective, but higher vitamin D intakes (averaging 663 IU/day) were associated with a lower risk of stress fracture. When only girls participating in high-impact activities were examined, higher vitamin D intakes were associated with a lower incidence of stress fracture; higher calcium intakes (~1900 mg/day) were associated with a greater risk of stress fracture. These results were consistent with other studies that have found that very low calcium intakes are bad for bones but that higher calcium intakes do not reduce the risk of fracture in children or teens. This study and other studies suggest that it is important for teenage girls to have adequate vitamin D. Vegan sources of vitamin D include fortified plant milks and supplements.

Should women who have had breast cancer eat soy products? Concerns have been raised because of soy's isoflavone content which could interfere with tamoxifen, a drug commonly used to treat breast cancer, and because of the potential for isoflavones to stimulate cell growth.

Researchers from the U.S. and China used data from 4 studies in the U.S. and China to examine the use of soy foods after women were diagnosed with breast cancer. Consumption of 10 mg or more of isoflavones per day (the amount in about 3.5 ounces of soymilk) was associated with a reduced risk of breast cancer recurrence and a slightly reduced risk of dying from breast cancer. These effects were seen in Chinese, U.S., and U.S. non-Asian women. In addition, soy food did not appear to interfere with tamoxifen therapy. Although these results are promising, breast cancer survivors should discuss any dietary changes with their healthcare providers.

In the United Kingdom, food, especially meat and dairy products, is responsible for about 20% of all greenhouse gas production. The UK's Climate Change Act of 2008 mandates reduction of greenhouse gas emissions to 80% below 1990 levels by 2050. One factor in meeting this mandate is to modify food consumption. An advisory committee created three possible scenarios for dietary changes that could reduce greenhouse gas emissions.

Ideally, in addition to helping the environment, dietary changes would also offer positive health benefits. A recent report estimated the health effects of each of the possible scenarios.

Scenario 1 calls for a 50% reduction in consumption of meat and dairy products and an increase in fruits, vegetables, and cereals. This is estimated to reduce greenhouse gas emissions by 19% and to result in 36,910 deaths per year being delayed or averted due to the healthier nature of the diet.

Scenario 2 calls for a 75% reduction in use of cow and sheep meat and replacing these foods with chicken and pork. This is estimated to reduce greenhouse gas emissions by 9% and to delay or avert 1,999 deaths per year.

Scenario 3 calls for a 50% reduction in use of meat from pigs and chickens and replacement of those foods by fruits, vegetables, and cereals. This is estimated to reduce greenhouse gas emissions by 3% and to result in 9,297 deaths per year being delayed or averted.

It is clear that Scenario 1 has the greatest impact on the environment and on human health. We can only wonder how much greater effect would be seen if 100% of meat and dairy products were to be replaced with fruits, vegetables, and cereals.

An analysis similar to this should be conducted in the United States and other countries so that dietary recommendations can be developed that represent best practices with regards to the environment and health.