Weakening an Argument About Vegetarian Diets

A leading dietary expert recently claimed that adopting a vegetarian diet can lead to significant health improvements. The expert pointed out that...

Following a vegetarian diet can reduce the risk of heart disease, lower cholesterol levels, and help in controlling body weight. Given this perspective, a nutritionist in the same conference countered this claim by stating that eliminating meat from one's diet may lead to nutritional deficiencies, particularly in iron and vitamin B12, which are primarily found in animal products. The nutritionist argued that a balanced diet should include moderate amounts of lean meats.

Which of the following, if true, would most seriously weaken the nutritionist's argument?