Heme (blood) iron, cancer, and cardiovascular disease.
Iron encourages production of free radicals which can damage DNA and presumably increase cancer risk. In a study of over 14,000 individuals, high iron intake and high iron body stores were both positively linked to the risk of colon cancer. Higher levels of iron were associated with higher incidence of colon polyps, possible forerunners of colon tumors. However, cancer patients themselves had low levels of stored iron, indicating that cancer itself can deplete iron stores. 
Controversy has surrounded the question as to whether too much iron in your diet raises your risk for heart disease. A new study from the Harvard University School of Public Health brings new insight to the debate. Lasting for 4 years, this research involved more than 50,000 male health professionals. It was found that total iron intake was not associated with heart disease risk. But the source of the iron came was the principle factor. High levels of heme iron raised risk for heart disease twofold. Heme iron is the type of iron found in meat, chicken and fish.
Plant foods contain non-heme iron which appears to not be associated with risk for heart attack. Traditionally, many nutritionists used to consider non-heme iron to be inferior to the iron found in animal products, because non-heme iron is somewhat less well absorbed. But new evidence suggests that non-heme iron seems to be preferable.
When the body is low in iron, it can increase absorption of non-heme iron, and it can reduce adsorption when it already has sufficient amounts. The heme iron in meats tends to pass quickly right through the adsorption mechanism, thus entering the blood stream whether it is needed or not. Since vegetarians generally have adequate iron intake, it is clear that non-heme iron can easily meet nutritional needs. Also, plant iron doesn’t create the health risks of heme iron.
Iron increases heart disease risks because heme iron acts as a pro-oxidant, causing LDL-cholesterol — the ‘bad’ cholesterol — to react with oxygen. This reaction is involved in the formation of plaques in the arteries and therefore increases one’s risk of cardiovascular problems. The chart: http://www.ecologos.org/iron.htm