recent studies have linked low-protein diets to a longer life span. Protein is one of the three macronutrients that humans need to survive. The other two “macros” are carbohydrates and fat.

While we require these nutrients to survive and thrive, how much we need of each may vary from person to person. Your own ideal macronutrient distribution may depend on several factors, including your genetic profile, your current health goals, and your age.  Some people are genetically predisposed to burn fat more efficiently, and may do better on a high-fat diet. Adults over the age of 65 often lose some of their ability to absorb protein from food, which means they need to increase their protein intake.

Ideal protein intake is an area that tends to create a lot of confusion. High-protein, low-carb diets are often recommended for weight loss, because excess carbohydrates are converted into fat and stored in the body. But, a long-term high-protein diet may not be the best choice for optimizing health and increasing longevity. In fact, recent studies have linked diets that are lower in protein to a longer life span.

High-protein diets and cancer: a troubling link

Diets that are high in animal protein can boost a hormone in the body known as insulin-like growth factor (IGF-1). IGF-1 is similar to insulin, and is produced by the liver. It promotes cell growth, which is a good thing in children. But when we become adults, we need lower levels of this hormone, as it’s only necessary to replace old cells that have died off.

Too much IGF-1 in adults stimulates other things — unwanted things — to grow, such as tumors. A report in Cell Metabolism states that “high protein intake is linked to increased cancer, diabetes, and overall mortality.”

IGF-1 also may encourage abnormal cells to break off and move to other areas of the body. In other words, this hormone can encourage cancer to spread once it has developed. When this happens, it is known as metastasizing. When a tumor metastasizes, it often latches on to critical organs such as bones, lungs, liver, or brain.

A large study of more than 6,000 adults supports the benefits of a low-protein diet for younger adults. The study found that:

• Adults ages 50 to 65 who ate a high-protein diet were 4 times more likely to die of cancer than those who ate a low-protein diet.
• Adults ages 50 to 65 who ate a moderate-protein diet were 3 times more likely to die of cancer than the adults on a low-protein diet.
• Adults of all ages who ate a high-protein diet were 5 times more likely to die from diabetes.

These associations were adjusted for other health factors, including smoking, waist circumference, and other diseases, and they still held true. But, the links were only found when the proteins came from animal foods, not plant sources.

Note that adults over age 65 may benefit from more protein. Older adults who ate high amounts of protein had a 60 percent lower risk of dying from cancer.