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High Animal Protein Diets: Is There a Cancer Connection?

December 18, 2014Food

Best Food Facts recently posted this blog:

A recent study (referred to in this article as Levine et al (2014)) followed more than 6,300 adults over the age of 50, to see what effect high-, medium-, and low-protein diets had on lifespan. A high-protein diet was defined as 20 percent of a person's daily calories coming from protein, a moderate-protein diet is 10-19 percent of calories from protein, and a low-protein diet consists of less than 10 percent protein. People in the study ate, on average, 16 percent protein, with two-thirds coming from animal sources. According to the researchers, this is typical of an American diet.

The findings? People aged 50-65 who ate high-protein diets were four times more likely to die of cancer compared to people who ate low-protein diets. Even those who ate moderate-protein diets were three times as likely to die from cancer. And people who ate high-protein diets were 75 percent more likely to die from any cause, including three times as likely to die from diabetes. The research team calculated that reducing protein intake from moderate to low would reduce the risk of death by 21 percent.

We reached out to Dr. George Blackburn, Professor of Surgery and Nutrition, Associate Director of the Division of Nutrition at Harvard Medical School, for some insight on the study.

What is your reaction to the Levine et al (2014) study?

Dr. Blackburn: It doesn't give us cause and effect but it shows an association. They used a single 24-hr recall as their dietary intake measurement. Using this approach incorporates considerable measurement error, especially when trying to link this intake with a long-term outcome like mortality. Additionally, there was no information on how animal protein was assessed for use in their models. As considerable protein is consumed in mixed dishes, it would be important to document rules used to assign protein in foods to animal or plant origin.

 The bottom line is these data should be replicated using more acceptable methods, including the incorporation of all adult subjects without existing disease like that in Yang et al (2014)*, and if results are shown to be null then the journal should withdraw publication of the study.

Other minor issues in Levine et al (2014) include:

  • They should have made sure all subjects with prior disease were excluded form analyses.
  • Selection of co-variates (probably should have used saturated fat rather than total fat) is not as complete as those in Yang et al (2014).
  • Separation of subjects by <10, 10-19, and ¡Ý20% of protein in their diets created unequal groups with very few subjects in the low protein group (this would be even a bigger issue for the analyses based on age).

* (Yang et al (2014) is a Centers for Disease Control and Prevention study examining the relationship between the consumption of added sugar and death from cardiovascular disease.)

One of the authors says, "... study after study shows near-indisputable health benefits from eating a plant-based diet." Is this a fair statement?

Dr. Blackburn: I think that's probably correct. Studies have shown that vegetarian protein is associated with better health than animal protein. They are all epidemiologic studies, so they don't relate to cause and effect, but I think we want to consume animal protein in moderation with a mixture of vegetable protein.

The study also showed that people above the age of 65 who ate more protein had a reduced risk of death. Why would this be?

Dr. Blackburn: It's the frailty of getting old. We didn't use to have a lot of people over the age of 65. Now we've got a huge population all the way up to 90 and older. They've got a different dietary requirement to meet, and we've learned a lot more in recent years about frailty and sarcopenia (the degenerative loss of skeletal muscle mass) associated with growing older. Dietary requirements change as you get older.

What are your thoughts on the contention in a news release on the Levine study that the cancer risk from eating a diet high in animal protein is about the same as smoking?

Dr. Blackburn: There's no mention of smoking in this study, so I have no comment on that. It's not an appropriate correlation to this study.  

What's your advice to people who read about this study and are concerned about their diet?

Dr. Blackburn: My advice is to consume a healthy diet that you can stick to. I don't think we need to concentrate on protein intake as much as we do on healthy foods. There needs to be proper balance. Focus on fruits and vegetables, whole grains, and fiber in balance with your protein choice. A good mixture of vegetable proteins, poultry, fish and milk make a lot of sense.

It's another study to take under consideration. We would not want to be overly focused on protein to the detriment of what's really needed, which are fruits, vegetables and whole grains.