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The scientific report guiding the US dietary guidelines: is it scientific?

BMJ 2015; 351 doi: https://doi.org/10.1136/bmj.h4962 (Published 23 September 2015) Cite this as: BMJ 2015;351:h4962

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Re: The scientific report guiding the US dietary guidelines: is it scientific?

Ms. Teicholz’s thoughtful and important critique inspired me to examine the DGAC report myself to form my own opinion. I focused on the portion of the report I am best qualified to analyze—the section pertaining to depression risk. The DGAC based its recommendations on a total of 19 studies—17 epidemiological studies and 2 randomized controlled trials. The committee concluded:

“Patterns emphasizing seafood, vegetables, fruits, and nuts, were generally associated with reduced risk of depression.”

“Patterns emphasizing red and processed meats and refined sugar were generally associated with increased risk of depression.”

Allow me to confess my food biases (something each member of the DGAC should also be required to do). I am a meat-eating psychiatrist, and believe, based on my personal experience and years of reading and writing about nutrition research, that meat is good for people. I have never found any scientific proof that red meat causes any human health problem, nor have I ever come across any plausible physiological mechanism to explain how red meat could cause a human health problem. I therefore read each of the 19 source studies to see how the committee formed its opinion that red meat increases risk for depression.

I set aside the fact that epidemiological studies cannot prove cause and effect and can therefore only generate hypotheses that then need to be tested. I also set aside the fact that it was easy to find high quality journal articles on the topic of dietary patterns and depression that were, for whatever reason, not included in the committee’s review. For the sake of this exercise, I simply took the evidence the DGAC used as the foundation of its report at face value. Briefly, here is what I found (each study is represented by its DGAC reference number):

3 studies did not evaluate meat (175,182,197).
7 studies implicated red meat in depression risk (189,194,196,200,201 203,205). All of them except for one (205) lumped meat in with dietary patterns that include junk foods high in refined carbohydrate.
2 studies lumped meat in with junk foods high in refined carbohydrates but nevertheless found no association between (lumped) meat and depression risk (193,198).
6 studies (190,191,192,195,199,202), including one RCT (202), found no association between meat and depression risk.
1 RCT (204) concluded that increased red meat consumption REDUCED risk for depression.

In summary, 16 studies look at meat. One of them suggests meat increases risk for depression. Six of them suggest that meat mixed with junk foods increases risk for depression. NINE studies specifically exonerate meat, including BOTH of the RCT’s, and one of the RCT’s found that eating MORE red meat was actually PROTECTIVE against depression.

How could the committee conclude, based on these studies, that meat consumption increases depression risk, when more than half of the studies exonerate meat? If a detailed examination of this small portion of the report is any indication of how the rest of the report was generated, then I wholeheartedly agree with Ms. Teicholz’s concern that the DGAC’s general conclusion that avoiding meat is better for our health is not grounded in science; in the case of depression, it is clearly not even grounded in the science the committee specifically selected to support its recommendations.

Summaries of each article (listed by DGAC report reference number) are below:

175: Meat not evaluated.
182: Negative study about HEI-C adherence and risk for depression (meat not evaluated).
189: Processed foods pattern associated with higher risk for depression than whole foods pattern. Unprocessed red meat and poultry were not included in either of the dietary patterns examined.
190: Higher ratio of red meat to white meat NOT associated with increased risk for depression
191: Negative study that specifically found no association between high meat/high fish dietary pattern and depression risk.
192: “Healthy” dietary pattern associated with lower risk for depression compared to an unhealthy pattern. The only component of the “unhealthy” diet that was associated with depression risk was refined carbohydrate, specifically not the meat.
193: Negative study (meat lumped together with Western junk foods but still no association)
194: “Prudent diet” better than “Unhealthy diet” (in which meat was lumped together with “chips, crisps, and soft drinks”).
195: Diet highest in animal protein did not increase risk for depression in women (high animal protein diet not tested in men).
196: “Inflammatory Dietary Pattern” (in which red meat was lumped together with sugar-sweetened soft drinks, refined grains, diet soft drinks, margarine, other vegetables, and fish) associated with increased risk for depression.
197: Higher adherence to DQI-I does not reduce risk for depression. Meat not studied (the DQI-I doesn’t include a meat category).
198: Dietary pattern containing red and processed meats not associated with depression risk.
199: Meat/processed meat dietary pattern did not increase risk for depression.
200: “Prudent diet” better than Western diet (which lumps meat in with “sweet snacks like ice-cream, candies and chocolate, sweet soft drinks and juices, baked potatoes and French fries, French rolls, processed foods, high fat cheese and eggs”).
201: Higher adherence to Mediterranean diet associated with reduced risk for depression. Difference in meat consumption between high adherence and low adherence group was only 43 g or 1.3 oz of “meat and meat products” whereas high adherence group ate 194 fewer grams of dairy per day, 373 g more vegetables, 307 more g of fruit, and 64 more g of fish. [Therefore: 66% more legumes, 60% more cereals, 264% more fruit/nuts, 216% more vegetables, 61% less dairy and only 22% less meat.]
202 (an RCT): In healthy subjects, the AHA diet (which includes lean red meats) no different from Mediterranean diet with or without nuts (the Mediterranean diet emphasizes white meats over red meats) with respect to depression risk.
203: Mediterranean diet associated with lower risk for depression (however the Mediterranean diet is not only lower in meat than standard diets but is also lower in refined carbohydrates).
204: (an RCT): “Vitality Diet” (containing 6 servings of lean red meat per week) reduced risk for depression when compared to a “Healthy Diet” (containing 3.5 servings of lean red meat per week).
205: “Healthy diet” without meat reduced risk for depression in pregnant women compared to a “Brazilian diet” (containing an unquantified amount of meat) or a “Processed” diet (containing an unquantified amount of processed meat).

Respectfully,

Georgia Ede MD

Competing interests: No competing interests

06 October 2015
Georgia A. Ede
Psychiatrist, Nutrition Consultant, Nutrition Blogger (www.diagnosisdiet.com)
Smith College
Smith College Health Service, 21 Belmont Ave, Northampton, MA 01063