Thank you for the thoughtful response Dr. Ludwig.
I agree that food availability data are problematic, for some of the reasons that I discussed and you pointed out as well. So I also included self-reported intake data, which I think told a similar story. Still, I agree this data, too, is flawed. My hope was that by using both data-sets, despite their limitations, at the least we can say what the data say. Obviously, reality could be different from the data. And if it is, we all want to know that and understand why that should be.
This means that the picture I have tried to draw could be qualified or even refuted if there was another data-set or interpretation and a compelling justification for why that data-set or interpretation is better. However, I am not persuaded that the data-set that has been offered as an alternative is better.
- Stephen and Wald use self-reported data and thus, as self-reported data, this data-set in principle has the same flaws as the NHANES data. Why should we use it over NHANES?
- Stephen and Wald’s set extends from 1920–1984, covering only 4 years after Guidelines were published.
- S&W’s data-set includes studies using heterogeneous assessment methods. This could be a weakness if each method had particular biases yet different methods were used more or less frequently over time.
- More importantly, I am not sure that the S&W data-set support the “low fat” argument. First, the decline in fat intake seemed to have begun before the dietary guidelines were released. And second, fat intake was projected at around 31% in 1920, yet the 1920 obesity rate was dramatically lower than in 2000, when the fat intake was higher, around 33%. See: https://pbs.twimg.com/media/DTjWBwkV4AEle_T.jpg
- As everyone knows the CDC data *are* the NHANES data. They show that % fat fell from ~37% to ~33% from 1980 to 2000. As we both must agree: this is around a 4% change. Again, as we also both agree, this decrease in % corresponded to ZERO change in intake. The % decrease happened because carb intake increased, but the driver here was more carbs. Total grams fat intake stayed constant from 1980 to 2000 according to the NHANES data.
- Finally, I do not think that using USDA survey data for first data point in 1960 and combining it with the NHANES data for 2000 represents best statistical practice. I think comparisons between years should be made within data-sets, using the same methodology. If this is done, I believe that best estimate, barring a new analysis from a different data-set that covers the years in question (1980 to 2010), is that fat intake changed about 4%. Again, this is consistent between USDA and NHANES, using within-data-set comparisons.
Moving on, we agree that in, e.g. the 1980 DGA, the brochure recommended increasing carbs. But ONLY if one limited fats. Moreover, it advised eating complex carbohydrates, not refined, and advised against sugars. See screencap: https://pbs.twimg.com/media/DTjaOtCV4AAD0w1.jpg
And original source: https://health.gov/dietaryguidelines/1980thin.pdf
A diet rich in complex, unrefined carbohydrates is compatible with good health, as e.g. some of the macrobiotic diet trials for T2DM have recently shown (see my recent Twitter feed). But Americans took the message “carbohydrates,” and ignored the “complex” part.
The question should also be asked: Is it possible that Americans ate more refined carbohydrates for reasons quite apart from their (mis)understanding of the guidelines? I cannot answer this question, but I think it is a legitimate one.
Last, there is no disagreement that, in the context of the standard American diet, overconsuming refined carbohydrates is harmful to health, and that this is what Americans are doing.
We also agree that present low-fat recommendations should continue to be a subject of debate. Clearly we need to debate every aspect of the dietary guidelines. Again, my claim is only an historical one: that it is difficult to interpret the dietary trend data as one of decreasing fat, and thus, that it is difficult to blame the guidelines’ previous low fat recommendations for the obesity epidemic.
I remain open (and eager) to revise my positions. If we want to get policy right, clearly the facts need to be right too. I believe that doing this and making a positive impact on others’ lives is what both of us are primarily interested in achieving.
I thank you for the opportunity to have discussed this issue with a researcher of such a high level of scientific achievement and important cultural legacy. It is an honor.