Animal Protein vs Plant Protein - the illusion of scale in diet epidemiology.

This graph appeared in Jason Fung's excellent Intensive Dietary Management blog here. I don't really want to disagree with Jason's statement that animal protein raises insulin more than plant protein, as I haven't looked into the evidence for that or what it means - I merely want to point out that this graph, and the paper it comes from, do not by themselves provide evidence that eating animal protein is associated with a higher risk of developing type 2 diabetes than eating plant protein.

The paper, by Sluijs et al, is titled "Dietary Intake of Total, Animal, and Vegetable Protein and Risk of Type 2 Diabetes in the European Prospective Investigation into Cancer and Nutrition (EPIC)-NL Study" and states

"During 10 years of follow-up, 918 incident cases of diabetes were documented. Diabetes risk increased with higher total protein (hazard ratio 2.15 [95% CI 1.77–2.60] highest vs. lowest quartile) and animal protein (2.18 [1.80–2.63]) intake. Adjustment for confounders did not materially change these results. Further adjustment for adiposity measures attenuated the associations. Vegetable protein was not related to diabetes. Consuming 5 energy % from total or animal protein at the expense of 5 energy % from carbohydrates or fat increased diabetes risk.
Diets high in animal protein are associated with an increased diabetes risk. Our findings also suggest a similar association for total protein itself instead of only animal sources. Consumption of energy from protein at the expense of energy from either carbohydrates or fat may similarly increase diabetes risk. This finding indicates that accounting for protein content in dietary recommendations for diabetes prevention may be useful."

Leaving aside the implausibility of the finding for the moment, there's an inconsistency in this abstract. If vegetable protein isn't "related to" diabetes, why is total protein a problem?

I knew from reading Song et al recently that the quartiles for vegetable protein actually represent quite small amounts. In the graph above, the upper quartile of vegetable protein is eating 33g/day, while the lower quartile of animal protein is eating 35g/day. So if you want to compare similar amounts of these proteins, you need to compare upper quartile vege with lower quartile animal, and they have exactly the same association with diabetes. And total protein (meat and vege combined) actually had a stronger association with diabetes than animal protein (1.67 in model 3, vs 1.58 for animal protein).

In real life, most people were eating both sorts of protein. Across the animal protein quartiles in Table 1, vegetable protein stayed very constant (people ate much the same amount of wheat). Unfortunately, there is no baseline data that tells us how much animal protein the quartiles of vegetable protein ate.

But let's take a common-sense approach to this data. Model 3, which I cited earlier, isn't adjusted for BMI and waist circumference. The highest protein quartile reports eating fewer total calories than the others, but has significantly greater BMI and waist circumference. And when these are adjusted for (Model 4, Table 2), voila, the association between protein and diabetes disappears from the quartile calculations; only the per 10g association remains. And this, though small, is greater for total protein (1.16) than for animal protein (1.13).

Amount of total protein across quartiles is 64g, 72g, 79g, and 88g. This range hardly seems excessive. Why it would be associated with diabetes at all is, frankly, a mystery. And what would happen if this population ate 64g, 72g, 79g, or 88g of plant protein is completely unknowable.

EDIT: some afterthoughts

The abstract of this paper only reports the completely unadjusted HRs. That is, not even age or sex adjusted. "Attenuated" really means "Disappeared".

The upper protein quartile ate fewer calories, were more active, and had significantly higher BMI (2 points) and waist circumference (4cm).
In other words, either this study breaks the sacred rules of diet thermodynamics, or diet was not reported accurately.
The quartile who ate least vegetable protein had to have eaten more animal protein than the others, just to survive. So why is their risk of diabetes so low?

Court of last appeal - the early history of the high-fat diet for diabetes

It's a long story, and not a proud one. Seeing an email in my inbox from the Journal of Diabetes & Metabolism, which seemed like the title of a journal I'd investigated earlier, I impulsively sent off a draft of my history of Louis "Harry" Newburgh and the Michigan diet. I just emailed the unformatted pdf to them, and never engaged any portal or website.
The journal replied, in terrible English, that my article would be accepted with changes requested by one of "two reviewers" supplying a short paragraph each. This request was that I shorten and focus the abstract, and format references.

At this stage I realised this was an OMICs journal, of predatory reputation. I sent off the formatted article with no change but a small edit to the abstract to see what would happen.
The next thing I knew, I was sent an author proof. And an invoice for US$4,000. For some reason the illustration of Newburgh had been titled as being of Frederick Allen, a name which appears in the text but was never attached to any picture. I corrected this and received an authorproof with errors corrected.
Here is this proof, which I consider to be an accurate version of the paper, albeit I believe it was not properly peer-reviewed.

https://www.dropbox.com/s/l3pxp9lazyu5v04/2155-6156-7-696_Authorproof.pdf?dl=0

I never paid the fee, and received numerous reminders, not all addressed to me. I still receive emails from other OMICs journals requesting my input. I was never asked to sign a COI declaration of any sort, nor any other agreement (the work is Open Commons). I had read a thread on researchgate in which an author describes seeing their work published despite not paying the fee.
https://www.researchgate.net/post/Can_I_trust_OMICS_publishing_group

So I searched for my paper online and found it here.
http://www.omicsonline.org/open-access/court-of-last-appeal--the-early-history-of-the-highfat-diet-for-diabetes-2155-6156-1000696.php?aid=78354

Not only is the picture of Newburgh captioned Frederick Lewis Allen 1932, the references in the HTML version (but luckily not the pdf) are imported from some other paper, probably from a different journal.
I feel like this is a punishment to make an example of defaulting authors!

Anyway, the history of Newburgh is published, which is the main thing, but I feel ethically unclean, and whether anyone can ever now cite this article in a proper paper is uncertain. At least OMICs have not claimed exclusive rights in the matter, so republication is not out of the question. But then, if I'd thought this necessarily detailed history would be easy to publish in a proper journal, it would never have ended up in the hands of OMICs.

I'd like to thank Ash Simmonds and Zooko for introducing me to the work of Prof Newburgh.