Does animal protein consumption cause inflammation?

Question

This article by Michael Greger, M.D. claims:

We’ve known for 14 years that a single meal of meat, dairy, and eggs triggers an inflammatory reaction inside the body within hours of consumption.

I've heard similar claims (1) from many other plant-based diet advocates, such as John McDougall, M.D. and Caldwell Esselstyn, M.D.

Do animal proteins in our food cause inflammation (especially compared to plant proteins)?

Are there contradictory studies? (2)

EDIT: The recently released documentary Game Changers also makes this claim, but provides no references.

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(1) To be clear, the claim is that animal protein, in general, (as opposed to other protein, and as opposed to some specific sources of animal protein, such as red meat, or processed meat) causes inflammation.

(2) It's fairly easy to track down specific claims. The problem is that the field of nutrition, and medical science, in general, is fraught with weak evidence (epidemiological correlations) and contradictions, so it's the overall weight of the evidence that matters. It's usually addressed in meta-studies and such, if at all.

Answer 1

The claims, like "meat, dairy, and eggs trigger an inflammatory reaction inside the body" arise from the studies in which they observed increased blood levels of "inflammatory markers," like C-reactive protein (CRP), in individuals on different diets. For example, in one study Dietary Red and Processed Meat Intake and Markers of Adiposity and Inflammation (J Am Coll Nutr, 2015) they observed an association between the high intake of red meat and increased CRP levels in women, but this does not already mean an association with any actual inflammatory disease.

In another study Vegetarian-Based Dietary Patterns and their Relation with Inflammatory and Immune Biomarkers: A Systematic Review and Meta-Analysis (Advances in Nutrition, 2019) they found an association between a vegetarian diet and reduced levels of inflammatory markers:

This study provides evidence that vegetarian-based dietary patterns are associated with lowered serum C-reactive protein, fibrinogen, and total leukocyte concentrations.

It could take years before high consumption of animal protein would result in an inflammatory disease, but it is not possible to make a study in which one group of participants would eat only animal and the other group only plant protein (and nothing else) for several years, so currently it is not possible to show a clear cause-effect relationship between animal protein and chronic inflammation.

Further, if a study says that a certain disease is associated with high animal protein consumption, it could be also associated with high animal fat or heme iron (from meat and fish) consumption, for example.

Examples of diseases with chronic inflammation:

• Atherosclerosis (and consequently cardiovascular disease)
• Cancer
• Rheumatoid arthritis
• Inflammatory bowel disease

Studies:

1) Cardiovascular disease:

A review: Plant Protein and Animal Proteins: Do They Differentially Affect Cardiovascular Disease Risk? (Advances in Nutrition, 2015):

...evidence to date is inconclusive and is likely to remain so, because it is difficult to isolate the independent effects of specific proteins.

2) Cancer

Vegetarian, vegan diets and multiple health outcomes: A systematic review with meta-analysis of observational studies (CRSFN, 2017):

This comprehensive meta-analysis reports a significant protective effect of a vegetarian diet versus the incidence and/or mortality from ischemic heart disease (-25%) and incidence from total cancer (-8%). Vegan diet conferred a significant reduced risk (-15%) of incidence from total cancer.

3) Rheumatoid arthritis

Nutrition Interventions in Rheumatoid Arthritis: The Potential Use of Plant-Based Diets. A Review (Frontiers in Nutrition, 2019):

Excessive body weight and diets that include animal products (e.g., dairy, red meat) exacerbate the RA symptoms likely due to their pro-inflammatory effects. In contrast, diets rich in vegetables, fruits, and fiber are associated with lower BMI, have anti-inflammatory properties and help reduce pain and inflammation in these patients.

4) Inflammatory bowel disease:

A prospective study including 67,581 women aged 40-65 years, in France: Animal protein intake and risk of inflammatory bowel disease (Am J Gastroenterol, 2010):

High total protein intake, specifically animal protein, was associated with a significantly increased risk of IBD [inflammatory bowel disease], (hazards ratio for the third vs. first tertile and 95% confidence interval being 3.31 and 1.41-7.77, and 3.03 and 1.45-6.34 for total and animal protein, respectively). Among sources of animal protein, high consumption of meat or fish but not of eggs or dairy products was associated with IBD risk.

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In conclusion, many studies have found an association between animal protein and inflammation. None of these studies have proven that the protein itself causes inflammatory diseases, though. From animal foods, the consumption of eggs (a review one, two) and dairy (a review one, two, three, four) has usually not been associated with inflammatory diseases, but the consumption of meat, especially processed meat* (a review one, two) has been.

*What is processed meat? According to World Health Organization:

Processed meat refers to meat that has been transformed through salting, curing, fermentation, smoking, or other processes to enhance flavor or improve preservation. Most processed meats contain pork or beef, but processed meats may also contain other red meats, poultry, offal, or meat by-products such as blood. Examples of processed meat include hot dogs (frankfurters), ham, sausages, corned beef, and biltong or beef jerky as well as canned meat and meat-based preparations and sauces.

In this context, the term meat processing does not include freezing and meat preparation directly before use (boiling, frying, salting, adding spices, etc.).

In individual studies they may not specifically define processed meat, but they refer to meats that contain substances that have been either added or formed during food processing (phosphates, nitrates, nitrites, -nitroso-compounds, polycyclic aromatic hydrocarbons, etc.). This is to show that the observed effects of high consumption of processed meat are not necessary due to protein but possibly due to added substances.

Answer 2

This meta-analysis showed that whey protein supplementation can actually reduce markers of inflammation.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4344580/

ABSTRACT

Whey supplementation is beneficial for human health, possibly by reducing the circulating C-reactive protein (CRP) level, a sensitive marker of inflammation. Thus, a meta-analysis of randomized controlled trials was conducted to evaluate their relationship. A systematic literature search was conducted in July, 2014, to identify eligible studies. Either a fixed-effects model or a random-effects model was used to calculate pooled effects. The meta-analysis results of nine trials showed a slight, but no significant, reduction of 0.42 mg/L (95% CI −0.96, 0.13) in CRP level with the supplementation of whey protein and its derivates. Relatively high heterogeneity across studies was observed. Subgroup analyses showed that whey significantly lowered CRP by 0.72 mg/L (95% CI −0.97, −0.47) among trials with a daily whey dose ≥20 g/day and by 0.67 mg/L (95% CI −1.21, −0.14) among trials with baseline CRP ≥3 mg/L. Meta-regression analysis revealed that the baseline CRP level was a potential effect modifier of whey supplementation in reducing CRP. In conclusion, our meta-analysis did not find sufficient evidence that whey and its derivates elicited a beneficial effect in reducing circulating CRP. However, they may significantly reduce CRP among participants with highly supplemental doses or increased baseline CRP levels.

So, there is, in fact, contradictory evidence, for some protein sources (whey protein is one that's most commonly used as a supplement)

Answer 3

According to Telegraph, inflammation occurs because of a sialic acid molecule called Neu5GC (N-Glycolylneuraminic acid) that triggers an immune response which causes inflammation.

Mice were used for the study which found that all the evidence linking Neu5Gc to cancer was circumstantial or indirectly predicted from experimental setups. According to the scientists, this is the first time they mimicked the exact situation in humans through feeding non-human Neu5Gc and inducing anti-Neu5Gc antibodies. This increased spontaneous cancer in mice.

This sugar can be found in red meats (pork, beef, and other livestock), cow’s milk and certain cheeses. Because the human body is not capable of producing this sugar naturally when the sugar is absorbed into the tissues, it is perceived as a foreign invader and activates the immune system. It is suspected that over time, the chronic inflammation caused by the immune system response plays a role in the development of cancer.

Thus, those who consume red meat on a regular basis are likely to suffer a stronger reaction than those who ingest red meat occasionally.