Is cooked protein less digestible than raw protein?

Question

I saw the following claim in several places:

At Raw Glow

Cooking meat changes the molecular structure of some of its proteins, rendering them unusable by the body and making cellular healing, reproduction and regeneration difficult. The protein molecules become bound, making them harder to digest. Up to 50% of cooked proteins that one eats will coagulate and cross-link. Cross-linking of proteins is associated with Alzheimer’s disease.

Because of coagulation, the protein is 50% less assimilable, as research showed at the Max Planck Institute for National Research in Germany. This means that a person needs to eat twice as much protein if it is cooked as opposed to raw.

At Sacred Source Nutrition

According to the Max Planck Institute, cooking foods coagulates at least 50% of the protein[ii], making them less bio-available to the body.

I also heard this in the film "Food Matters".

I'm having a very hard time believing this, as I would expect denaturation of proteins to help with digestion, as it makes it easier for proteases to cut them up into amino acids. The idea that cooking proteins hinders their digestion sounds rather far-fetched to me. But I was unable to find the study from the Max-Planck Institute that is referenced, none of the articles directly link it.

Does this study exist and does it support the claim? Is there any other data on the effect of cooking on the digestibility and bioavailibity of proteins?

Answer 1

It depends on the particular protease and cooking temperature.

From (Santé-Lhoutellier 2008):

Meat cooking affected myofibrillar protein susceptibility to proteases, with increased or decreased rates, depending on the nature of the protease and the time/temperature parameters. Results showed a direct and quantitative relationship between protein carbonylation (p < 0.01) and aggregation (p < 0.05) induced by cooking and proteolytic susceptibility to pepsin. However, no such correlations have been observed with trypsin and α-chymotrypsin.

The change of an amino group by a carbonyl group can modify the redox properties of proteins and so can disturb the recognition of proteins by proteases.

From (Bax 2012), an in vitro study:

At 70 °C, the proteins underwent denaturation that enhanced the speed of pepsin digestion by increasing enzyme accessibility to protein cleavage sites. Above 100 °C, oxidation-related protein aggregation slowed pepsin digestion but improved meat protein overall digestibility.

From (Bax 2013), an in vivo study on "the effect of meat cooking temperature on the parameters of protein digestion in the small intestine, using the minipig as a model animal":

An in vitro approach has revealed that cooking temperature is one of the key determinants of digestion speed. Relative to raw meat, the speed of digestion was increased at a cooking temperature of 70°C, and decreased at a cooking temperature above 100°C. (Summarizing the previous study.)

This effect was explained by a progressive denaturation of proteins, which exposes cleavage sites to digestive enzymes, at low temperatures, and oxidation leading to protein aggregation, which hides cleavage sites, at high temperatures.

[I]n the present study, very similar variations to those recorded in vitro were observed, with the highest speed of digestion observed at a cooking temperature of 75°C.

This study shows that the speed of protein digestion, a parameter of increasing interest in nutrition, can be modulated by meat preparation, a slower digestion being observed with high cooking temperature.

References

Marie-Laure Bax, Laurent Aubry, Claude Ferreira, Jean-Dominique Daudin, Philippe Gatellier, Didier Rémond, and Véronique Santé-Lhoutellier. Cooking Temperature Is a Key Determinant of in Vitro Meat Protein Digestion Rate: Investigation of Underlying Mechanisms. Journal of Agricultural and Food Chemistry 2012 60 (10), 2569-2576

Bax M-L, Buffière C, Hafnaoui N, Gaudichon C, Savary-Auzeloux I, et al. (2013) Effects of Meat Cooking, and of Ingested Amount, on Protein Digestion Speed and Entry of Residual Proteins into the Colon: A Study in Minipigs. PLoS ONE 8(4): e61252. doi:10.1371/journal.pone.0061252

Veronique Santé-Lhoutellier, Thierry Astruc, Penka Marinova, Eleonore Greve, and Philippe Gatellier. Effect of Meat Cooking on Physicochemical State and in Vitro Digestibility of Myofibrillar Proteins. Journal of Agricultural and Food Chemistry 2008 56 (4), 1488-1494