I have seen in news that some bodybuilder died of taking steroids; when I went through details I learned that "low quality proteins" contributed to their death. I have studied about linkages in proteins but I don't understand what makes them stronger or weaker. So what is the difference between high quality proteins or low quality proteins? How does low quality protein affect muscular activity or growth?
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1$\begingroup$ I'm voting to close this question as off-topic because it belongs to fitness.SE not biology.SE. $\endgroup$– Remi.bJan 23, 2017 at 18:30
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$\begingroup$ @Remi.b I tried to save this question for the OP - I think there is an on-topic question contained there once the personal health component is removed. There might be an additional question to send to fitness.SE about how/whether to use protein supplements that wouldn't be appropriate here. $\endgroup$– Bryan Krause ♦Jan 23, 2017 at 20:54
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$\begingroup$ Without a link to the news story, and subsequently the investigation into this individual's death, it's impossible to know what exactly "low-quality" means in this context. It could be what @BryanKrause talks about in his answer, or it could mean "low-quality" as in it was procured from a disreputable source that didn't take adequate care to verify the protein's purity, or adulterated the product somehow like the melamine scandal regarding milk powder in China beginning in 2008. $\endgroup$– MattDMoJan 24, 2017 at 1:48
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"Low quality" vs. "high quality" protein does not refer to anything about the linkages between amino acids in proteins. Instead, these terms are used in a nutritional context to refer to whether an individual protein source is sufficient as a sole source of protein in someones' diet.
Essential amino acids are the amino acids that humans cannot synthesize; other amino acids can be synthesized from these, but they do not need to be part of the diet. Not all sources of protein have sufficient quantities of all of the essential amino acids. Low-quality protein sources are also referred to as "incomplete" and high-quality sources as "complete."
Meat products are typically "high quality" because they contain all of the essential amino acids together. Therefore, if you subsisted on various foods but only got your protein from one animal source, you would be okay.
Some plant products do not have all the essential amino acids in large concentrations. However, if you combine protein sources from different "low quality" plant sources that together cover these deficiencies, there is no disadvantage compared to a single "high quality" source.
In the digestive process, all you take out of the proteins you eat is the individual amino acids (generally): that means that only the amino acid composition matters, not the strength or quality of the bonds between amino acids. If someone tries to tell you otherwise, they are probably just trying to sell you something.
Here is a paper that talks about some of these issues, and addresses some of the myths about balancing proteins. The quick summary is that problems develop if you get your sole protein from a source that is low in a particular essential amino acids over a long time period, but there is no need to ensure every meal contain all the amino acids.
It is possible for people on unusual protein diets, either because they are avoiding animal products or using incomplete protein supplements, to develop a particular amino acid deficiency. Otherwise, as long as all amino acids are supplied, there is no difference on health or muscles to consuming "low quality" vs "high quality" proteins.
References:
Young, V. R., & Pellett, P. L. (1994). Plant proteins in relation to human protein and amino acid nutrition. The American journal of clinical nutrition, 59(5), 1203S-1212S.
answered Jan 23, 2017 at 20:52
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$\begingroup$ My understanding is that amino acids are not generally stored. Say you eat just corn which is first-limiting in lysine. Once you've built what proteins you can before the lysine is exhausted, the remaining amino acids are further metabolized, mostly for energy, as opposed to being incorporated into proteins. My source dates to my Biochem degree in the '70s, and textbooks from the '90s. Maybe something new has been discovered about storage of amino acids? (Still, +1 on your post.) $\endgroup$– bpeditJan 23, 2017 at 22:19
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$\begingroup$ @bpedit Thanks, I think that's a good point, to my understanding you are correct that amino acids aren't "stored" the way fats are (at least not in humans) - and I didn't say anything about storage in my answer. However, you have a huge storage of amino acids in the form of all the proteins in your body. When proteins are degraded they aren't just junked, you can reuse a lot of the amino acids. The lysine you take in from a single meal is just a fraction of your total body lysine. So, it takes awhile for the shortage to catch up and cause problems. $\endgroup$– Bryan Krause ♦Jan 23, 2017 at 22:50
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$\begingroup$ @bpedit ...From the source I linked and just a general survey of "advice on the internet" it seems like a general guideline for vegetarians is to try to get a complete protein intake within the timeframe of a day, rather than worrying about being complete at each and every meal. I'm sure there is no immediate problem missing a day or two at a time, either, because fasting on those timescales isn't harmful (to my knowledge). $\endgroup$– Bryan Krause ♦Jan 23, 2017 at 22:53
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$\begingroup$ @bpedit You mention "many" but only gave one example. Many cultures have also suffered from things like rickets and scurvy at various points. I'm not sure what your perspective is, but I'd caution against the logical fallacies that people in the modern "paleo" movements employ, assuming that cultures they are less familiar with must have it all "figured out." Surely there are good evolutionary/anthropological reasons why there aren't extant cultures that subsist solely on corn with no other protein source, but I wouldn't read much further than that. $\endgroup$– Bryan Krause ♦Jan 23, 2017 at 23:16