Why your body may not be optimising the protein you eat

‍Editor’s Note: From grocery lists, to fitness priorities, and even healthy snacking, protein is everywhere—but do we truly understand it? In this series, the Good Food Movement breaks down the science behind this vital macronutrient and its value to the human body. It examines how we absorb protein from the food we consume, how this complex molecule has a role to play in processes like immunity, and the price the Earth pays for our growing protein needs.

You’ve mixed your rajma with your rice, measured your servings of fish and milk, and are all set to conquer your protein needs. But somewhere along the line you notice that all this meticulous planning is not resulting in better energy, muscles, or hair. What’s going wrong? 

Perhaps it is not your protein intake that’s faltering, but rather your protein absorption. The protein may be entering your body, but is it getting broken down into a form that can be processed and used appropriately? 

How protein goes kaput

How do our bodies usually absorb protein? Imagine it to be a knotted ball of yarn. Digestion starts when the stomach releases hydrochloric acid and the enzyme, pepsin. Hydrochloric acid denatures the protein (unfolding its complex three-dimensional structures) —essentially, it untangles the yarn. Pepsin starts the process of breaking protein into polypeptides, i.e. cutting the untangled yarn into manageable chunks. 

The body can only process around 20-30g of protein every 3 hours. So, you need to space out your intake to ensure your body can use it.

Polypeptides (long chains of amino acids, which are the building blocks of protein) are then passed onto the pancreas, which is the main centre of protein digestion. Proteases is one of the major protein-digesting enzymes released by the pancreas, and it breaks down polypeptides into tripeptides, dipeptides, and lastly, to individual amino acids. Think of the breakdown of polypeptides as separating the yarn into groups of 2–3 threads, or into individual threads. These journey onwards to the small intestine, which breaks down any remaining tripeptides and dipeptides into individual amino acids. These individual amino acids are then absorbed into the bloodstream. They all report to the liver, from where they are deployed across the body for different functions. 

But what happens if some protein does not break down into absorbable amino acids? Well, they continue their journey onto the large intestine. The large intestine does not have the enzymes to break down proteins. Sometimes, they may pass through your body unfettered, but they could also become tempting food for the bacteria of the colon. As the bacteria metabolises protein, it releases certain byproducts (including ammonia, sulphides, and valeric acid) which can contribute to various ailments, from inflammation to DNA damage. So, not only does a lack of absorption cut off our access to protein, it risks becoming an active health risk. 

Also read: Is there an ‘ideal’ amount of protein that must be consumed?

Protein 1, human body 0?

Why would our bodies refuse to digest protein? There could be multiple reasons. For one, you could be trying to finish off your protein requirement for the day in one sitting. The body can only process around 20-30g of protein every 3 hours. So, you need to space out your intake to ensure your body can use it. Similarly, certain disorders (like maple syrup urine disease) or medications (like antacids) might interfere with protein absorption.

The trickier issue is that not all protein is equally digestible. Its digestibility differs based on its source as well as processing method.  For example, dairy proteins are easier to digest than meats (though both have more than 90% digestibility). Plant proteins tend to be a little harder to digest, at only around 70-90% digestibility. One of the main reasons plant proteins are less digestible is that they are more likely to have anti-nutritional factors—compounds which obstruct digestion in some way. These compounds occur naturally in the plant, typically forming part of its immune system. 

The lesson here is not to cut out these foods from your meals, but rather to be mindful of how you incorporate them into your meals.

Cereals like wheat and rice, and vegetables and fruits like potatoes and tomatoes, are examples of plant proteins with common anti-nutritional factors. They have certain chemicals that bond with trypsin, pepsin, and other protein-digesting enzymes and inhibit their functioning.  

Tea and coffee share a certain astringency with fruits like grapes and pears, because they all have tannins. Tannins bind with the proteins we eat (especially collagen proteins—the proteins which build up our skin, bones, ligaments and tissues) forming a cluster (a process called aggregation). Binding changes their chemical composition such that digestive enzymes are unable to break them down. Garlics, onions and other alliums contain saponins, which also tend to form complexes with proteins resulting in reduced digestibility. 

Nuts and seeds form an interesting case, since their impact on protein digestion is indirect. They contain phytic acid which forms complexes in our bodies. These complexes inhibit the absorption of minerals like zinc and calcium, which are in turn crucial in the synthesis of digestive enzymes. Without adequate enzymes, our absorption of nutrition (protein or otherwise) suffers.

Also read: The science behind bodily protein: What are complete and incomplete sources?

The power of intuition and mindfulness

Don’t be alarmed: this doesn’t mean that plant proteins are a less reliable source of nutrition. The way we process proteins in our kitchens can improve how our bodies digest them. Age-old practices of soaking, germinating, heating, and pressure-cooking all increase digestibility of plant proteins by reducing the potency of their anti-nutritional factors. One study from as far back as 1989 found that soaking mung beans improved in vitro protein digestion by 4% in 6 hours and 21% in 18 hours by lowering anti-nutritional factors (phytic acid, saponins, and polyphenols) in the legume seed. 

Counterintuitively, a lot of common ways to process meat—mincing, salting, irradiation, ageing—are associated with lower digestibility. These processes tend to oxidise proteins, causing them to form strong bonds with each other (called cross-links), which makes them less susceptible to digestion; effectively, the yarn of wool becomes knotted up and harder to unravel. A 2017 study found that long-term pickling and drying, as well as extended high-temperature cooking, reduced the sensitivity of pork protein to pepsin digestion, and consequently the absorption of its protein content by the body.

Age-old practices of soaking, germinating, heating, and pressure-cooking all increase digestibility of plant proteins by reducing the potency of their anti-nutritional factors.

On the bright side, just like some proteins come with anti-nutritional factors, others are rich in enzymes and contribute to protein digestion. Ginger, pineapple, papaya, and honey all contain different protein-digesting enzymes like bromelain, papain, zingibain. 

It’d be a natural reaction to read this article and worry about your morning cup of tea, or the tomatoes you’d been enthusiastically incorporating into your salads. But fret not—there are many ways to ensure that the protein you’re feeding your body is being properly absorbed and processed. For one, you can use workarounds: eating cooked instead of raw tomatoes, soaking nuts and seeds before eating them, or shifting your tea time to three hours after your meal can go a long way. But more importantly, food is not a simple binary. Tea and tomatoes are both great sources of antioxidants—as are several other foods with anti-nutritional factors. The lesson here is not to cut out these foods from your meals, but rather to be mindful of how you incorporate them into your meals. 

Our bodies undergo thousands of reactions during the process of digestion which interact with each other in ways that are hard to predict and simplify. Moreover, human biology is varied, and our digestive systems respond differently to the same food. We do not yet have a predictable (or affordable) way of measuring our individual rates of protein absorption, which leaves a lot up to us observing our bodily reactions to foods and adjusting our approaches accordingly. It is nice, in a way, to be engaged in a continuous dialogue with our body, don’t you think?

Also read: Protein’s seen and unseen benefits: How it affects metabolism, muscle repair

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