Spotlight on: Protein (and are you really getting enough?)

Protein is a fundamental macronutrient essential for the proper functioning of the human

body. In the human body, proteins are a part of every cell and tissue, and our bodies are constantly recycling proteins daily. From building and repairing tissues to supporting immune function and hormone production, its importance cannot be overstated. Adequate consumption of dietary protein is critical for the maintenance of optimal health during normal growth and aging. In this evidence-based article, we delve into the role of protein in the diet, exploring its benefits, recommended intake, sources, and potential health implications.

Benefits of protein

Muscle growth and repair

While it has many vital roles in the body, protein is crucial for muscle growth because it helps repair and maintain muscle tissue.  In addition to the normal protein turnover, a key role of protein is to facilitate muscle growth and repair; particularly during physical activity when muscle protein turnover (both muscle synthesis and breakdown) is increased. Consuming adequate protein (which supplies essential amino acids) is critical to repairing and rebuilding exercise-induced muscle damage, promoting muscle growth and strength. However, increasing growth and strength can only occur when enough protein is consumed to create a positive net protein balance, meaning the building of new muscle exceeds muscle breakdown (Krzysztofik et al., 2019). Under normal conditions, the increase in muscle breakdown outweighs the muscle synthesis which ultimately results in a negative net protein balance. That is why consuming protein is crucial to shift the balance towards increased muscle synthesis; resulting in a positive net protein balance (Joanisse et al., 2021). Increasing one’s protein intake, especially during resistance training, can lead to greater strength and muscle mass gains (Stokes et al., 2018). A recent meta-analysis showed significant positive associations between consuming protein directly after conducting resistance exercise, with a decrease in fat, and an increase in muscle (Morton et al., 2018). Outside of the exercise setting, a high-protein diet has also been found to help reduce the rate of age-related muscle loss (Mitchell et al., 2017).  

Weight management and satiety

There is abundant evidence to support a high-protein diet as a powerful weight management, and loss tool (Moon & Koh, 2020). Primarily, it helps individuals to feel fuller for longer, therefore reducing overall calorie intake, and preventing muscle loss during a weight loss diet. Accordingly, inadequate protein intake may lead to an increased appetite as the body seeks to ensure it receives enough essential amino acids, potentially resulting in overconsumption of calories. (Simpson & Raubenheimer, 2005; Carbone & Pasiakos, 2019). There is also a stronger satiety effect in consuming protein, as opposed to carbohydrates and fat. This effect is especially profound in solid protein-rich foods (as opposed to a protein shake for example) (Rand et al., 2003).

Secondly, the body expends more energy digesting and metabolising protein compared to carbohydrates and fats. Higher-protein diets have been promoted to increase overall energy expenditure through an increased energy expenditure directly following a meal, and resting metabolism. For example, around 20–30% of the calories in dietary protein are used in the process of metabolising and/or storing it in the body. This is substantially higher than the 5-10%, and 0-3% used by carbohydrates and fats, respectively (Westerterp-Plantenga et al., 2009). Additionally, it is common to experience a reduced metabolism when on a calorie-restricted diet (Heilbronn et al., 2006). Consuming more protein can help prevent this decline in resting energy expenditure (Halton & Hu, 2004; Eisenstein et al., 2002)

Finally, in addition to a lower metabolic rate, individuals on a weight-loss diet also experience muscle loss (Weiss et al., 2017). Consuming higher amounts of protein during calorie-restricted weight loss diets helps preserve muscle mass (Longland et al., 2016). In diets that involve around a 500–750 kcal/day deficit, protein intakes of around 1.6 g/kg/d or more have been found effective in preserving muscle mass (Carbone et al., 2019).

So how much protein should I eat?

The recommended dietary allowance (RDA) established by the World Health Organization is defined as the minimum amount of protein required to meet essential amino acid requirements, establish nitrogen balance, and prevent muscle mass loss for almost all adults. However, this is often misrepresented and misinterpreted as a recommended optimal intake. Instead, the commonly quoted RDA is the minimum amount of protein required to prevent deficiency, rather than an amount that allows for optimal health.

There are calls from professional organisations and experts to double the current RDA for physically active individuals, which would mean it would be recommended to consume between 1.2–2.0 g/kg/d of protein for optimal health (Carbone & Pasiakos, 2019). 

Protein sources and quality?

Protein can be obtained from both animal and plant-based sources. The one key consideration when choosing a protein source is its amino acid content. Amino acids are molecules used in our body to make all proteins in the body. The human body needs 20 different amino acids to function correctly. Nine of these are essential amino acids that must come from the diet (the rest can be made by the body if it is not supplied in the diet).

Animal protein

The key benefit of most animal proteins is that almost all of them contain all 9 essential amino acids and are called “complete proteins”. The only exception is animal-derived collagen, which is lacking tryptophan.

Some common animal protein sources are:

• Lean meats (chicken, turkey, beef, pork)

• Fish and seafood (salmon, tuna, shrimp)

• Eggs (particularly the whites)

• Dairy products (milk, yogurt, cheese)

Plant-based protein

Plant proteins are a little more complicated in that they often contain less and lack certain essential amino acids when compared to animal proteins (Krajcovicova-Kudlackova et al., 2005).  Generally, they also do not contain as much protein per serving as animal products. Protein sources lacking one or more essential amino acids are “incomplete proteins”. If an effort is not made to account for this, it can result in lower protein synthesis.

Some common plant protein sources are:

• Legumes (beans, lentils, chickpeas)

• Nuts and seeds (almonds, walnuts, chia seeds)

• Tofu and tempeh

• Quinoa (complete protein)

• Soy products (complete protein)

• Buckwheat (complete protein)

If you are looking to avoid animal proteins, it is recommended to consume a variety of different plant proteins, as opposed to aiming to eat only complete plant-based proteins. This helps ensure you are getting enough of all the essential amino acids in your diet. Plant-based protein powders can be a great complement to a non-animal diet as they often provide a higher concentration of protein and have all the essential amino acids added.

For more information about different protein powders see: https://www.healthfullymyself.com/post/protein-powders-which-form-offers-optimal-bioavailability

If you are interested in reading more about the potential and shortfall of plant-based protein sources you can check out this review:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7760812/

But is there such a thing as too much protein?

Short-term health effects of a high-protein diet

Short-term, it has been speculated that a high protein diet may cause harmful effects on the kidneys, and liver. This appears to be largely due to a study conducted around a century ago where renal damage was found in a small group of rats on a high-protein diet (Jackson & Moore, 1928). However, claims appear to be unfounded. A recent study examined the effect of a high-protein diet for one year where the participants were consuming 2.5 – 3.3g per kilogram per day. This study found no harmful effects on measures of blood lipids, liver, or kidney function (Antonio et al., 2016). Additionally, even though the group of participants consumed more calories, their body fat did not increase.

However, it should be noted that the study had several key limitations that mean these results should not be taken out of context. Primarily, the source of the extra protein added to the participants’ diet was not stated (and as we will go on to learn, different protein sources have their own benefits and limitations so this is important to consider when interpreting the results). Additionally, although there was no statistically significant change in the participants' blood lipid profile, the measurements before and after the study were trending negatively (this raises the question of whether the results would have reached significance if the study had been continued for longer).

Long-term health effects of a high-protein diet

Concerns surrounding high-protein diets and potential long-term health implications persist; especially when discussing longevity, metabolic health, cardiovascular health, and cancer risk. However, these concerns are predominantly related to the protein source rather than simply a diet high-protein from various sources.

One recent review linked a low-protein/high-carbohydrate diet to increased longevity and metabolic health. Similarly, several observational studies suggest that a low protein intake (<10% of calories from protein) may be protective against cancer- and diabetes-related mortality (Levine et al., 2014). However, this effect has been attributed to a diet high in specific amino acids (methionine or branched-chain amino acids) which are associated with the regulation of ageing and metabolism and found in particularly higher levels in red meat (Kitada et al., 2019). Additionally, animal proteins, especially red meat, contain higher levels of saturated fats than other proteins which are undisputedly linked to negative health outcomes. When Levine et al. (2014) separated the protein sources, their results indicated that while high levels of animal proteins (particularly red meat and processed meat) promote mortality and other health issues. The study also found that plant proteins have a particularly beneficial effect on health.

Multiple studies support these conclusions. Red meat and processed meat consumption are associated with the risk of developing cardiovascular disease, chronic kidney disease, cancer, and diabetes (Wolk, 2017; Lew et al., 2017). A meta-analysis by Wolk (2017) agrees, indicating that high red meat consumption increases the risk of cardiovascular mortality, and cancer and that high processed meat consumption increases the risk of cancer, cardiovascular mortality, and diabetes.

Recommendations

There is no evidence to suggest a high-protein diet has any negative health implications. However, it is generally recommended that animal meat consumption (red and processed meat in particular) should be limited, and choosing lean protein sources can help reduce the outlined health risks of animal protein consumption. Overall, balance appears to be key to obtaining the benefits both animal and plant proteins offer. Several authors suggest substituting more plant proteins into the diet in place of animal proteins, rather than adopting an ‘all or nothing’ approach to either source.

In summary

Protein is a vital nutrient that plays numerous crucial roles in the body. All evidence suggests that an intake of 1.2g/kg/day or more of protein is beneficial to most people. There is no one “best” protein source, instead ensuring an adequate intake of protein from a variety of sources is essential for overall health and well-being. Evidence suggests that while red meat should be limited, animal protein can be consumed as part of a balanced diet, and a wide variety of nutritious protein-rich foods from animal and plant sources should be incorporated into the diet, along with other healthy dietary habits.

We invite any questions or comments in the section below!

*Disclaimer*

The information presented in this article is for educational purposes only and is based on our interpretation of the current research. Individuals are recommended to consult with a qualified healthcare professionals or registered dietitians for personalised guidance and recommendations tailored to their individual needs and circumstances.

References

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