Background: about protein
Almost every activity inside human body utilizes proteins. Protein is categorized as a type of macronutrient. Macronutrient is defined as one of the nutrients able to provide energy. For most people, providing energy is protein’s least significant role in the human body since body will prioritize the energy provision by metabolizing fats and carbohydrates. Protein is regularly spared from energy provision for its unique functions. Its components, the amino acids, are the building blocks of the human body, from our hair, bones, muscles to all the organs. It contributes to immunity in which antibodies are made from protein, digestion which uses protein as digestive enzymes, muscles which are built from protein and enable movements…These all emphasize the vitality of protein in human diet.
Amino acids are linked by polypeptide bonds and form a chain. These chains differ in the length as well as sequences in which these amino acids are linked. To make a protein, one or multiple chains of amino acids cluster and form a specific structure which matches the protein’s specific function. The variation in the structure of protein allows them to carry out many functions vital for human body, for example, the globular shape of hemoglobin facilitates its role of carrying oxygen. Amino acids are also categorized into essential (must be consumed in the diet) and non-essential (do not have to be consumed because they can be made inside the body using other amino acids) (Hou & Wu, 2018). The types and ratios of these amino acids vary in different foods. Higher proportion of protein is available in meat products, and they also contain a ratio of essential amino acids which is more like the dietary requirement of human.
Protein is present in almost every kind of food, varying in amount and the type of amino acids. Meat products such as beef, pork, different kinds of fish are high in protein. Some plant-based food also contains relatively large amount of protein such as soy and beans. There has been a large amount of discussion and research about the comparison between protein sources such as their amino acids composition and other effects on health, which will be further discussed in the following sections.
About Sarcopenia
Sarcopenia, which is defined as the loss of muscle mass along with a decline in its functional quality, usually occurs in older population due to the effect of aging on muscle (G. Abellan Van Kan 2009). Sarcopenia caused mainly by aging is defined as primary and those caused by other factors such as lack of motor activity, disease, malnutrition are termed as secondary sarcopenia. It has been an involving concept since Irwin Rosenberg initially defined it in 1989. It has been officially defined as a disease in 2016 by World Health Organization. Consequences of sarcopenia include decline in functional performances and onset of mobility limitations (G. Abellan Van Kan, 2009). Not only are the symptoms concerning in themselves, but also the health care cost is a growing issue. The direct health care cost of the condition in 2000 was estimated to be about 18.5 billion dollars (Janssen, I. 2004). And there was estimation that the cost of hospitalization of individuals with sarcopenia would be doubled (Steffl, M. 2017). Based on the estimation of WHO, an increase of 38% of population above 65 years old is expected in 2025, which means there would be more elder people and consequently more sarcopenia cases in the future.
The general meaning of sarcopenia has been made clear. Multiple measurements and standards have been developed since 1989 such as magnetic resonance imaging and Dual energy X-ray Absorptiometry which is the most accepted approach of quantifying muscle mass (G. Abellan Van Kan 2009). One of the most common methods of measuring strength is testing handgrip strength. Some other methods are available but each of them has certain drawbacks. Due to the variation in measurements as well as the blurring lines between sarcopenic and simply having low muscle mass and functional quality, there has not been a consensus on the operational definition applicable across clinics, research, and the population. As a result, there is a noticeable variation between sarcopenia prevalence investigated by different research.
The main factors affecting muscle mass are muscle atrophy, decreased mitochondrial function, increased oxidative stress, impaired satellite cell function and inflammation (Distefano,G. 2018), which are all age-related. One study has shown that a person loses muscle at a rate of 0.3-0.8 percent every year after an approximate age of 30 (Paddon-Jones, D 2009), along with a decrease in muscle strength with a rate ranging from 2 to 4 percent annually, which becomes significant after 50-60 years old. The rate of decrease of strength is greater than the rate of decrease of muscle mass. If the declines of the two are linear, then the loss of strength can be said to be completely caused by the loss of muscle mass. This suggests not only the muscle mass but also the functional quality of muscles declines. As a result, this provides a justification for using both factors, and muscle strength can be a better predictor of disability and increased mortality rate than muscle mass (Distefano & Goodpaster, 2018).
This paragraph will mainly focus on sarcopenia prevalence in older (generally above 60) Chinese people. The Asian Working Group of Sarcopenia 2019 consensus (AWGS 2019) refined the definition of possible sarcopenia, sarcopenia, and severe sarcopenia. As a result, the results from studies before 2019 may no longer be valid, and the results from studies after 2019 will be more applicable across each other. A study in 2021 analyzed 6172 subjects from 28 provinces, and the results showed that the prevalence rate of possible sarcopenia is 38.5%; the prevalence rate of sarcopenia is 18.6%; the prevalence rate of severe sarcopenia is 8.0% (Wu et al., 2021). Shandong University has published a meta-analysis in 2022 saying that Chinese people over the age of 65 have a sarcopenia prevalence of 23.2%(Xiaoyu et al., 2022). It is also reavealed a link between age and sarcopenia prevalence, showing that it is 10.4% for people aged 65-69, 22.4% for people between age 70-79, and 40.6% for people over 80. It is hard to state the comparison between the sarcopenia prevalence in China and other regions due to the differences in measurements and standards. However, considering the traditional dietary pattern of China tend to underrate the importance of protein in general, it makes sense if Chinese population has a higher sarcopenia prevalence than the west.
Available methods of treatment and prevention all focus on consuming suitable amount of protein and using resistance training (Cruz-Jentoft, A.J. 2020), which focus on compensating or even reversing the adverse effects of aging. Lean mass is positively correlated to overall health (Wolfe, R.R. 2017) and its maintenance positively influences the decline in muscle function due to aging (Bradlee, M.L. 2018). Meanwhile, to a lesser extent, gait speed and hemoglobin concentration were positively associated with decreased risk of sarcopenia. (Xin, Wu. 2021). All the factors mentioned will be improved from engagement in physical activities and the consumption of adequate protein.
Circumstances about protein consumption in older Chinese people
The circumstances of protein consumption will focus on three aspects: protein injestion amount, protein distribution, and protein source.
In general, the protein intake amount is below the recommended take, hence a relatively higher possibility of sarcopenia. The Dietary reference intake of China states that protein should account for 15% of the total energy consumption. However, a study in 2015 revealed that the average protein energy percentage was below 15% in all the four regions – Northeast, east coast, central, and western ((Xu et al., 2015). A meta-analysis on sarcopenia prevalence (Xiaoyu et al., 2022) stated that more than 80% of older Chinese people have intake levels blow the recommended intake of protein, while the consumption of other two macronutrients, fats and carbohydrates, are more ideal. A longitudinal study (Wany et al., 2019)summarized data from China Health and Nutrition Survey from 1991 to 2015 and concluded that the protein consumption among elderly of Chinese (above 60) in the nine provinces investigated has decreased and replaced by more fat in the diet. The average protein intake was 52.2 g/day, which can hardly be considered adequate. However, an urban-rural difference of protein consumption is noticed. Since this study focused on only 9 provinces, it may not be as valid as expected. Another study (Zhao et al., 2021) collected data from the China National Chronic Non-Communicable Disease and Nutrition Surveillance of adults in 2015, but this time from 31 provinces and for people above 65. The average amount of protein intake is 47.9 g/day, which is more inadequate. The consumption of protein as well as other nutrients varies according to regions. People with higher incomes (live in urban) generally consume more protein and fat (Wang, Liusen. 2019; Qihe, Wang. 2022 (Xu et al., 2015) - we don’t know whether this is due to access to food, education, or preferences. This emphasizes a need to promote more rural and low-income older people to choose a more balanced diet.
According to a native study in 2021 (Ouyang et al., 2022), the protein distribution pattern of Chinese people is 25% in morning and 35%-36% for both lunch and dinner. These results may fluctuate since culture which include dietary pattern varies among different places and nations. However, this study applied multistage sampling method to minimize selection bias to the greatest extent.
According to a review focusing on meat product consumption in the East (Nam et al., 2010): Speaking from a historical aspect, meat product and its processing industry in the East are not as well developed as those in the west. Although such issue has improved from economic growth and the import of western wood, culturally speaking traditional eastern diet favor more grains and vegetables and failed to realize the importance of protein. Such an issue regarding tradition is more applicable to the elders, who are the ones who should be more aware of this due to the increase in sarcopenia prevalence.
How should the diet be modified to promote muscle health and prevent sarcopenia?
The requirement for protein of an individual varies according to age, gender, and activity (World Health Organization: Geneva, Switzerland, 2007). A study (Loenneke et al., 2016) suggests people over 50 should beware of protein consumption to prevent sarcopenia. Having inadequate protein contributes to incomplete muscle synthesis therefore increases the loss of muscle and more likely lead to sarcopenia in long term; having more than enough protein does not mean the negative changes in the muscle would be moderated to a greater extent (and most of them would be stored as adipose tissue, which may potentially in turn contribute to sarcopenia) compared to having the recommended amount of protein. However, the discussion around the optimal amount of protein hasn’t reached a consensus. The current international Recommended Dietary Allowance for protein is 0.8 g/kg. But there have been studies such as this review (Lonnie et al., 2018) suggest that the current recommendation may be insufficient for necessary muscle maintenance. A literature review (Putra et al., 2021) recommended a daily intake of 1.2 g/kg specifically for older adults in order to prevent sarcopenia after summarizing the studies from the past decade. And according to the autoreactive nutritional science review published in 2023 which represents the consensus of Chinese nutritionists (老年人肌少症防控干预中国专家共识(2023), 2023) the intake for sarcopenic individuals should reach as much as 1.5g /kg body weight. It should be noticed that meeting the recommended standard might be harder for elders since their decrease in consumption volume and appetite, if so then protein supplementation can be applied.
More frequent and evenly distributed protein consumption is recommended since it seems to be associated with increased muscle mass by promoting muscle protein synthesis. Having most of the daily protein in a single meal leads to a greater retention of fat-free mass compared to a more even distribution. Eating protein over two meals rather than 1 or zero gives you better knee strength and lean muscle mass (Loenneke et al., 2016). And the study also indicates that the optimal per meal protein consumption lies between 30 and 45 grams. Another review focusing specifically on how protein consumption promotes muscle building also imply that even distribution seems to have the best effect on muscle growth (Schoenfeld & Aragon, 2018).
Protein comes from food products in various forms, which can be categorized as animal source, plant source, and supplements. Each kind of food’s constituents vary accordingly, from the type of amino acids present to its relative proportion. Animal sources contain higher proportion of essential amino acids and fat (Hou & Wu, 2018). They also contain fat-soluble vitamins within them. Animal sources can be further categorized into red meat, white meat, and dairy products. Although they all provide a substantial amount of essential amino acids, red meat such as beef contains higher proportion saturated fatty acids while white meat such as fish contains higher proportion of unsaturated fatty acids. Dairy products contain higher proportion of calcium and vitamin D. They contain all the essential amino acids. Plant protein contain less fat than animal protein but more fiber and water-soluble vitamins. Fiber is well known for its health benefits, and it is recommended by the Chinese dietary guideline to increase the intake considering the current dietary pattern of the population. Most of them do not contain all the essential amino acids. The application of highly plant-based or even vegetarian diet requires a careful planning to ensure the consumption of all types of amino acids in adequate amount. However, a study in 2021 states a higher level of vegetable protein intake is positively associated with higher muscle mass(Ouyang et al., 2022). This is probably the result of having less saturated fatty acids and more unsaturated fatty acids in the diet. Simply considering from nutritional aspects both of them provide other vital nutrients and they contain different proportions of amino acids, therefore a balanced consumption has complementary effects between different foods, which is one of the general guidelines to promote healthy eating.
Protein supplements are protein extracted from certain sources such as mild, egg and soy. As its processing suggests, they do not contain other nutrients, and they are recommended only when excess protein is required. Some supplements are categorized according to their production process such as whey while some other are categorized according to the type of amino acids they contain such as leucine. Supplementary essential amino acids especially leucine (老年人肌少症防控干预中国专家共识(2023), 2023)benefit older adults by promoting muscle synthesis. However, another study states that even with protein supplementation, if not combined with changes in behavior, namely participation in physical activities especially resistance training, there is no much effect on the muscle size, strength, and function(Mertz et al., 2021). The fact that protein supplementation alone does not benefit preservation of muscle mass and strength emphasized the that not only eating pattern contribute to muscle health, but lifestyle also plays an important role as well, which is the role that a majority of Chinese older people neglect.
There are other nutrients besides proteins, no matter in the form of supplements or in foods, that positively influence muscle such as vitamin D, omega-3 fatty acids, magnesium, and antioxidants (Putra et al., 2021). Although they are beyond the scope of this article, this fact again emphasized that a healthy diet comes from variation and balance.
Exercise should accompany recommended protein consumption approach
Besides adequate protein intake, the vitality of exercise has also been mentioned (Rogeri et al., 2022; Seo et al., 2021; Wu et al., 2021; 老年人肌少症防控干预中国专家共识(2023), 2023). The effects of exercise on muscles include better regeneration due to decreased fibrosis formation, less intermuscular adipose tissue which presents the decrease in insulin sensitivity of mitochondria, and consequently promoted muscle growth and functional quality ((Distefano & Goodpaster, 2018). As a result, exercise has the ability to moderate or even reverse the negative effects of aging on muscles. Resistance training is more effective than other training methods in promoting muscle growth and increase in muscle strength, the pattern applies to people at all stages of life.
Conclusion
Protein consumption among elderly Chinese people is evaluated from multiple aspects, and the result is that it is below the recommended level, which is associated with prevalence of sarcopenia. To prevent or treat sarcopenia, increased protein consumption and exercise are required, which are both not met by most of the elderly population. Multiple sources of protein are evaluated, and it is currently recommended to best consume them in a balanced way. Resistance training is the most suitable approach for sarcopenia prevention and treatment. It requires time for the field of nutrition and sport to further develop. More of these people should be educated and find a way to promote their health and therefore improve their life quality.