We examined the relationship between dietary protein sources and protein adequacy, body composition, and prevalence of SO according to weight status in South Korean populations. It was found that lower consumption of plant-based protein foods, such as beans and tofu, or poultry and egg consumption among people who consumed excess daily dietary protein was strongly associated with SO prevalence. A positive association was observed between poultry and egg consumption and muscle strength in the sarcopenia group. Furthermore, red meat consumption showed a negative effect on imbalanced body composition by increased fat mass (kg and %) and decreased ASM (%) in participants with normal weight.
Obesity Risk Factors in Sarcopenic Obesity Prevalence
Our results are consistent with those of other studies on sarcopenia in the South Korean population (28, 29), using the weight-adjusted ASM to identify the prevalence of SO. Obesity is a low chronic inflammation factor that may lead to the development of SO by the infiltration of lipid deposition in the muscle tissue from extensive fat accumulation (30). In line with this, the combination of sarcopenia and obesity more significantly accelerates deteriorating muscle health when compared with the entities associated with chronic disease states in South Korean adults (31). Similarly, our results showed higher risks of body composition changes and prevalence of SO in the OB group.
From the perspective of muscle physiology, sarcopenia is independently associated with physical dysfunction and disability (32). We observed that higher physical inactivity reduced muscle function and mass, whereas it increased fat accumulation in the SO and sarcopenia groups. Currently, the coronavirus disease 2019 (COVID-19) pandemic has served as a potential risk factor for the onset of sarcopenia by interrupting physical activity, dietary habits, and sleep patterns in thousands of people who have been isolated, with excess time spent at home and owing to social distancing (33). Sarcopenia can lead to acute body composition changes, mitochondrial dysfunction, and insulin resistance, and may also increase the risk of COVID-19 infection and severity of symptoms.
A previous study reported decreased lean body mass and increased fat mass in the overweight/obese group, compared with healthy lean controls (34). There were positive associations with intermuscular fat mass and high sensitivity C-reactive protein (hs-CRP) and cortisol concentrations, as well as a negative association with SMM (34). Similarly, the SO and OB groups included more Taeeum-type individuals with predisposing metabolic risk factors (12, 13), compared with the BMI < 25 kg/m2 group. Our cross-sectional results from a previous cohort study showed associations with a higher prevalence of pre-MetS and higher hs-CRP levels in Taeeum-type South Korean populations (12, 13). This reflects obesity-related physiological characteristics associated with the Taeeum-type, which can be positively associated with SO prevalence in people who are overweight and obese. Contributing lifestyle factors include the consumption of uncontrolled or western-type diet and physical inactivity or sedentary lifestyle, which lead to imbalanced inflammatory mediators stimulating muscle tissues that aggravate low-grade inflammation (35).
Effects of Dietary Protein Sources and Adequacy on Muscle Physiology and Body Composition
Dietary protein ingestion is a key modulator of muscle mass and function (36). We found that the consumption of poultry and eggs positively affected grip strength (kg) in the sarcopenia group, while fat mass percentage increased with meat intake (g/d), whereas ASM (%) decreased in the normal group. It could be argued that the energy balance and protein adequacy might be problematic dietary risk factors in the participants who already had excess intake of energy (S [male: 2303.51 ± 46.86, female: 2072.89 ± 27.95] and normal [male: 2303.51 ± 46.86, female: 2072.89 ± 27.95], kcal/day) and protein (S: 1.28 ± 0.01 and normal: 1.22 ± 0.01, g/kg) compared with their dietary reference intake.
In a previous study, we found an approximately 2-fold higher meat intake increase in people with SO (PR: 1.93, 95% CI: 1.07–3.50), compared with that of the lowest tertile in older South Koreans with cardio-metabolic diseases (37). Consistent with these findings, our results showed that, participants who consumed < 18.5 g/day of poultry and eggs in the excessive protein intake individuals (> 1.2 g/kg/day) showed 3.4 times higher risk of developing SO compared with those in the highest intake group (Q5: 137.5 g/day). Similarly, with beans and tofu consumption, there was a 2.5-fold higher risk of developing sarcopenia in participants who had < 5.7 g/day intake in the excessive protein intake individuals.
In muscle protein synthesis, leucine is the major contributor to postprandial muscle anabolic synthesis (38). Compared with animal proteins, plant-based proteins have low leucine content, which may affect the anabolic properties (39). Nevertheless, plant-based proteins have various health-related benefits, including decreased BMI, reduced body fat, decreased caloric intake, increased energy expenditure, reduced insulin resistance, and prevention of obesity, T2DM, and CVDs (40). Cardiometabolic diseases often coexist with chronic inflammation and obesity, which are linked to poor diet quality and excess body weight. Therefore, an adequate amount of high-quality protein and supplementation with more plant-derived foods, such as legumes and beans, and low-saturated fat diet, could reduce the risk of underlying health conditions.
Nutritional Recommendations for Populations with Sarcopenic Obesity to Improve Clinical and Nutritional Statuses
Plant-based diets have been recommended to prevent chronic diseases by the Joint Food and Agriculture Organization of the United Nations and World Health Organization expert consultations since 2004 (41). In the EPIC-Oxford cohort study, individuals who were vegetarian or vegan and had a higher intake of fiber and polyunsaturated fatty acid and a lower intake of saturated fatty acid (SFA) had a decreased risk of ischemic heart disease, compared with meat-eaters (42).
A balanced diet is recommended, with anti-inflammatory effects derived from the consumption of plentiful vegetables and fruits, as well as the moderating effects of whole grains, low-fat dairy products, fish, legumes, and poultry and eggs consumption while avoiding SFAs found in sugars and snacks. Finally, individuals who have metabolic high-risk factors, such as obesity, insulin resistance, and metabolic syndrome, should have a healthy diet to improve their health. Moreover, having a healthy and active lifestyle is necessary for achieving weight control and lowering the glucose levels and lipid level parameters in people with obesity.
Strength and Limitations
To the best of our knowledge, this is the first study to evaluate the associations between SO prevalence and dietary protein sources by considering the predominant personality type with complex and different lifestyles, including diet, using the KM type. We also used valid and reliable nationally-representative, multilayer sampling data from the KDCC study. Finally, a wide range of covariates were considered in the adjustment of the multivariate analyses, including age, sex, health-related behaviors (smoking status, alcohol consumption, and physical activity level), KM type, and energy intake (kcal/day). These methods yielded descriptive results which greatly enhanced our understanding of SO prevalence and risk factors.
This study has some limitations. First, this cross-sectional study cannot provide causal inferences due to the limited evidence. Second, we did not consider the residual confounding factors, such as genetic or medical history, as covariates. Lastly, reverse causality might exist because the individuals with SO tended to consume lesser protein sources than their counterparts. In particular, lower energy intake was described in male participants with SO than in male participants of in the normal group. However, there was no variation in dietary intake between groups according to the status of sarcopenia and obesity in the participants; however, protein (g/kg) consumption of the participants varied between groups. The reason could be that people with higher BMI values did not consume adequate and/or high-quality protein relative to their body weight. It is also predicted that they consumed higher percentages of fat- or carbohydrate-rich foods in their diet. Therefore, careful interpretation is required to ensure the generalizability of our findings.