Animal Protein Intake and Functional Capacity in the Elderly
This study, conducted in Japanese community-dwelling elderly adults, found that higher protein intake, particularly animal protein intake, was associated with lower risk of future higher-level functional decline. Some of the strengths of this study include the large sample size of community-dwelling elderly people, long follow-up (7 years), and careful adjustment for potential confounders.
The results indicated that daily intake of animal protein may help prevent functional decline in elderly men. These results appear to be consistent with findings from prospective cohort studies that have shown a positive association between dietary protein intake and risk of falls and frailty, although these studies focused on the risk of falls and frailty, which were characterized as affecting the most basic ADLs. The TMIG assesses not basic functional disabilities, such as risk of falls and frailty, but impaired physical, psychological, and social functioning and can identify people who are at high risk of developing disabilities. To the best of the knowledge of the authors of the current study, no data are available on the associations between dietary protein intake and early functional decline in elderly people.
Result from the Health, Aging, and Body Composition Study suggest that animal protein intake is associated with lean mass change in elderly people, although such an association was not seen with plant protein intake. Animal protein provides all of the essential amino acids and is a source of higher-biological-value protein than plant protein. The essential amino acids, which cannot be synthesized, are required to meet the body's needs and are primarily responsible for stimulation of muscle synthesis. This supports the hypothesis that increasing animal protein intake may confer protection against future higher-level functional decline in elderly people. The findings were further clarified by showing that the highest quartile intake of fish, but not of meat, was associated with a lower risk of future higher-level functional decline.
A diet high in fish is favorable for the prevention of diabetes mellitus, stroke, and other lifestyle-related diseases. Particularly in elderly adults, the consumption of fish, and not of meat, is associated with a lower risk of dementia. Better social performance is considered to be a consequence of better physical and mental health. Because fish contain several types of nutrients, it is possible that the combined effect of several nutrients, rather than intake of a single nutrient is helpful in preventing functional decline in elderly adults. Thus, nutritional programs and public health activities that encourage the consumption of fish could prevent future functional decline and lifestyle-related diseases in elderly adults.
The intake of animal protein and TMIG subscale decline was also analyzed. A weak association was found between social role subscale decline and animal protein intake, but not with the intellectual activity and IADL subscales. It is thought that many factors contribute to sarcopenia, including central nervous system decline, intrinsic loss of muscle mass, reduction in dietary protein, and decreased level of physical activity. Sarcopenia, defined according to muscle mass, muscle strength, and physical performance, has a significant association with IADL disability in Japanese elderly adults. The consequences of sarcopenia, including decreases in muscle strength and decline in resistance-type physical activities, have not been clarified. One study recently found that decline in social role, but not in IADLs, was associated with low frequency of exercise. Thus, increasing animal protein intake could be related to increasing muscle mass and may lead to maintenance of social roles in elderly adults. The current study findings indicate that animal protein intake may affect early functional decline before affecting IADLs.
The association between animal protein intake and future higher-level functional decline was observed only in men in this study. This finding was consistent with results from the Women's Health Initiative Observational Study, in which associations between the risk of physical frailty and animal protein intake in elderly women were not found. Several investigators have suggested that loss of skeletal muscle in elderly adults may lead to greater risk of functional impairment. Two longitudinal studies showed an association between protein intake and muscle mass in older adults. Furthermore, loss of skeletal muscle mass with age was greater in men than in women. From these reports, higher animal protein intake appeared to be related to preservation of skeletal muscle mass and was associated to a greater degree with lower risk of future functional decline in men than in women.
Men with the lowest intake of animal protein tended to have greater intake of energy, carbohydrates, and carbohydrate-containing food (rice, bread, noodles) and lower intake of potassium, vegetables, and fruits than men who had the highest intake of animal protein. Some reports have demonstrated that high blood glucose levels and type II diabetes mellitus were related to greater cognitive decline and dementia. High carbohydrate intake may be a risk factor for these metabolic outcomes. Furthermore, high carbohydrate consumption is related to cognitive abnormalities. Because subjects with lower animal protein intake tended to consume large amounts of carbohydrates, these dietary patterns may have caused more-severe functional declines and future cognitive abnormalities.
In addition, it seems that men who have lower intake of animal protein have a tendency to eat a less-healthy diet. Because people's diets consist of a variety of foods with complex combinations of nutrients, the examination of only single foods could result in identification of erroneous associations between dietary factors and functional decline in elderly adults.
In future studies, the dietary pattern approach using factor and cluster analysis could provide more information regarding the risk of potential functional decline in elderly adults.
The current study had several limitations. First, information regarding food and nutrient intake was obtained on the basis of dietary recall. The association between the food frequency questionnaire and typical diets has been well established, although there are several problems; for example, the questionnaire has a limited number of items and minimal information about portion sizes. Second, information regarding dietary intake modification during the 7-year study period was not taken into consideration. Although participants who reported deterioration in their functional capacity because of illness were excluded from the baseline sample, some people in the broader population with past medical histories of diseases such as diabetes mellitus and hypertension would probably modify their lifestyle because of illness. Third, the information regarding METS exercise expenditure cannot be collected. Frequency of exercise was assessed instead of METS exercise expenditure. The relationship between TMIG decline and frequency of exercise, which are important factors that can affect elderly adults, was also analyzed; adjustment for frequency of exercise did not significantly modify the findings.
Greater intake of animal protein was associated with lower risk of higher-level functional decline as assessed using the TMIG in men but not in women. The characteristics of subjects who had higher levels of protein intake were different between men and women, which suggested that these characteristics and unmeasured or residual confounders may have caused the sex-based differences in the results. To the knowledge of the authors, this is the first longitudinal cohort study to associate animal protein intake with future functional decline in Japanese men and women aged 60 and older. The results indicated that low animal protein intake may contribute to early functional decline and that higher animal protein intake may reduce the risk of higher-level functional decline in elderly people.
Discussion
This study, conducted in Japanese community-dwelling elderly adults, found that higher protein intake, particularly animal protein intake, was associated with lower risk of future higher-level functional decline. Some of the strengths of this study include the large sample size of community-dwelling elderly people, long follow-up (7 years), and careful adjustment for potential confounders.
The results indicated that daily intake of animal protein may help prevent functional decline in elderly men. These results appear to be consistent with findings from prospective cohort studies that have shown a positive association between dietary protein intake and risk of falls and frailty, although these studies focused on the risk of falls and frailty, which were characterized as affecting the most basic ADLs. The TMIG assesses not basic functional disabilities, such as risk of falls and frailty, but impaired physical, psychological, and social functioning and can identify people who are at high risk of developing disabilities. To the best of the knowledge of the authors of the current study, no data are available on the associations between dietary protein intake and early functional decline in elderly people.
Result from the Health, Aging, and Body Composition Study suggest that animal protein intake is associated with lean mass change in elderly people, although such an association was not seen with plant protein intake. Animal protein provides all of the essential amino acids and is a source of higher-biological-value protein than plant protein. The essential amino acids, which cannot be synthesized, are required to meet the body's needs and are primarily responsible for stimulation of muscle synthesis. This supports the hypothesis that increasing animal protein intake may confer protection against future higher-level functional decline in elderly people. The findings were further clarified by showing that the highest quartile intake of fish, but not of meat, was associated with a lower risk of future higher-level functional decline.
A diet high in fish is favorable for the prevention of diabetes mellitus, stroke, and other lifestyle-related diseases. Particularly in elderly adults, the consumption of fish, and not of meat, is associated with a lower risk of dementia. Better social performance is considered to be a consequence of better physical and mental health. Because fish contain several types of nutrients, it is possible that the combined effect of several nutrients, rather than intake of a single nutrient is helpful in preventing functional decline in elderly adults. Thus, nutritional programs and public health activities that encourage the consumption of fish could prevent future functional decline and lifestyle-related diseases in elderly adults.
The intake of animal protein and TMIG subscale decline was also analyzed. A weak association was found between social role subscale decline and animal protein intake, but not with the intellectual activity and IADL subscales. It is thought that many factors contribute to sarcopenia, including central nervous system decline, intrinsic loss of muscle mass, reduction in dietary protein, and decreased level of physical activity. Sarcopenia, defined according to muscle mass, muscle strength, and physical performance, has a significant association with IADL disability in Japanese elderly adults. The consequences of sarcopenia, including decreases in muscle strength and decline in resistance-type physical activities, have not been clarified. One study recently found that decline in social role, but not in IADLs, was associated with low frequency of exercise. Thus, increasing animal protein intake could be related to increasing muscle mass and may lead to maintenance of social roles in elderly adults. The current study findings indicate that animal protein intake may affect early functional decline before affecting IADLs.
The association between animal protein intake and future higher-level functional decline was observed only in men in this study. This finding was consistent with results from the Women's Health Initiative Observational Study, in which associations between the risk of physical frailty and animal protein intake in elderly women were not found. Several investigators have suggested that loss of skeletal muscle in elderly adults may lead to greater risk of functional impairment. Two longitudinal studies showed an association between protein intake and muscle mass in older adults. Furthermore, loss of skeletal muscle mass with age was greater in men than in women. From these reports, higher animal protein intake appeared to be related to preservation of skeletal muscle mass and was associated to a greater degree with lower risk of future functional decline in men than in women.
Men with the lowest intake of animal protein tended to have greater intake of energy, carbohydrates, and carbohydrate-containing food (rice, bread, noodles) and lower intake of potassium, vegetables, and fruits than men who had the highest intake of animal protein. Some reports have demonstrated that high blood glucose levels and type II diabetes mellitus were related to greater cognitive decline and dementia. High carbohydrate intake may be a risk factor for these metabolic outcomes. Furthermore, high carbohydrate consumption is related to cognitive abnormalities. Because subjects with lower animal protein intake tended to consume large amounts of carbohydrates, these dietary patterns may have caused more-severe functional declines and future cognitive abnormalities.
In addition, it seems that men who have lower intake of animal protein have a tendency to eat a less-healthy diet. Because people's diets consist of a variety of foods with complex combinations of nutrients, the examination of only single foods could result in identification of erroneous associations between dietary factors and functional decline in elderly adults.
In future studies, the dietary pattern approach using factor and cluster analysis could provide more information regarding the risk of potential functional decline in elderly adults.
The current study had several limitations. First, information regarding food and nutrient intake was obtained on the basis of dietary recall. The association between the food frequency questionnaire and typical diets has been well established, although there are several problems; for example, the questionnaire has a limited number of items and minimal information about portion sizes. Second, information regarding dietary intake modification during the 7-year study period was not taken into consideration. Although participants who reported deterioration in their functional capacity because of illness were excluded from the baseline sample, some people in the broader population with past medical histories of diseases such as diabetes mellitus and hypertension would probably modify their lifestyle because of illness. Third, the information regarding METS exercise expenditure cannot be collected. Frequency of exercise was assessed instead of METS exercise expenditure. The relationship between TMIG decline and frequency of exercise, which are important factors that can affect elderly adults, was also analyzed; adjustment for frequency of exercise did not significantly modify the findings.
Greater intake of animal protein was associated with lower risk of higher-level functional decline as assessed using the TMIG in men but not in women. The characteristics of subjects who had higher levels of protein intake were different between men and women, which suggested that these characteristics and unmeasured or residual confounders may have caused the sex-based differences in the results. To the knowledge of the authors, this is the first longitudinal cohort study to associate animal protein intake with future functional decline in Japanese men and women aged 60 and older. The results indicated that low animal protein intake may contribute to early functional decline and that higher animal protein intake may reduce the risk of higher-level functional decline in elderly people.