Energy and macronutrient intakes at breakfast and cognitive declines in community-dwelling older adults: a 9-year follow-up cohort study

Exploring causal links in the gut-brain axis: a Mendelian randomization study of gut microbiota, metabolites, and cognition

The causal mediation effects of metabolites between gut microbiota and cognitive phenotypes remain unclear. Guided by the gut-brain axis mechanism, this study employed systematic Mendelian randomization (MR) to investigate these mediation pathways and their implications for functional food development. Univariate MR analysis was performed to estimate the causality of 211 gut microbial taxa (n = 18 340) and 452 serum metabolites (n = 7824) on general cognitive (n = 257 700), non-cognitive (n = 510 795), and specific cognitive phenotypes (n ≈ 2500) using genome-wide association study data. Inverse-variance weighted estimation was adopted as the primary method, with MR sensitivity analyses performed to complement the results. Metabolic pathway analysis was employed to enrich metabolic profiles, while two-step MR was used to screen mediation pathways. We revealed seven causal associations between microbiotas or metabolites and cognitive phenotypes (FDR < 0.05). Increased abundance of the order Clostridiales id.1863 was associated with better cognitive traits (OR = 1.14, 95%CI = 1.06-1.22, P = 2.06 × 10-4), while 1-linoleoylglycerophosphoethanolamine was also positively associated with cognitive traits (OR = 1.61, 95%CI = 1.33-1.95, P = 8.17 × 10-7). Seven significant metabolic pathways were enriched, including alpha-linolenic acid and linoleic acid metabolism, highlighting the potential role of omega-3 and omega-6 fatty acids in cognitive health. We further identified two significant mediation pathways linking the gut microbiota to cognitive phenotypes through metabolites. Notably, homostachydrine (39.1%) was found to mediate a proportion of the impact of the genus Turicibacter on emotion recognition (indirect effect: β = 0.105, 95%CI = 0.006-0.259, p = 2.60 × 10-2). This study provides evidence for causal relationships between gut microbiota, serum metabolites, and cognitive function, supporting the gut-brain axis mechanism. Our findings suggest potential targets for the development of functional food and personalized nutrition to improve cognitive health.

Association between dietary diversity and healthy aging: a systematic review

OBJECTIVE

The proportion of older people has been enhanced in the past half-century around the world, consequently leading to the increasing overall prevalence of age-related health conditions. The present study aims to summarize the current evidence regarding the association between dietary diversity (DD) and the components of healthy aging.

METHODS

The databases, including Scopus, PubMed, Web of Science, and Google Scholar, were searched using relevant keywords and without date restrictions up to November 2023. All original articles written in English evaluating the association between DD and healthy aging components were eligible for this systematic review.

RESULTS

Totally, 2728 records were obtained in electronic search databases after removing duplicates and irrelevant studies based on the title and abstract; the full text of the 64 articles was critically screened, and 32 cross-sectional studies and 17 cohort studies were included in this review. Except for two cases, all of these studies (95.9%) reported that high DD was associated with lower cognitive and physical frailties, low sleep disturbance and mental disorders, and good nutritional status among the elderly population.

CONCLUSION

Findings suggest higher DD may contribute to healthy aging through improved nutrient intake and reduced risk of age-related diseases, but heterogeneity in study designs limits conclusive evidence. Enhancing healthy dietary behaviors for older people is suggested to increase their awareness and consequently DD. Further research is needed to confirm these results and the causal relationship.

Effects of Protein Intake on Cognitive Function in Chinese Older Adults: A 10-Year Follow-Up Study

BACKGROUND

As the global population ages, there is an increasing prevalence of mild cognitive impairment and dementia. Protecting and preserving cognitive function in older adults has become a critical public health concern.

METHODS

This study utilized data from four phases of the Chinese Longitudinal Healthy Longevity Survey conducted from 2008 to 2018, encompassing a total of 2454 participants. Latent growth curve modeling was employed to analyze the trajectory and role of protein intake frequency and cognitive function.

RESULTS

The frequency of protein intake among older adults tends to rise, with individuals exhibiting higher initial levels experiencing smaller subsequent increases. Conversely, cognitive function generally declines, with those starting at higher levels experiencing more pronounced decreases. Notably, the initial frequency of protein intake is positively correlated with the initial level of cognitive function (β = 0.227, 95% CI: 0.156 to 0.299, p < 0.001), but does not significantly influence the rate of change in cognitive function (β = -0.030, 95% CI: -0.068 to 0.009, p = 0.128). The rate of change in protein intake frequency is positively associated with the rate of change in cognitive function (β = 0.152, 95% CI: 0.023 to 0.280, p = 0.020).

CONCLUSIONS

The alterations in protein intake frequency are linked to alterations in cognitive function among older adults. Maintaining a stable high frequency of protein intake or increasing the frequency of protein intake may contribute to stabilizing cognitive function as well as reducing the risk of cognitive impairment and dementia in older adults.

Application of a new definition of sarcopenic obesity in middle-aged and older adults and association with cognitive function: Findings from the National Health and Nutrition Examination Survey 1999-2002

BACKGROUND & AIMS

The role of sarcopenic obesity (SO) in impaired cognitive function has been investigated in several observational studies, but results have been mixed. This study applied the proposed European Society for Clinical Nutrition and Metabolism (ESPEN)-European Association for the Study of Obesity (EASO) definition of SO to a representative population aged ≥50 years to identify the association between SO and cognitive function.

METHODS

Data from the National Health and Nutrition Examination Survey 1999-2002 waves were used. At the screening phase, body mass index or waist circumference were used to evaluate obesity; sarcopenia was identified using the SARC-F questionnaire. At the diagnostic phase I and II, sarcopenia was assessed using knee extensor isometric strength and appendicular lean mass, and fat mass percent was used to assess obesity. Cognitive function in older participants (60-85 years) was assessed using the Digit Symbol Substitution Test. A self-reported memory question was used in middle-aged individuals (50-59 years).

RESULTS

The sample included 2356 participants (men, 44.7%). The prevalence of SO was 32.3%, 21.2% and 15.0% at the screening, diagnosis I, and diagnosis II, respectively. Significant associations between SO and cognitive impairment were observed in individuals aged 60-85 at diagnosis I (OR: 2.3, 95%CI 1.4-3.8, P = 0.007) and diagnosis II (OR: 2.7, 95%CI 1.5-4.9, P = 0.004).

CONCLUSION

The new ESPEN-EASO definition of SO identified a high prevalence of SO cases. A significant association between SO and poor cognitive function in older individuals was observed.

Nutrition intake modifies the association between pulmonary function and cognitive performance among elderly Americans from NHANES 2011-2012

PURPOSE

Nutrition intake is one of the modifiable risk factors for cognitive decline. Whether energy and protein intakes alter the association between pulmonary function (PF) and cognition has not been studied.

METHODS

We made use of information from the U.S. National Health and Nutrition Examination Survey (NHANES) 2011-2012. PF measures, including forced expiratory volume in one second (FEV1), forced vital capacity (FVC), and peak expiratory flow (PEF), were calculated, whereas cognitive function was assessed through four tests: the Immediate Recall test (IRT), the Delayed Recall test (DRT), the Animal Fluency test (AFT) and the Digit Symbol Substitution test (DSST). Energy and protein intakes were measured using the 24-h dietary recall method. Weighted generalized linear regression was performed upon adjustment for covariates. Further interaction analyses were conducted to investigate the effect of energy and protein intakes on the association between PF and cognition.

RESULTS

We finally included 803 participants aged ≥ 60 years (54.4% female, weighted value). After adjusting for covariates, multiple measures (including FEV1, FVC, PEF, and composite PF) were all positively associated with better global cognition and the DSST score (P < 0.05). A stronger positive association between the DSST score and FEV1 (P for interaction = 0.001), FVC (P for interaction = 0.004), PEF (P for interaction = 0.003), and composite PF (P for interaction = 0.001) in lower energy intake. Similar results were observed in lower protein intake (all P for interaction < 0.05).

CONCLUSION

Higher PF was independently associated with improved specific components of cognitive function (i.e., the DSST score). The positive association between PF and the DSST score was stronger in individuals with lower energy and protein intakes.

Unveiling the influence of daily dietary patterns on brain cortical structure: insights from bidirectional Mendelian randomization

Cognitive impairment is a significant concern in aging populations. This study utilized Mendelian randomization analysis to explore the impact of dietary habits and macro-nutrients on cortical structure. A bidirectional Mendelian randomization approach was employed, incorporating large-scale genetic data on dietary habits and brain cortical structure. The results did not reveal significant causal relationships between dietary factors and overall cortical structure and thickness. However, specific dietary factors showed associations with cortical structure in certain regions. For instance, fat intake affected six cortical regions, while milk, protein, fruits, and water were associated with changes in specific regions. Reverse analysis suggested that cortical thickness influenced the consumption of alcohol, carbohydrates, coffee, and fish. These findings contribute to understanding the potential mechanisms underlying the role of dietary factors in cognitive function changes and provide evidence supporting the existence of the gut-brain axis.

MIND diet and cognitive function and its decline: A prospective study and meta-analysis of prospective cohort studies

BACKGROUND

Evidence on the association of the Mediterranean-DASH Intervention for Neurodegenerative Delay (MIND) diet with cognitive aging is limited and inconsistent.

OBJECTIVE

We examined how the MIND diet is related to cognitive function and its decline among middle-aged and older adults.

METHODS

We included 4066 participants with baseline dietary assessment and at least one cognitive test from the China Health and Nutrition Survey (CHNS) in 1997, 2000, 2004, and 2006, with a median follow-up of 3 years. The modified MIND diet score (range: 0-12) was calculated based on nine healthy and three unhealthy food groups. Linear mixed-effect models were used to examine the association of adherence to the MIND diet with z-scores of cognitive function and cognitive decline. We also conducted a meta-analysis including our findings and seven other cohort studies.

RESULTS

At baseline, median MIND diet scores for the increasing tertile were 3.0, 4.0, and 5.5, respectively. Participants with higher MIND diet scores had significant better global cognitive function. The adjusted difference in global cognitive function z-score for every three-point increment of MIND diet scores was 0.11 (95% CI, 0.06, 0.16, p-trend<0.001), which was approximately equivalent to being one year younger in age. Consumption of nuts, fish, red meats, and tea showed independent positive associations with cognitive function, and fried food consumption exhibited negative associations. In the meta-analysis of 26,103 participants, one standardized deviation increment of the MIND score was associated with 0.042 (95 % CI 0.020, 0.065) units higher in global cognitive function z-score and 0.014 (95 % CI -0.010, 0.037) units slower in annual cognitive decline.

CONCLUSIONS

Our findings suggest that higher adherence to the MIND diet was associated with better cognitive function and potentially slower cognitive decline in later life. Further large-scale observational and interventional studies are warranted to elucidate the cognitive effects of the MIND diet.

PROSPERO REGISTRY NUMBER

This meta-analysis was registered at PROSPERO as CRD42022330417.

Effects of Culinary Spices on Liking and Consumption of Protein Rich Foods in Community-Dwelling Older Adults

Insufficient protein intake is a common challenge among older adults, leading to loss of muscle mass, decreased function and reduced quality of life. A protein intake of 0.4 g/kg body weight/meal is recommended to help prevent muscle loss. The purpose of this study was to assess whether the protein intake of 0.4 g/kg body weight/meal could be achieved with typical foods and whether culinary spices could enhance protein intake. A lunch meal test was conducted in 100 community-dwelling volunteers; 50 were served a meat entrée and 50 were served a vegetarian entrée with or without added culinary spices. Food consumption, liking and perceived flavor intensity were assessed using a randomized, two-period, within subjects crossover design. Within the meat or vegetarian treatments, there were no differences in entrée or meal intakes between spiced and non-spiced meals. Participants fed meat consumed 0.41 g protein/kg body weight/meal, while the vegetarian intake was 0.25 g protein/kg body weight/meal. The addition of spice to the vegetarian entrée significantly increased liking and flavor intensity of both the entrée and the entire meal, while spice addition only increased flavor for the meat offering. Culinary spices may be a useful tool to improve the liking and flavor of high-quality protein sources among older adults, especially when used with plant-based foods, although improving liking and flavor alone are insufficient to increase protein intake.

The association of energy and macronutrient intake at breakfast and cardiovascular disease in Chinese adults: From a 14-year follow-up cohort study

Background

We aimed to examine the associations between energy and macronutrient intakes at breakfast and the incidence of cardiovascular events among Chinese adults.

Methods

There were 12,937 participants from the China Health and Nutrition Survey who met the study criteria and completed six rounds of questionnaires in 1997, 2000, 2004, 2006, 2009, and 2011. Combined weighing methods with 24-h dietary recall were used to measure dietary intake throughout the day. Intakes of macronutrients at breakfast were calculated using energy provided by nutrients as a percentage of breakfast energy. We calculated hazard ratios using a multivariable Cox frailty model with random intercepts to account for household clustering.

Results

During follow-up, we documented 453 (3.6 per 1,000 person-years) major cardiovascular events, 195 (1.5 per 1,000 person-years) myocardial infarctions, and 293 (2.3 per 1,000 person-years) strokes. In Chinese adults, more breakfast carbohydrates or less proteins intake was associated with the reduced risk of cardiovascular diseases. Especially for women, higher intake of breakfast carbohydrates was associated with a lower risk of major cardiovascular events (quintile 5 vs. quintile 1, HR 0.47 [95%CI 0.30-0.74]; p _trend = 0.0008) and stroke (quintile 5 vs. quintile 1, HR 0.48 [95%CI 0.26-0.88]; p _trend = 0.0006). Higher intake of breakfast proteins was associated with a higher risk of major cardiovascular events (quintile 5 vs. quintile 1, HR 1.77 [95%CI 1.12-2.79]; p _trend = 0.1162), myocardial infarction (quintile 5 vs. quintile 1, HR 2.49 [95%CI 1.21-5.11]; p _trend = 0.2641). There was a significant association between breakfast fat intake and cardiovascular diseases in the adult population, but less significant correlation was found in Chinese men or women. Breakfast fat intake was positively associated with the risk of major cardiovascular events (quintile 5 vs. quintile 1, HR 1.74 [95%CI 1.27-2.36]; p _trend = 0.0070), myocardial infarction (quintile 5 vs. quintile 1, HR 2.03 [95%CI 1.23-3.37]; p _trend = 0.0168), and stroke (quintile 5 vs. quintile 1, HR 1.64 [95%CI 1.12-2.41]; p _trend = 0.0732). There was a significant reduction in major cardiovascular events and stroke when breakfast energy intake was moderated, even if the independence of skipping breakfast.

Conclusion

High carbohydrate intake and low protein and fat intake at breakfast may contribute to cardiovascular health while maintaining a moderate energy intake.

Diet in the Prevention of Alzheimer's Disease: Current Knowledge and Future Research Requirements

Alzheimer's disease is a progressive brain disease that is becoming a major health problem in today's world due to the aging population. Despite it being widely known that diet has a significant impact on the prevention and progression of Alzheimer's disease, the literature data are still scarce and controversial. The application of the principles of rational nutrition for the elderly is suggested for Alzheimer's disease. The diet should be rich in neuroprotective nutrients, i.e., antioxidants, B vitamins, and polyunsaturated fatty acids. Some studies suggest that diets such as the Mediterranean diet, the DASH (Dietary Approaches to Stop Hypertension) diet, and the MIND (Mediterranean-DASH Intervention for Neurodegenerative Delay) diet have a beneficial effect on the risk of developing Alzheimer's disease.

Breakfast Skipping and Declines in Cognitive Score Among Community-Dwelling Older Adults: A Longitudinal Study of the HEIJO-KYO Cohort

Previous studies outlined the correlation of adverse effects of breakfast skipping with cognitive function. However, the majority of these studies have focused on the short-term effects; to date, the long-term effect of breakfast skipping on cognitive function among older adults remains unclear. In this prospective cohort study of 712 older adults (mean age, 70.8 years), breakfast skipping was defined as skipping breakfast one or more times per week, and declines in cognitive score was defined as decreases in Mini-Mental State Examination (MMSE) score of two or more in the observed period. During follow-up (median, 31 months), 135 of 712 participants developed declines in cognitive score. Poisson regression models revealed that the incidence rate for declines in cognitive score was significantly higher in breakfast skipper (n = 29) than breakfast eaters (n = 683) [incidence rate ratio (IRR), 2.10; 95% CI, 1.28-3.44]. Additional propensity score adjustments related to breakfast skipping from baseline parameters (age, gender, smoking and drinking status, BMI, household income, educated level, depressive symptoms, hypertension, diabetes, sleep medication, physical activity, caloric intake, and baseline cognition) produced consistent results (IRR, 2.21; 95% CI, 1.33-3.68). Sensitivity analysis, when the cut-off value of decreases in MMSE score was changed to three points, suggested a significant and stronger association (IRR, 3.03; 95% CI, 1.72-5.35). Regarding daily intakes of food groups, breakfast skippers consumed a significantly lower amount of vegetables, fruits, and fish than breakfast eaters. In conclusion, our findings suggest that breakfast skipping is longitudinally associated with declines in cognitive score among older adults.

Association between egg consumption and cognitive function among Chinese adults: long-term effect and interaction effect of iron intake

The association between egg consumption and cognitive function is inclusive. We aimed to assess the association between egg consumption and cognitive function in Chinese adults and tested the interaction between egg consumption and Fe intake. The data used were from a nationwide sample (n 4852, age ≥ 55 years) from the China Health and Nutrition Survey between 1991 and 2006. Assessment of cognitive function was conducted in 1997, 2000, 2004 and 2006. Dietary egg intake was obtained by 24-h dietary recalls of 3 consecutive days during home visits between 1991 and 2006. Multivariable mixed linear regression and logistic regression were used. Egg intake was positively associated with global cognitive function. In fully adjusted models, across the quartiles of egg intake the regression coefficients were 0, 0·11 (95 % CI -0·28, 0·51), 0·79 (95 % CI 0·36, 1·22) and 0·92 (95 % CI 0·43, 1·41), respectively. There was a significant interaction between egg intake and Fe intake. The association between high egg intake and cognitive function was stronger among those with low Fe intake than those with high Fe intake. In addition, there was a significant interaction between egg consumption and sex, with the association mainly observed in women but not men. Furthermore, compared with non-consumers, those with higher egg consumption (Q4) had the OR of 0·93 (95 % CI 0·74, 1·19), 0·84 (95 % CI 0·69, 1·02) for self-reported poor memory and self-reported memory decline, respectively. Higher egg intake is associated with better cognition in Chinese adults among those with low Fe intake.

Macronutrient Intake and Risk of Dementia in Community-Dwelling Older Adults: A Nine-Year Follow-Up Cohort Study

BACKGROUND

Little is known about the association between macronutrient intake and incident dementia.

OBJECTIVE

To identify an optimal range of macronutrient intake associated with reduced risk of dementia.

METHODS

Our analysis included 93,389 adults aged 60-75 years from the UK Biobank. Diet was assessed using a web-based 24-h recall questionnaire between 2009-2012. Dementia was ascertained using hospital inpatient, death records, and self-reported data up to January 2021. We calculated a macronutrient score based on associations between an individual's macronutrient intake and incident dementia.

RESULTS

During a median follow-up of 8.7 years, 1,171 incident dementia cases were documented. We found U-shape relationships for carbohydrate, fat, and protein intake with incident dementia. Compared to individuals with optimal carbohydrate intake, those with high intake (HR (95%CI): 1.48(1.15-1.91)) but not low intake (1.19(0.89-1.57)) had a higher risk of dementia. In the multivariable analysis, a low-fat intake (HR (95%CI): 1.42(1.11-1.82)) was associated with a higher risk of all-cause dementia. After adjustment for covariates, a high (HR (95%CI): 1.41(1.09-1.83)) but not low protein intake (1.22(0.94-1.57)) was associated with an increased risk of dementia. Individuals in quintiles 3-5 of optimal macronutrient score had a lower risk of dementia compared with those in quintile 1 (HR (95%CI): 0.76(0.64-0.91) for quintile 3, 0.71(0.60-0.85) for quintile 4, 0.74(0.61-0.91) for quintile 5). The association between macronutrient score and incident dementia was significant across subgroups of age, gender, education, and smoking.

CONCLUSION

Moderate intakes of carbohydrate, fat, and protein were associated with the lowest risk of incident dementia.

Associations of Dietary Pattern and Sleep Duration with Cognitive Decline in Community-Dwelling Older Adults: A Seven-Year Follow-Up Cohort Study

BACKGROUND

A few studies have linked dietary patterns and sleep to cognitive decline.

OBJECTIVE

To examine the independent and joint associations of dietary patterns and sleep with cognitive decline.

METHODS

Our analysis included 2,307 participants aged 55- 89 years at baseline from the China Health and Nutrition Survey. Dietary intake was assessed using weighing methods in combination with 24 h dietary recalls for three consecutive days. Exploratory factor analysis was applied to identify major dietary factors. Cognition was assessed in 1997, 2000, 2004, 2006, and 2015.

RESULTS

Five dietary patterns were identified: dairy-fruits-fast foods, grains-vegetables-pork, plant-based food, beans-mushroom, and beverages-nuts patterns. Beans-mushroom pattern and sleep duration of 8 h/day were defined as healthy habits. There was a positive association between the beans-mushroom pattern and change in the global cognitive Z-score over seven years (β (95% CI) for quintile 5 versus quintile 1:0.17 (0.05, 0.30)). Compared to individuals with sleep duration of 8 h/day, those with sleep duration of≤5 h/day (β (95% CI): - 0.23 (- 0.45, - 0.00)) or > 10 h/day (- 0.52 (- 0.73, - 0.32)) had a greater decrease in global cognitive Z-score. Compared to individuals with no healthy patterns, those with a healthy dietary pattern only (β (95% CI): 0.18 (0.08, 0.28)), healthy sleep pattern only (0.13 (0.04, 0.23), and both healthy dietary and sleep patterns (0.19 (0.08, 0.31)) had a relative increase in global cognitive Z-score.

CONCLUSION

Our findings highlight the importance of involving both diet and sleep as intervention priorities for the potential prevention of cognitive decline.

Long-term dietary protein intake and subjective cognitive decline in US men and women

BACKGROUND

Diet is one of the modifiable risk factors for cognitive decline. However, studies on dietary protein intake and cognitive decline have remained limited and inconclusive.

OBJECTIVES

In this study, we aimed to investigate the associations between long-term dietary protein intake and subsequent subjective cognitive decline (SCD).

METHODS

We included 49,493 women from the Nurses' Health Study (NHS) (1984-2006) and 27,842 men from the Health Professionals Follow-up Study (HPFS) (1986-2002). For the NHS, average dietary intake was calculated from 7 repeated semi-quantitative FFQs (SFFQs), and SCD was assessed in 2012 and 2014. For the HPFS, average dietary intake was calculated from 5 repeated SFFQs, and SCD was assessed in 2008 and 2012. Poisson regression was used to examine the associations between dietary protein, amino acids, and various protein food sources with subsequent SCD.

RESULTS

Higher protein intake compared with total carbohydrates was associated with lower odds of SCD. When substituting 5% energy from protein for the equivalent percentage of energy from total carbohydrates, the pooled multivariable-adjusted ORs (95% CIs) were 0.89 (0.85, 0.94) for total protein, 0.89 (0.84, 0.94) for animal protein, and 0.74 (0.62, 0.88) for plant protein. When substituting 5% of energy from animal protein with plant protein, the OR was 0.84 (95% CI: 0.72, 0.97). For protein food sources, higher intakes of beans/legumes, fish, and lean poultry were significantly associated with lower odds of SCD, but higher intake of hotdogs was associated with higher odds of SCD.

CONCLUSIONS

Higher protein intake was associated with lower odds of SCD when compared isocalorically with carbohydrate. Plant protein sources were also associated with lower odds when compared with animal protein sources. Our findings suggest that adequate protein intake, and choices of protein sources could play a role in the maintenance of cognition and should be studied further.

Dietary Diversity and Healthy Aging: A Prospective Study

Population aging is a global phenomenon. The present study determined the effects of dietary diversity score (DDS) and food consumption on healthy aging. A subset of the data of the China Health and Nutrition Survey was utilized in this study. DDSs were calculated using the dietary data collected in the years 2009 and 2011. A healthy aging score (HAS) was calculated by summing the standardized scores on physical functional limitation, comorbidity, cognitive function, and psychological stress based on the data collected in the year 2015, with a lower HAS indicating a healthier aging process. Life quality was self-reported in the year 2015. This study found that DDS was inversely associated with HAS (T3 vs. T1: β −0.16, 95%CI −0.20 to −0.11, p-trend <0.001). The consumption of meat and poultry, aquatic products, and fruits was inversely associated with HAS, and participants in the highest tertile of staple foods consumption had a higher HAS than those in the lowest tertile. HAS was inversely associated with good self-reported life quality and positively associated with bad life quality. In conclusion, food consumption may influence the aging process, and adherence to a diverse diet is associated with a healthier aging process in elderly people.