Exploration of adulthood diet quality in relation to walking speed indicating physical and cognitive function in later life

Rationale: The proportion of adults aged above 60 is rapidly increasing and by 2050 they will comprise almost one fourth of total world population. However, with moderate disability rates also increasing, there is great need for older people to maintain overall well-functioning. Physical and cognitive functioning play a major role in daily activities and determine life quality of older people. Maintaining walking ability is crucial for independent living and walking speed is predictive of overall health, survival and better physical and cognitive functioning. Rates of functional decline vary interindividual and between sexes, with women living longer but with higher comorbidity rates than men. Part of this variability in functional trajectories is attributable to modifiable lifestyle factors, with diet being of great interest. Specific nutrients and presumably healthy diets have been associated with better physical and cognitive performance in older people. Research gap: Evidence for diet quality in line with current dietary guidelines and objective measures of well-functioning, important for older people, is limited. In addition, considering well-known differences in eating and ageing trajectories between men and women, it is highly underexplored whether the effect of diet on overall functioning of older people is sex-specific. Finally, evidence for whether accumulation of healthy dietary exposure over time may exert additional benefits on overall functioning in later life, is scarce. Aim: To investigate the overall and sex-specific prospective associations between diet quality in late midlife and over a 27-year period in adulthood and objective measures of walking speed in older life, indicating survival and physical and cognitive functioning. Methods: This thesis used dietary data (five-day food diaries) of men and women from four time points of the 1946 MRC National Survey of Health and Development birth cohort (N=5,362) when participants were at ages 36, 43, 53 and 60-64 and measures of walking speed from the Insight 46 (N=502), its clinical sub-study, when participants were at age 69- IV 71. Diet quality was defined by the Healthy Eating Index-2015 at each age (low: 0 - high: 100). Prospective associations with walking speed at age 69-71 were assessed for diet quality (a) in late midlife (age 60-64) and (b) over adulthood (from age 36 to 64), considering major demographic, health and lifestyle factors from age 36 to 64. Sample size: At least one valid dietary assessment was available for n=480. Walking speed was available for n=474 and among men (51%) and women in the Insight 46, dietary data was available as follows: n=313 (age 36), n=369 (age 43), n=346 (age 53), n=444 (age 60-64). Final sample size ranged from n=331 (Chapter 4) to n=437 (Chapter 6). Results: Diet quality improved over adulthood (age 36: HEI=47 ± 11 vs age 64: HEI=62 ± 13). Women had higher diet quality than men at each age (β=4 HEI points, 95% CI: 3, 6; p<0.05). Men had faster absolute values of walking speed than women at age 69-71 (1.39 m/s ± 0.15 vs 1.32 m/s ± 0.14, p<0.05), which was attenuated after controlling for anthropometric differences (p= 0.85). Main analysis showed there was a null overall association between diet quality in late midlife (β: 0.01 m/s, 95% CI: -0.01, 0.02) and over adulthood (β: 0.02 m/s, 95% CI: -0.02, 0.03) and walking speed in later life. Sex-specific analysis showed a positive association between diet quality in late midlife and faster walking speed in women independently of other factors (β: 0.02 m/s, 95% CI: 0.01, 0.04), but null associations for men (β: -0.01 m/s, 95% CI: -0.03, 0.01). When walking and performing a cognitive task, there was indication of a positive association between diet quality and walking speed in women (β: 0.03 m/s, 95% CI: 0.01, 0.05) and an inverse association in men (β: -0.02 m/s, 95% CI: -0.05,-0.01). Finally, starting with and maintaining high diet quality over adulthood was associated with faster walking speed in later life in women (β: 0.04 - 0.06 m/s, 95% CI: 0.01, 0.11), thus reflecting better functioning and longer survival, but not men (β: -0.01 m/s, 95% CI: -0.02, 0.01); yet the association was not independent of other lifestyle factors. V Contribution to knowledge: This thesis provided novel insights of the impact of diet quality across various stages over adulthood on measures of overall functioning, which predict survival and are clinically relevant for older people. Furthermore, it added original evidence for sex differences modifying this relationship and most importantly it suggested that achieving and maintaining high diet quality the longest possible over adulthood is likely to yield additional benefits towards better age-related physical functioning, in particular in women.