The main finding from this study was that higher total plant nitrate intake, largely driven by consumption of vegetables and grains, was associated with lower odds for frailty in community-dwelling older Australian women. This is the first study, to our knowledge, which reports an association between habitual nitrate intake and frailty. Specifically, women with higher plant nitrate intakes (Q2-Q4) were 31–34% less likely to be frail when compared with women with the lowest (Q1) plant nitrate intake. There did not appear to be any additional benefit once plant-nitrate intakes of ~ 64 mg/day were attained. No association was found between total animal nitrate intake and frailty.

The mechanisms through which dietary nitrate impacts frailty are likely to be multifactorial, and consequent to a nitrate-related increase in the bioavailability of NO [10, 11]. NO has pleiotropic actions in the human body and is implicated in the function of a range of different body systems. Namely, as a potent neurotransmitter and modulator of cerebral blood flow, NO exerts effects on cognitive function [19, 37]. NO also impacts muscle function and metabolism, as increased NO bioavailability consequent to nitrate intake has been linked to increased efficiency of mitochondrial respiration, alterations to muscle Ca2+ handling, a reduced ATP cost of muscle force production, and increased blood flow to the muscle [14, 18, 38, 39]. Dietary nitrate intake has also been shown to modify aspects of cardiovascular health, reducing endothelial dysfunction, vascular stiffness and blood pressure [9, 34, 40, 41]. Most importantly, in the current cohort, dietary nitrate intake (especially from plants) is inversely related with muscle function measures (e.g., grip strength, timed-up-and go) [14] and atherosclerotic cardiovascular disease [34]. Considering this captures key components of the FI, it provides evidence for the underlying mechanisms to support our results. Given that higher nitrate intakes have been reported in other work to benefit these components of frailty [9, 13, 15, 42] , including cognitive health [43, 44], this relationship likely reflects an accumulation of the aforementioned multi-system benefits.

Our data suggests that an intake of > 64 mg/d of plant-derived nitrate, largely from consumption of vegetables and grains, may be sufficient to maximise beneficial associations with frailty, with no additional reductions in the likelihood of frailty observed at higher intakes. In contrast, acute and short-term studies exploring the impact of nitrate on health markers have typically suggested much higher doses of nitrate are required to maximise (or even elicit) beneficial effects. For example, a meta-analyses investigating the effects of acute dietary nitrate intake on various markers of cardiovascular risk factors recommended that a 130–259 mg bolus dose of nitrate was required to reduce systolic blood pressure [45]. In another meta-analysis reporting the acute positive effects of dietary nitrate on muscle power, a range of 400–985 mg of nitrate, mostly obtained from beetroot juice, was provided across all the studies [13]. However, recent evidence has suggested that when ingested habitually, dietary nitrate intake of 100–200 mg/day may be more effective for reducing systolic blood pressure than substantially higher intakes (400 mg/day) [46]. It is currently unclear why this discrepancy in optimal dose is present. However, Babateen et al., suggested that high doses of nitrate over prolonged periods may result in the development of tachyphylaxis, resulting in the downregulation of vascular eNOS activity, and decreased NO production via oxidation of L-arginine [46, 47]. We speculate that similar mechanisms may be the reason for the nadir observed in the current study, and as such any dietary nitrate intake above that of ∼ 64 mg would have no additional benefit in reducing the likelihood of frailty. Alternatively, it is possible that low doses of nitrate in the habitual diet allow the accrual of small, incremental benefits over time which may not be detectable in acute or short-term studies against background measurement error.

No association was recorded between dietary nitrate intake from animal sources and frailty. Notwithstanding the prospect that animal nitrate truly has no influence on frailty, there may be various explanations for this finding in the current study. Firstly, assuming that dietary nitrate from both plant and animal sources exerts a similar effect per mg, nitrate intake from animal sources may not be sufficient to affect frailty. For example, median (IQR) daily total plant and total animal nitrate was 72.1 (55.6–90.0) mg/d and 3.5 (2.2–5.1) mg/day, respectively. Therefore, it may be considerably harder to establish any influence of animal nitrate without a larger range of intakes, especially considering intake from plant sources is always likely to confound results. Of course, it is possible that plant nitrate intake also has a stronger association with frailty compared to animal sources due to the food matrixes that are inherent to plant-based sources of nitrate which have consistently been shown to be beneficial for overall health [48,49,50,51]. Plant-based sources of nitrate also contain other active components such as polyphenols and other antioxidants that might lead to (i) a greater NO yield per mg of nitrate, and (ii) prolonged NO bioactivity by scavenging superoxide and other free-radicals [52,53,54]. Of note, greater effects of plant-based nitrate on blood pressure, compared to an isolated supplement of sodium nitrate (an allowed additive for processed meats), have previously been observed [55]. Consuming higher amounts of ultra processed foods, which include processed meats, has also been shown to increase risk of incident frailty, assessed using the Fried frailty phenotype, in a prospective cohort of older Spanish adults over 3.5 years (n = 1822, ≥ 60 years, 51.3% female) [56]. Similarly, when considering physically frail middle-aged adults from the UK Biobank (n = 19,913, 58 years, 59.4% female), frequent consumption of processed meat was also associated with increased mortality risk [57].

From a public health perspective, the relatively low dose of dietary nitrate (> 64 mg/day) that may be required to reduce the likelihood of frailty in older community-dwelling women is encouraging. To put these findings into perspective, just one serving of green leafy vegetables, or approximately 50 g of beetroot, would provide 75 mg of nitrate. Alternatively, individuals with low nitrate intakes (e.g., 35 mg/day, median of Q1), consuming approximately half a serve extra per day of green leafy vegetables would ensure a nitrate intake of more than 64 mg. Potential benefits are supported in our analysis substituting plant nitrate with green leafy vegetable intake that demonstrated approximately 35% lower odds for frailty when as little as 1 cup of green leafy vegetables (e.g., ~ 20–30 g of spinach) was consumed. Using vegetables as a means of increasing dietary nitrate is well-accepted amongst older adults with very high compliance (97–98%) in studies lasting 4–5 weeks where participants were required to consume > 200 mg/day [58, 59]. Nitrate-rich vegetables are also relatively inexpensive and easily accessible. Indeed, in 2023, 64 mg of nitrate could be obtained from commercially available vegetables costing as little as 0.08 GBP (USD 0.10) [60]. Given that increased dietary nitrate intake has been linked to multiple domains of frailty such as cardiovascular health, muscle strength, and cognitive function, older individuals may be recommended to increase their intake of nitrate-rich vegetables as a means of reducing the likelihood of frailty [12]. Although increasing nitrate intake above this threshold is not likely to reduce frailty odds further, diets rich in vegetables are generally considered to be healthier [5] (e.g., due to the provision of fibre and other micronutrients implicated in various aspects of human health), and hence, 1–2 serving of green-leafy vegetables should not be considered an upper limit, but rather a lower-bound.

The recruitment of a large cohort of Australian community-dwelling older women that were representative of their demographic is a major strength of this study. Whilst this is the first study to provide evidence for an inverse non-linear association between plant-derived dietary nitrate and frailty assessed using the validated cumulative deficits model [27], there are several limitations to the work. For example, the data are observational and cross-sectional in nature, therefore causality cannot be established. Future studies should examine this association in a longitudinal design, to establish whether nitrate intake is associated with changes in frailty. To minimise the potential for residual confounding, we included a range of dietary covariates including protein, calcium, vitamin C, fibre and vitamin E intake, as well as two measures of overall diet quality (NRFI as well as ADG) and still report comparable results. However, the addition of vitamin E and fibre to the multivariable-adjusted model recorded significantly lower odds for frailty only in individuals in Q2 and Q3 (but not Q4) of plant nitrate intake, when compared to the Q1. As vegetables are a rich source of nitrate, vitamin E and fibre, this may have influenced our results. Alternatively, it should be acknowledged that there are other components in vegetables that could be contributing to the observed associations. Additionally, although nitrate intake was estimated from validated food databases for the current study, some error may be introduced due to variations in cooking methods [61, 62] or growing conditions of the plants [63, 64], so that actual nitrate intake could vary. To minimise error, we adopted recently compiled nitrate databases which also took cooking method into account.

In conclusion, this investigation highlights the potential detrimental impact of low plant nitrate intakes for the likelihood of frailty in community dwelling older women. Notably, small increases in intakes of nitrate derived from both vegetables and grains appear most beneficial. Public health messaging should continue to promote 5–6 serves per day of vegetables, whilst highlighting the importance of including at least one serve per day of nitrate-rich vegetables (e.g., spinach, beetroot, rocket, pak choi). Habitual consumption of such vegetables (75 g per day) would provide sufficient dietary nitrate to obtain levels typically linked to a range of health benefits [12]. Finally, further work with longitudinal research designs is needed to support these findings before population-specific recommendations may be considered.