Daily consumption of dietary supplements and other dietary modifications are commonly proposed as a way to improve overall health. In Canada, 65% of women between 51 and 70 years of age use such supplements, but only 42% of men in the same age group do so [1]. While the use of supplements is widespread, there is less information regarding their effectiveness at improving health in older adults. One proposed use for supplements is to reduce frailty or attenuate the negative effects of age on health. While many of these interventions are similar to clinical ones [2], this re following sections view will focus on the evidence found in pre-clinical models. 

2. Vitamin Supplements

Many of the interventions designed to attenuate frailty in pre-clinical models use vitamins. Vitamins are biologically active compounds that are important for health and that may or may not be partially synthesised endogenously. While all vitamins are crucial, only a few have been tested as frailty interventions, as discussed below.

2.1. Vitamin D

Many studies have used vitamins as an intervention to attenuate frailty. The most common intervention is vitamin D₃ (25-hydroxyvitamin D). Vitamin D₃ is a prohormone formed in skin by the combination of ultra-violet light and a cholesterol derivative ^[3][32]. Interestingly, vitamin D₃ is not readily found in food and must instead be synthesised. This makes vitamin D₃ more like a hormone than a traditional vitamin ^[3][32]. Vitamin D₃ has multiple physiological functions, such as maintaining skeletal muscle health ^[4][33], increasing bone density ^[5][34], and preserving cardiovascular health ^[6][35]. This is not surprising, as vitamin D receptors (VDRs) are found throughout the body ^[7][36]. The absence of vitamin D₃ has also been linked to multiple pathologies. VDR knockout (KO) mice have higher mortality, lower weight, increased alopecia, and bone malformations when compared to wildtype controls ^[8][37]. VDR KO mice also tend to develop secondary hyperparathyroidism even when fed a high calcium, high phosphorus rescue diet ^[9][38]. Another important function of vitamin D₃ is its role in maintaining calcium and phosphate homeostasis ^[3][32]. It regulates calcium absorption in the gut and controls serum levels of calcium ^[10][39]. Interestingly, similar pathological phenotypes occur in VDR KO mice, even when they are fed a high calcium rescue diet ^[11][40]. Another genetic mouse model of low vitamin D₃ involves the hepatic CYP2R1 enzyme, which converts vitamin D₃ into circulating 25-hydroxyvitamin D (25-OHD) ^[12][41]. CYP2R1 KO mice have enlarged livers, and very low circulating levels of calcium and phosphorus ^[12][41]. Interestingly, levels of the enzyme CYP2R1 decrease with age naturally in mice, leading to low levels of 25-OHD ^[13][42]. This suggests that aging mice may greatly benefit from vitamin D₃ supplementation. These multi-system effects of vitamin D₃, along with aging pathologies linked to vitamin D₃ deficiency, make it a prime target as an intervention to mitigate frailty. The importance of vitamin D₃ in skeletal muscle health suggested to some researchers that it might reduce physical frailty. Studies in mice show that chronic vitamin D₃ deficiency reduces skeletal muscle contractility ^[14][43], and more recent work shows that skeletal muscle metabolism is disrupted in VDR KO mice ^[15][44]. The use of vitamin D₃ to improve physical health was investigated by Seldeen et al. ^[16][45] when young male mice (6 months old) were given diets either deficient in vitamin D₃ (125 IU) or with sufficient levels of vitamin D₃ (1000 IU) for 12 months ^[16][45]. Mouse health was assessed by several physical performance measures (grip strength, balance, endurance, and time to exhaustion), but the physical phenotype was not assessed. Mice deficient in vitamin D₃ had lower uphill sprint exhaustion times, reduced stride length and grip endurance but no change in grip strength ^[16][45]. These changes in physical performance were associated with an increased expression of genes that code for muscle atrophy pathways in the quadriceps ^[16][45]. However, there are no changes in serum markers of inflammation in these vitamin D₃ deficient animals ^[16][45]. A follow-up study by the same group used older male mice (24 months old) and measured frailty with the frailty phenotype ^[17][46]. Instead of studying vitamin D₃ insufficiency alone, they added another group with a high vitamin D₃ diet (8000 IU). After the 4-month exposure period, mice with both insufficient and normal levels of vitamin D₃ had higher frailty ^[17][46]. Importantly, this was not seen in the high vitamin D₃ group ^[17][46]. Interestingly, they noted no increase in bone mineral density as might have been expected with high levels of vitamin D₃ supplementation. Similarly, Liu et al. ^[18][47] measured frailty using a modified frailty index in middle-aged male rats (13 months) fed a vitamin D₃ supplemented (1.8 IU/kg) diet for 8 months ^[18][47]. Rats that took vitamin D₃ had significantly lower frailty index scores than their age-matched controls ^[18][47]. Unlike the work by Seldeen et al. ^[17][46], they did find a protective effect of vitamin D₃ on bone mineral density in older rats ^[18][47]. The difference in results of vitamin D₃ supplementation on bone mineral density may be due to the use of different doses (8000 IU vs. 1.8 IU/kg), varying timeframes (4 vs. 7 months), or differences in species (mouse vs. rat). Taken together, these studies indicate that vitamin D₃ supplementation is a promising intervention to mitigate frailty, even if it is started later in life. This also highlights the importance of having sufficient vitamin D₃ levels, as a lack of this essential nutrient may increase frailty. Importantly, these studies used only male rodents, which limits the applicability of this work. Future work should determine whether vitamin D₃ supplements at similar doses and delivered over similar time frames are effective in older females. As there is still controversy on the precise mechanisms through which vitamin D₃ exerts these beneficial effects, more work in this area is warranted.

2.2. Vitamin C

Vitamin C or ascorbic acid is an essential vitamin that is obtained through the diet. It is absorbed through food and cannot be synthesised by humans. This makes vitamin C, unlike vitamin D, a true vitamin. Physiologically, vitamin C acts in a similar fashion to antioxidants and it is necessary for human health ^[19][48]. Vitamin C supplementation has been suggested to augment immune function either via antioxidant protection or by directly enhancing immune cell function ^[20][49]. For example, influenza virus A infected male mice show lower expression of proinflammatory cytokines in the lung when they are vitamin C deficient when compared to infected mice with adequate vitamin C levels ^[21][50]. By contrast, this result is not found in female mice ^[21][50]. There is also evidence that high doses of vitamin C kills cancer cells in mice ^[22][51] and that supplementation with this essential nutrient extends lifespan in murine models ^[23][52]. Combined, these studies suggest that vitamin C has the potential to affect frailty, especially via beneficial effects on the immune system. A complication related to vitamin C supplementation in mouse models is that, unlike humans, mice synthesise their own vitamin C ^[19][48]. Hence, many researchers use a Gulo KO model where the gulo enzyme (L-gulo-y-lactone oxidase), essential for vitamin C synthesis, is knocked out ^[24][53]. These mice have lower body weights, a significantly reduced lifespan, and higher serum cholesterol levels ^[24][25][53,54]. These findings suggest that increased levels of vitamin C may improve health by attenuating multiple underlying frailty mechanisms such as those involving inflammation. Animal studies have not yet explored vitamin C as an intervention for frailty, although some studies show promising effects on both lifespan and overall markers of health. To better investigate vitamin C’s antioxidant effects, Selman et al. ^[26][55] used female mice exposed to cold stress to increase oxidation. Young wildtype mice were kept in cold conditions (7 °C) and then administered lifelong vitamin C supplementation ^[26][55]. They found no improvement in energy expenditure, metabolism, or lifespan in cold-exposed mice fed vitamin C. Interestingly, this study also found that cold exposure alone had no effect on mouse lifespan, unlike previous work that has shown a decrease in lifespan when oxidation levels are increased ^[27][56]. Thus, these findings suggest that cold-induced oxidation may not be an ideal oxidation model ^[26][55]. Uchio et al. ^[28][57] used senescence marker protein 30 knockout (SMP30 KO) male mice to test this intervention. These SMP30 KO mice show increased tissue susceptibility to damage ^[29][58] and cannot produce vitamin C ^[30][59]. SMP30 KO mice were given either high or regular doses of vitamin C for 2 months before half the mice in each group were given dexamethasone as a glucocorticoid analog to mimic an increase in stress ^[28][57]. Mice fed high levels of vitamin C had preserved immune function, normal cytokine levels and preserved T-cell count after dexamethasone treatment ^[28][57]. This shows that vitamin C supplementation can maintain immune system function under stress. Thus, these studies show mixed results regarding the beneficial effects of vitamin C supplementation, with preservation of immune function in aging being the best characterised. Interestingly, while the study utilising male mice showed beneficial results ^[28][57], the one using females did not ^[26][55], suggesting possible sex-specific effects of vitamin C supplementation. Considering the detrimental effects of systemic immune dysfunction with age, future work could focus on vitamin C supplementation and its impact on inflammaging and frailty in both sexes.

2.3. Vitamin E

Vitamin E, or α-tocopherol, is an essential vitamin which is mainly found in animal fats and plant oils. It is generally categorised as an antioxidant. Like other supplements, vitamin E has numerous physiological effects. For example, there is evidence that vitamin E can alter cytokine production in human and animal models ^[31][60]. Vitamin E is also implicated in neurological development, as young mice fed a vitamin E deficient diet have reduced cognition and increased brain oxidation ^[32][61]. This was further examined using α-tocopherol transfer protein (TTP) knockout mice. TTP plays a role in controlling systemic levels of vitamin E. Adult male and female mice without the TTP protein show inhibition of neurogenesis and increased expression of neurodegeneration genes along with increased signs of anxiety ^[33][62]. This suggests the importance of sufficient vitamin E, particularly in maintaining neurological health, which may translate to protection against age-related cognitive decline and potentially also attenuate the degree of frailty. The impact of vitamin E supplements on frailty have not been fully investigated, but effects on lifespan and physical performance have been explored. Focusing on antioxidant effects, Navarro et al. ^[34][63] fed mice a lifelong vitamin E supplementation diet. Interestingly, they found a sex-specific effect on survival, where males fed vitamin E had lower mortality, but this was not seen in females ^[34][63]. Using only the male mice, they determined that vitamin E supplementation improved motor coordination and exploratory behavior compared to controls. As in previous work, they found that males given vitamin E had less oxidative damage in their brains compared to controls ^[34][63]. This suggests that many of the health benefits of vitamin E may be mediated through protection against oxidation; however, future work is required, especially as these beneficial effects may not occur in females.

2.4. Nicotinamide

Nicotinamide is the amide form of vitamin B₃ and is a key component in the nicotinamide adenine dinucleotide pathway (NAD+). This compound can be both obtained from the diet and endogenously synthesised ^[35][64]. Interestingly, NAD+ levels decrease with age and this is linked to cellular senescence ^[36][65]. Many other aging processes including DNA damage, cognitive impairment, and mitochondrial changes are linked to lower NAD+ ^[37][66]. These are highlighted in an NAD+ deficient mouse model, C57Bl/6RccHsd, which has a nicotinamide nucleotide transhydrogenase gene deletion. Male C57Bl/6RccHsd mice exhibit a reduction in insulin sensitivity and altered metabolism compared to controls ^[38][67]. However, there are sex differences in the NAD+ pathway, where female mice are resistant to the metabolic dysfunction resulting from a nicotinamide deficiency unlike males ^[39][68]. These beneficial effects are promising as healthspan interventions and suggest that nicotinamide may be a useful intervention to reduce frailty ^[40][69]. The effects of nicotinamide supplementation on overall markers of health in pre-clinical models have been investigated, but the effects on frailty directly have not been measured. Mitchell et al. ^[41][70] explored the beneficial effects of nicotinamide on metabolism. They fed 12-month-old male mice nicotinamide supplements with or without a high fat diet to induce obesity for their remaining life ^[41][70]. Neither of these diets resulted in a change in lifespan, but mice fed a high fat diet had improved locomotor activity when nicotinamide was also consumed ^[41][70]. This suggests that nicotinamide can offset some of the negative changes that occur with obesity in older male mice. However, when male mice are injected with nicotinamide supplements for 8 weeks, they develop insulin resistance and increased lipid accumulation in their skeletal muscle ^[42][71]. One reason for these differing results may be the use of different doses of nicotinamide (0.5 g/g and 1.0 g/kg in food vs. 100 mg/kg injected respectively). Beneficial effects were observed with lower doses while detrimental effects occurred at the higher doses, so the concentration-dependence of these effects should be further investigated. In addition, both studies used only male mice so future work should explore the effects of nicotinamide supplementation in females as well.