«Is a Vegan diet detrimental to endurance and muscle strength?» . This question was addressed by a group of Canadian researchers in a cross-sectional study (PMID: 32332862) from 2020 that compared a group of 56 young female omnivore (28) and vegan (28) athletes with regard to estimated maximal oxygen consumption (VO2 max), a sub-maximal endurance test (70% of VO2 max) and muscle strength (leg and chest press). What were the primary conclusions of this study?
According to the authors: «Both groups (i.e. omnivores and vegans) were comparable for physical activity levels, body mass index, percent body fat, lean body mass, and muscle strength. However, vegans had a significantly higher estimated VO2 max (44.5 ± 5.2 vs. 41.6 ± 4.6 ml/kg/min; p = 0.03, respectively) and sub-maximal endurance time to exhaustion (12.2 ± 5.7 vs. 8.8 ± 3.0 min; p = 0.007, respectively) compared with omnivores. (…) these findings contradict the popular belief from the general population»
Perhaps surprisingly the vegan group did not even seem to be having the most optimal vegan diet (whole-foods plant-based WFPB). You can tell it because the average fibre consumption for the vegan group was ~40 g/day. This is much better than the amount of fibre consumed by most of the population in Western countries such as the UK and USA (where 72% of the population fails to meet the adequate intake recommendations of 25-30 g a day) but typically below what you would get on a healthier WFPB dietary pattern where people typically easily obtain 50-100 g of fibre a day. Furthermore, the % of calories obtained from saturated fat (SF) in the vegan athlete group (~13 %) was also higher than the ideal maximum SF intake given by most health organizations (5-10%) which suggests that they may have been including coconut oil or some other type of processed vegan food richer in SF. This excess of SF could have been detrimental to optimal performance by having a negative effect on endothelial function/blood flow and insulin sensitivity for example.
In another recent 2022 cross-sectional study titled «Active Vegetarians Show Better Lower Limb Strength and Power than Active Omnivores» (PMID: 35088394), 58 participants (32 vegans and 26 omnivores) from 18 to 40 were compared to examine their dietary intake and physical performance (aerobic capacity, strength -dynamic, isometric and relative-, plus muscle power). The plant-based group was deemed to have a healthier dietary pattern when compared to the omnivores; they consumed more carbohydrates (as expected) and fibre, but less protein than the omnivores. In fact while omnivores were consuming the widely recommended (for resistance training) 1.6 g/kg of protein the vegan athletes were at only 1.04 g/kg. However, interestingly this did not seem to be detrimental to this group of vegan athletes. Saturated fat and cholesterol intake was much lower than the omnivore group. Not surprisingly suggest vegans had on average lower weight, BMI and fat mass.
In this particular study the authors found that for most parameters analyzed there were no significant differences between the trained omnivores and vegans even though this group of vegan athletes was shown to have greater lower limb strength and power than the omnivores in the study.
The following review from 2019 (PMID: 30634559) Barnard et al. hypothesized as to why plant-based diets could be beneficial especially with regard to improvements in endurance:
«Plant-based diets play a key role in cardiovascular health, which is critical for endurance athletes. Specifically, these diets improve plasma lipid concentrations, blood pressure, body weight, and blood glucose control, and, as part of a healthful lifestyle, have been shown to reverse atherosclerosis. The possibility that such diets may also contribute to improved performance and accelerated recovery in endurance sports is raised by their effects on blood flow, body composition, antioxidant capacity, systemic inflammation, and glycogen storage. These attributes provide a scientific foundation for the increased use of plant-based diets by endurance athletes.»
This idea is explored more deeply in a recent literature review of several studies on the effect of vegetarian & vegan diets of performance from 2021 (PMID: 34836139) in which the authors suggest that even though current research has not been able to clearly demonstrate consistent differences of performance between diets so far, there seems to be a trend towards improved performance after vegetarian and vegan diets for both endurance (especially when performing exercise intensities relying on higher carbohydrate usage) and even strength exercise. Endurance capacity relies on skeletal muscle mitochondrial/capillary density, the concentration of hemoglobin, endothelial function, antioxidant potential, functional heart morphology and availability of carbohydrates. Given the macro- and micronutrient composition of vegan/vegetarian diets, the authors speculate that these could possess potentially advantageous properties for endurance performance compared to an omnivorous diet. Furthermore the authors also hypothesize that by improving the quality of the gut microbiome (by increased fibre intake), vegan diets could also modulate signalling through the production of short-chain fatty acids (SCFAs). SCFAs can be used as energy-deriving substrate in tissues such as the muscle, indicating that they can contribute to enhanced skeletal muscle growth. Moreover, SCFAs are considered as putative signalling molecules for skeletal muscle adaptation and can directly phosphorylate and activate AMPK by increasing the AMP/ATP ratio in skeletal muscle (it basically leads to increases in ATP production, the main energy storage molecule in our cells).
Having said all this, there’s still a lack of higher quality studies on the effects of vegan/vegetarian diets on sports performance and as stated by the authors of this article, «future studies must carefully combine the analysis of molecular signalling networks in combination with physiological read-outs in extended time frames» before more definite claims can be made. We basically need more prospective longitudinal studies and randomized controlled trials (RCTs)…
I want to finish this post by making my intentions clear. I am not suggesting that a balanced (hopefully) plant-forward omnivore diet is sub-optimal for physical performance. My main argument is that, contrary to popular belief, a healthy WFPB diet is definitely not detrimental to sports performance and there is mounting evidence that it could actually be beneficial at (the very) least with regard to some types of endurance exercise. More research is needed as always but the trend is pretty clear given the current evidence.