Plant-based eating patterns and their association with exercise performance and chronic disease. Are there any nutritional risks?
Plant-based diets (PB), as their name reveals, mainly or only comprise plant foods, such as grains, legumes, nuts, seeds, vegetables, and fruits, excluding foods from animals, including dairy, meat, poultry, fish, and seafood. They vary in restrictiveness, from avoiding only red meat to avoiding animal products entirely. For example, a strict vegan will not consume anything from an animal, including honey and gelatin.
The proportion of the population reporting following a PB diet continues to increase, with approximately 5-10% in developed countries reporting following some type of PB.
Whether to include animal-derived protein in the diet may relate to concerns about physical health, environmental sustainability, socioeconomic considerations, ethics related to the worker or animal welfare, or religious convictions, among other motivations.
There are many subcategories under the plant-based umbrella term, including the following:
- Lacto-ovo-vegetarian is primarily plant-based, including dairy products and eggs, and excluding animal flesh (meat, poultry, fish).
- Vegan is strictly plant-based, excluding all animal products, from red meat and eggs to honey and gelatin.
- Pescatarian allows fish and seafood consumption, excluding red meat, chicken, dairy, and eggs.
- Flexitarian is primarily plant-based, occasionally including red meat, poultry, dairy, or fish.
These subcategories are the main directions of PB diets but are not exhaustive since each ramifies to additional subcategories, such as lacto-vegetarians, ovo-vegetarians, vegans who follow a raw food diet or a fruit diet, and people who abide by a macrobiotic diet.
The environmental benefits of PB diets
There is a considerable variation in the ecological footprint of animal-based products, with beef being especially detrimental to the environment compared to other products such as pork, chicken, or eggs. An increasing body of evidence suggests that the production of plant foods tends to be less resource-intensive and environmentally destructive for several reasons, mainly due to lower levels of greenhouse gas emissions (GHGs), compared to raising animals for human consumption.
The Academy of Nutrition and Dietetics advocated that ‘’PB diets are more environmentally sustainable than diets rich in animal products because they use fewer natural resources and are associated with much less environmental damage’’. Following a PB diet is often considered the most effective strategy for systemically reducing GHGs and agricultural land use related to food production and consumption, thus decreasing environmental impact.
A review study concluded that adopting lacto-ovo-vegetarian diets could reduce GHG emissions by 35%, land use by 42%, and freshwater use by 28%. Furthermore, a 25% reduction in meat consumption and a transition to a PB diet would minimize the impact of agricultural land expansion on the ecosystem’s biodiversity and carbon dioxide emissions. Altogether, embracing a PB eating pattern, especially in developed countries, could be an effective strategy for reducing the food system’s environmental degradation and our use of the earth’s resources.
The role of PB diets in health and disease
PB diets are considered nutritionally upgraded, compared to omnivorous diets, since they contain less saturated fat and cholesterol, and more folate dietary fiber, antioxidants, phytochemicals, and carotenoids.
Several studies have shown that plant-based dietary patterns are linked to a lower risk of cardiovascular disease (CVD) and cardiovascular disease mortality. Specifically, vegetarians are 32% less likely to develop coronary heart disease (CHD) than meat eaters. Moreover, PB diets have been shown to reverse atherosclerosis in individuals with established CVD. They also reduce both systolic and diastolic blood pressure and enhance insulin sensitivity, thus reducing the risk of type 2 diabetes and improving glycemic control in individuals with diabetes. Based on systematic reviews of randomized clinical trials, vegetarians tend to have a lower body mass index (BMI) and waist circumference, hence decreased obesity risk, reduced incidence of specific cancers, and lower all-cause mortality.
One study showed that adopting a low-fat vegan diet in post-menopausal women resulted in a mean weight loss of 5.8kg in 14 weeks, significantly greater than the control diet.
PB diets might act through multiple pathways in terms of their proposed health effects, including improvements in the lipid profile of vegetarians/vegans, including decreases in total cholesterol, LDL cholesterol, and triglycerides, reduced visceral fat, enhanced oxidative stress and inflammation markers, and reduced glycosylated hemoglobin (HbA1c).
However, these positive health outcomes stem from healthful plant food sources rich in dietary fiber, such as whole grains, fruits, vegetables, legumes, nuts, and plant protein, such as soy. PB diets can also be unhealthful, containing high amounts of refined carbohydrates, sugar, and highly processed mock meats. Therefore, an unhealthy PB diet may be equally compounding to an omnivorous diet in terms of their risk in health outcomes.
Besides the metabolic benefits of healthful PB diets, there is also evidence that they may be associated with a lower risk of chronic kidney disease (CKD). Specifically, a recent study found that when one serving of red and/or processed meat was replaced with plant proteins, the risk of CKD was significantly lower. It was also demonstrated that a healthful PB diet, emphasizing fruits, vegetables, whole grains, nuts, legumes, coffee, and tea, was associated with a slower GFR, the optimal marker to measure kidney function decline. However, the protective effect was lost when healthy plant foods were replaced with poor plant-based food choices, such as commercial fruit juice, refined grains, and sweets.
Considerations for the athletes consuming a PB diet
PB diets can meet the needs of athletes at all levels, from recreational to elite athletes. However, if not appropriately planned, a PB diet may provide insufficient amounts of certain macro- and micro-nutrients. Thus, depending on food preferences, athletes need to ensure adequate intake of the nutrients that are either found less abundantly in PB diets or are less well absorbed from plants compared to animal sources. Nutrition recommendations should consider each athlete’s training volume (intensity and frequency), sport, season, performance goals, and food preferences.
- Energy intake
Dietary fiber, abundant in PB diets, slows digestion, increases feelings of satiety, and decreases total daily energy intake. While such characteristics are beneficial for overall health, they could make meeting the energy requirements of elite sport challenging. Especially endurance running athletes should be conscious not to overconsume dietary fiber during competition since they have been correlated with gastrointestinal disturbances. To ensure adequate energy is consumed, athletes should monitor their weight and be aware of involuntary or sizeable degrees of weight loss. Some strategies for PB athletes to ensure adequate intake include eating five to eight meals and snacks daily, decreasing high-fiber foods, and choosing energy-dense food choices, such as nuts and seeds, nut and seed butter, avocados, dried fruits, hummus, and granola.
- Protein intake
Branched-chain amino acids (BCAAs), leucine, isoleucine, and valine, are particularly important for promoting muscle protein synthesis (MPS). Although they are more concentrated in animal-based protein compared to plant-based protein, intervention studies utilizing either a whey protein or a soy protein supplement in conjunction with resistance training yield negligible differences between groups in terms of muscle gain.
Nevertheless, some athletes may find it challenging to meet their protein needs from PB foods if their caloric requirements are exceptionally high. As mentioned before, due to the high fiber content, a PB diet tends to be very filling, and therefore, athletes may find it challenging to consume enough protein from plant food sources to support optimal muscle protein synthesis. Some studies suggest increasing the amount of protein consumed at each meal and consuming it evenly throughout the day. It is also recommended to mix plant-based protein sources, including soy foods, legumes, nuts, seeds, quinoa, and other grains, to ensure a more balanced amino acid profile of all essential amino acids and support most training needs.
Recent work has also shown that distributing protein consumption throughout the day and including a pre-sleep protein feeding promotes MPS overnight, especially when preceded by resistance training.
The American College of Sports Medicine recommends that athletes consume 1.2-1.7g/kg/day and, during times of energy restriction, to promote muscle mass retention, to consume up to 2.0g/kg/day. If an athlete still struggles to meet protein recommendations, a soy protein supplement may be particularly adjuvant due to its high leucine content.
- Omega-3 fatty acids
Most studies indicate that plasma levels of EPA and DHA are lower in PB athletes than in omnivores. Hence, athletes looking to optimize their performance may need to ensure they are achieving optimal intake levels from the omega-3 fatty acids, EPA, and DHA, primarily found in fatty fish, hence may be lacking in such diets. As a result, supplementation with an algal DHA supplement may improve muscle adaptation, energy metabolism, muscle recovery, and injury prevention.
Although iron is found abundantly in a PB diet, the bioavailability of iron in plant food sources (non-heme iron) is less than that found in animal sources (heme iron). In combination with the increased iron needs in athletes, especially females and endurance, this may result in lower ferritin levels and iron deficiency, which needs to be treated with an iron supplement so that exercise performance will not be compromised. To maximize their body’s non-heme iron absorption, individuals are suggested to pair plants’ iron food sources, such as whole grains, legumes, nuts, seeds, dried fruits, green leafy vegetables, and iron-fortified cereals, with a vitamin C source, such as lemon, citrus fruits, peppers, tomatoes, and strawberries. They are also recommended to cook acidic foods, such as tomatoes in cast-iron skillets, avoid the simultaneous consumption of iron-rich foods with calcium supplements and beverages rich in tannins, such as coffee, tea, and cocoa, as well as avoid exercise within two hours of consumption.
Research has shown that athletes with insufficient dietary zinc intake may negatively impact exercise performance in terms of both strength and endurance. However, PB athletes tend to have normal Zinc levels; hence supplementation is unnecessary. Vegetarian food sources for zinc include whole grains, legumes, tofu, tempeh, nuts, and seeds. However, the high phytate content of these foods decreases zinc absorption. Some practical food preparation techniques to increase zinc bioavailability include soaking and sprouting beans, grains, nuts, seeds, and leavening bread.
A well-planned PB diet provides sufficient calcium for the athlete to meet the recommended daily intakes. However, vegans especially consume substantially less calcium than other vegetarians and omnivores. Therefore, athletes with inadequate calcium intake should consistently use calcium-fortified foods, such as fortified breakfast cereals, fortified fruit juices, and fortified plant-based milk, or at least take a calcium supplement. Moreover, phytic and oxalic acids in plant-based calcium food sources are both inhibitors of calcium absorption. Hence, food choices with good absorption calcium rates include beans, almonds, tahini, dried figs, soy products, and low-oxalate vegetables such as kale, broccoli, Chinese cabbage, and bok choy. Boiling can also reduce oxalate content in high-oxalate vegetables, such as spinach and swiss chard.
- Vitamin D
Athletes following a PB diet may be at greater risk for inadequate vitamin D levels due to limited PB sources of vitamin D (fatty fish, fish oils, and egg yolk are the richest nutritional sources), particularly if they do not achieve sufficient sun exposure (10-30 minutes between 10 a.m. and 3 p.m.) and/or have darker skin. Since vitamin D is vital for athletes due to its role in immune function, inflammatory modulation, and exercise performance, plausible deficiencies should be taken care of. Fortified plant-based milk, fortified orange juice, fortified margarines, and fortified breakfast cereals provide modest amounts of vitamin D for PB athletes. Therefore, supplementation may be needed to achieve an adequate vitamin D status, depending on dietary intake and sunlight exposure, hence a healthy bone mineral density.
- Vitamin B12
Vitamin B12 is solely found in animal products and, therefore, requires supplementation. Notably, the mean dietary intake of vitamin B12 for vegan athletes falls well below the recommended allowance; hence supplementation is obligatory. Lacto-ovo-vegetarians, on the other hand, may have marginally normal, depending on the use of dairy products. Marginal amounts may also be available through nutritional yeast, fortified cereals, fortified vegan meat analogs, and plant-based milk, such as soy, almond, oat, etc., but such sources will not provide sufficient intake. In addition to detrimental health effects, such as megaloblastic anemia and fatigue, vitamin B12 deficiency is associated with lower creatine biosynthesis, which, along with the low creatine intake through plant-based foods, may eventually impair exercise performance. Therefore, all vegans should check their B12 status annually as a good preventative measure.
Creatine supplementation may optimize short-duration, high-intensity, and resistance exercise performance in athletes following a PB diet. Creatine supplementation may also benefit cognitive function and concussion recovery, especially for athletes in team skill-based sports.
PB diets and exercise performance
Carbohydrates are the primary energy source during moderate and high-intensity aerobic exercise. Compared to other dietary patterns, the high concentration of carbohydrates typically found in a PB diet has been reported to increase muscle glycogen concentration, improving endurance performance.
Although regular exercise reduces the risks for many chronic diseases, such as obesity, type 2 diabetes, and CVD, intense exercise performed by elite athletes can elicit an inflammatory response and increase oxidative stress, leading to muscle fatigue and delayed-onset muscle soreness syndrome (DOMS). Since muscle glycogen levels are directly correlated with time to fatigue in moderate (60-80% VO2max) intensity exercise, optimizing glycogen levels may delay fatigue in endurance and team sports. Furthermore, since phytochemicals such as polyphenols and antioxidants reduce inflammation and oxidative stress, PB diets that are abundant in these compounds have been considered superior for performance than animal-based dietary patterns.
Nevertheless, the limited evidence available suggests that PB diets neither hinder nor aids exercise performance in terms of strength, aerobic power (VO2max), or power performance. Additional research is needed to determine whether such diets contribute to improved exercise performance and/or accelerate recovery.
Potential nutritional risks associated with PB diets
Although PB diets can have many health benefits, as described above, a major criticism is the risk of nutrient deficiencies related to long-term use, particularly the more strict forms of PB diets, such as veganism. This notion is intensified for sensitive life cycle stages, such as childhood and adolescence, pregnancy, lactation, and older adulthood.
Pregnant women who adhere to vegan diets are at higher risk of protein deficiency, especially in the second and third trimesters of pregnancy. Hence, 25g of additional protein, including 2 ½ cups of soy milk and 1 ½ cups of lentils daily, is recommended. So, if protein consumption is adequate, PB dietary patterns have no difference regarding infant birth weight compared with omnivorous mothers.
In fact, PB dietary patterns are associated with a reduced risk of excessive weight gain, gestational diabetes mellitus, hypertensive disorders during pregnancy, and preterm birth. Pregnant and lactating vegetarians who are well-supplemented with vitamin B12 and the omega-3 fatty acid DHA derived from microalgae and also have sufficient sun exposure (see above) and a well-balanced diet by consuming a variety of nutrient-dense and fortified plant foods can effectively meet their energy and nutrient needs.
Vegetarian children and adolescents generally meet their protein needs when their diets contain adequate energy and a variety of plant protein sources. Other than vitamin B12, which needs to be consumed through fortified foods and/or supplemented, especially in vegans, deficiencies of other micronutrients are rarely seen when well-balanced PB diets are consumed.
The same applies to older adults consuming PB diets, with extra attention for possible vitamin D and calcium deficiencies, which, when present, should be corrected through the reinforced consumption of fortified foods and/or supplements.
A systematic review involving 37,134 subjects found vegetarians and vegans had lower bone mineral density at the femoral neck and lumbar spine compared to omnivores. The effect was greater in vegans, who also had higher fracture rates. Another review concluded that the balance between protective factors in PB diets and potential nutrient shortfalls might leave vegetarians, especially vegans, at increased risk of bone loss and fractures.
A meta-analysis found that vitamin B12 deficiency was associated with stroke, Alzheimer’s disease, vascular dementia, and Parkinson’s disease. Similar health damages may arise from iron insufficiency, another commonly assumed risk for plant-based dieters.
Although protein is abundant in PB diets, the amino acid profile in plants is often suboptimal compared to animal-based sources, particularly in BCAAs. However, the Academy of Nutrition and Dietetics has reported that consuming a wide variety of PB protein sources and adequate energy intake is sufficient to meet the required intakes of all essential amino acids.
All in all, like any diet, the nutrition quality of a PB diet lies on the spectrum from a minimally nutritious diet based on nutrient-poor, processed foods to a maximally healthy diet rich in whole food sources. Therefore, when appropriately planned, a PB diet consisting substantially of minimally processed and fortified foods can be nutritionally adequate and safe for all age groups and in all physiological conditions, including childhood, adolescence, pregnancy, lactation, and older adulthood. Particular attention should be given to calcium, iron, omega-3 fatty acids, vitamin D, and especially vitamin B12, the only micronutrient that may be missing entirely from a vegan diet unless supplemented.
- Plant-based diets are eating patterns that more or less, depending on the subcategory, encompass the consumption of plant foods, such as fruits, vegetables, grains, legumes, nuts, and seeds, while avoiding the inclusion of animal or animal-derived products, such as dairy, eggs, and honey.
- Well-planned and nutrient-dense PB diets, especially the pescatarian and lacto-ovo-vegetarian subcategories, can be adopted by individuals who seek to improve their overall health, particularly in terms of body weight, blood pressure, lipid, and metabolic profile, and cancer risk.
- All kinds of plant-based diets are potentially nutritionally adequate and safe for all population groups, from athletes to lactating women and children, provided they are appropriately planned and well-balanced.
- Any individual following a plant-based diet who thinks they cannot meet their macro- and/or micro-nutrient needs or have already established nutritional deficiencies should work with a registered dietitian to assist them, let alone athletes whose exercise performance strongly relies on that.