A 2018 Gallup poll recorded that 5% of Americans identify as vegetarian, and 3% as vegan [
13]. Some studies have reported that adherence to a plant-based dietary pattern is associated with lower risks of an array of chronic diseases. Although the dynamics of cultural heritage and genetic characteristics should also be considered, in general, these diets reduce risks for major diseases that disproportionately affect Blacks, such as heart disease, cancer, diabetes, obesity, and chronic kidney disease. Multiple reasons for their benefits exist, including the synergistic effects of nutrients typically found in whole plant foods, such as fruits, vegetables, whole grains, legumes, and nuts [
14,
15]. Here, we review the current evidence supporting a plant-based diet for disease protection and some of the associated mechanisms.
2.1. Heart Disease
Heart disease is the leading cause of death among men and women in the United States and globally [
16]. Major risk factors include elevated blood lipids, high blood pressure, and obesity [
16]. Currently, Blacks have higher deaths rates from heart disease than other ethnic groups and are disproportionately affected with its risk factors [
16]. For example, more than 40% of Black men and women are diagnosed with hypertension, and 82% of Black women are classified as either overweight or obese based on having a body mass index (BMI) of 25 or greater [
17,
18]. The American Heart Association identifies that reasons for cardiovascular disease disparities in Blacks include physical inactivity, poor quality sleep, cigarette smoking, and poor diet quality (preference for unhealthful “soul food” dishes) [
19]. Genetic differences between Blacks and Whites may also play a role, although more cohort studies are needed that identify the prevalence of risk alleles in minority populations, and their relationship to cardiovascular disease risk [
19].
Since the emergence of heart disease as a leading cause of death in the early 1900s and its reaching a peak in the 1960s [
20], diet has been studied for its potential role. The Diet–Heart hypothesis, coined by Ancel Keys in the 1950s, postulated that saturated fat in the American diet was a major culprit because it increased cholesterol levels [
21]. The 2015–2020 Dietary Guidelines for Americans recommends that “individuals should eat as little dietary cholesterol as possible” based on strong evidence showing that “eating patterns that include lower intake of dietary cholesterol are associated with reduced risk of cardiovascular disease” [
12]. The document further cautions that “intake of saturated fat should be limited to less than 10 percent of calories per day by replacing them with unsaturated fats” [
12]. According to the guidelines, these recommendations are also based on “strong and consistent evidence” showing that substitution of saturated fats with unsaturated fats is associated with reduced risk of both incidences of and mortality from cardiovascular disease [
12]. Although the hypothesis continues to be debated, a recent systematic review concluded that replacing saturated fats with poly and mono-unsaturated fats (commonly found in plant sources) is associated with lower coronary heart disease events [
22]. Some of the main sources of saturated fat in the U.S. diet include dishes containing cheese, meat, poultry, and seafood [
12]. Plant sources of unsaturated fats include olive oil and nuts [
12]. Diets that are high in sodium, red meat, and processed meats, and low in nuts/seeds, fruits, and vegetables are also related to increased cardiometabolic death, which may be influenced in part by a higher intake of saturated fats relative to unsaturated fats [
23,
24]. Although some meats have a lower saturated fat content than others, results from the 2019 APPROACH (Animal and Plant Protein and Cardiovascular Health) trial showed that high intakes of saturated fat were associated with increased cholesterol levels, regardless of meat type (white meat versus red meat) [
25]. The lowest cholesterol levels were among non-meat consumers, suggesting that plant-based protein sources may offer the greatest protection from heart disease [
25]. Gaush-Ferre et al. found similar results, where substituting red meat with high-quality plant proteins (legumes, soy, nuts) led to more desirable blood lipids and lipoproteins. These results were not seen when substituting red meat for fish or refined carbohydrates [
26].
In addition to saturated fats, other compounds in meats and animal byproducts have been associated with heart disease. For example, Wang and colleagues demonstrated a two-fold increase in systemic and urine trimethylamine N-oxide (TMAO) levels with chronic red meat consumption compared to white meat or non-meat consumption [
27]. Plasma TMAO levels were independent of saturated fat intake. As an atherogenic metabolite, TMAO production may increase the risk of cardiovascular disease in a dose-dependent manner [
28]. Wang and colleagues showed that plasma TMAO could be reduced within 4 weeks of discontinuing red meat consumption, thereby potentially lowering heart disease risk [
27].
Compared to omnivore diets that are high in meats and animal byproducts, plant-based diets are typically lower in saturated fats and cholesterol [
29,
30]. In addition, several nutrients and compounds found in foods of plant origin like fruits and vegetables are beneficial for the heart. Vitamin K, a fat-soluble vitamin abundant in dark green leafy vegetables, promotes proper blood clotting and protects the arteries [
31]. Vitamin K also aids the adequate removal of calcium to prevent calcification in the blood vessels [
32]. A recent meta-analysis demonstrated that consumption of leafy green and cruciferous vegetables was associated with a 15.8% reduced incidence of cardiovascular disease in eight studies [
33]. Dietary fiber, a non-digestible carbohydrate commonly found in oats, apples, beans, and peas, is also beneficial for the heart in several ways. First, soluble fiber lowers the absorption of cholesterol in the liver by binding bile acids during digestion. This disruption of cholesterol production decreases both total and LDL cholesterol to reduces heart disease risk [
34]. High fiber intake also increases satiety, which may promote weight management to reduce the risk of heart disease [
35,
36]. Heart-healthy mono and polyunsaturated fats, such as those commonly found in avocados, nuts, and olive oil, are favorable for the heart because they help to lower blood pressure and triglyceride levels [
22].
Additionally, phytochemicals may promote heart health, since many polyphenols decrease endothelial dysfunction to regulate proper heart function. For example, resveratrol, an antioxidant found in the skin of red grapes, works to scavenge free radicals that can cause rupture of plaque along the arterial walls [
37], and may reduce risk of hypertension, stroke, and ischemic heart failure [
37]. Other polyphenols, such as quercetin and EGCG, induce vascular relaxation to regulate blood pressure and lower the risk of heart disease [
38].
Plant-based dietary patterns have been shown to promote heart health, although it is unknown if these benefits persist after accounting for genetic variations. For example, Dean Ornish showed in the late 1990s that a low-fat plant-based diet emphasizing fruits, vegetables, and whole grains promoted a regression of coronary atherosclerosis [
39]. Plant-based diets that are inclusive of customary levels of fats have also recorded positive results in the management and potential reversal of coronary heart disease [
40]. The Portfolio dietary pattern, a plant-based diet created by David Jenkins and colleagues, demonstrated its effectiveness for reducing cholesterol levels after a 4-week intervention. Results were comparable to a very low saturated fat diet that was supplemented with 20 mg of lovastatin, a cholesterol-lowering drug [
41]. More recently, the Portfolio diet was associated with a 13% risk reduction of coronary heart disease after 10 years [
42]. Thus, the positive association between plant-based diet adherence and heart health is an important finding for the nearly 50% of Black adults who already have some form of heart disease [
43].
2.2. Cancer
Behind heart disease, cancer is the second leading cause of death in the United States [
44]. Some modifiable risk factors include low physical activity, poor diet, and obesity [
44]. Blacks are disproportionately affected with cancer compared to Whites and generally have lower survival rates [
44]. The incidence and death rates for all cancers are higher in Black men than White men (9% and 22%, respectively), and Black women have a 13% higher risk of dying from cancer than White women, even though they have a 7% lower risk of being diagnosed. For specific cancers, the American Cancer Society reports that while the probability of developing breast cancer is higher among White women (13.2%) than Black women (11.5%), Blacks are more likely to die from a diagnosis (3.1%) than Whites (2.6%). Similarly, Black men have a higher probability of developing prostate cancer than White men (14.8% versus 10.6% respectively), and higher death rates than Whites (4.0% versus 2.2%, respectively). Death rates for cancers of the colon and rectum are also higher in Blacks than in Whites (55.2 per 100,000 versus 44.6 per 100,000, respectively) [
44].
The relationship between poultry/fish consumption and cancer has yielded conflicting results. Some studies have found possible benefits for esophageal squamous cell carcinoma, along with oral cavity and oropharyngeal cancer subtypes with fish consumption [
45,
46], although more research is needed that considers nutrient variations by fish type [
45,
46]. For example, the role that omega-3 fats in fish oil play in tumor suppression continues to be investigated, and more interventions are needed on omega-3 dosage and subtypes (DHA versus EPA) [
47]. Further, European and Asian cohorts have recorded possible risk reduction of colorectal and breast cancers with omega-3 fish consumption [
48,
49], but more research is needed in Blacks. Brasky et al. found no significant association between omega-3 and fish consumption and endometrial cancer risk in 47,602 African American women, although non-significant hazard ratios were more favorable among normal weight women (HR = 0.53; 95% CI = 0.18, 1.58) than in overweight women (HR = 0.88; 95% CI = 0.56, 1.31) [
50].
The World Cancer Research Fund (WCRF) and American Institute for Cancer Research (AICR) both caution that certain fish may increase cancer risk, especially those prepared by salting, smoking, or curing [
51]. Salted fish, for example, is associated with increased nasopharyngeal cancer risk [
51]. The California Collaborative Prostate Cancer Study observed that pan-frying, oven-broiling, and grilling methods were associated with increased risk of advanced prostate cancer with regular white fish consumption in men (
p = 0.001) [
52]. Today, the most common preparation methods of fish and other meats in Black communities is fried or salted, which may not be associated with risk reduction as in other populations [
53,
54]. Overall, large prospective studies are needed to further examine the effects of fish intake on cancer risk [
55]. Additionally, more evidence is needed on the exposure amounts or specific thresholds where effects may be observed [
51].
Other meats have also been linked to cancer. In 2015, the International Agency for Research on Cancer from the World Health Organization published findings on the carcinogenicity of red and processed meat consumption [
56]. Processed meats were classified as carcinogenic, increasing colorectal cancer risk by 18% with each 50 g portion consumed daily, and red meat was classified as a probable carcinogen [
56]. The 2018 WCRF/AICR report further describes that red meat consumption probably increases colorectal cancer risk by 12% with each 100 g daily portion, and limited evidence was also found for increased nasopharyngeal, lung, and pancreatic cancer risks [
51]. More recently, the Nutritional Recommendations (NutriRECS, Halifax, Canada) Consortium challenged the WCRF/AICR recommendations. The Consortium reviewed both randomized controlled trials and cohort studies that were conducted in 1000 or more adults for at least 6 months, and concluded that there was insufficient evidence to suggest that consuming unprocessed red meat and processed meat is detrimental [
57]. The recommendation to continue consuming these meats was made based on the view that “the certainty of evidence for the potential adverse health outcomes associated with meat consumption was low to very low, supported by the similar effect estimates for red meat and processed meat consumption from dietary pattern studies as from studies directly addressing red meat and processed meat intake” [
58]. However, in response to these findings, Harvard University School of Public Health commented that the methodology used to classify the evidence as “low” or “very low” was inappropriate because of the panel’s use of the GRADE criteria, which is more appropriate for drug trials, instead of the more commonly used HEALM or USDA criteria for large, long-term dietary randomized clinical controlled trials [
57]. The University also cautioned that “this recommendation runs contradictory to the large body of evidence indicating higher consumption of red meat—especially processed red meat—is associated with higher risk of type 2 diabetes, cardiovascular disease, certain types of cancers, and premature death” [
57]. In a review of 42 published meta-analyses, Lippi et al. identified an increased risk of cancer in consumers of large amounts of red and processed meats, although the risks were not increased for white meat or poultry consumption [
59]. More specifically, colorectal, lung, gastric, esophageal, and bladder cancers were identified with increased risks. A recently published longitudinal study of over 42,000 participants identified a 23% increased breast cancer risk with increased red meat consumption [
60]. Red/processed meat intake was also positively associated with hepatocarcinomas in men and women in another 2019 study [
61].
There are several proposed mechanisms to explain the increase in cancer risk with regular red/processed meat consumption. For processed meats, N-nitroso compounds added during preparation may encourage cancer development endogenously [
62]. Steinberg showed a dose-dependent relationship between red meat consumption and nitroso compounds in the feces (249 ± 167 µg nitroso concentration with 420–600 g daily red meat consumption, compared to 54 ± 7 µg nitroso concentration with no daily meat consumption and 87 ± 55 µg with 420–600 g white meat consumption) [
63]. Heme iron within red/processed meats can catalyze this reaction, thereby accelerating tumor progression [
63]. N-nitrosamines are also found in fried pork, fried bacon, and fried/grilled poultry [
64]. Grilled and/or fried meats release heterocyclic amines and polycyclic aromatic hydrocarbons during preparation. Heterocyclic amines have been associated with breast, colorectal, liver, lung, and prostate cancers, while polycyclic aromatic hydrocarbons have been associated with colorectal, breast, and pancreatic tumors [
65]. In a recent review of in vivo evidence, Crowe et al. discussed that although most studies report an increased risk of colorectal cancer with consumption of nitrite-containing processed meats, “future epidemiological studies should consider more specifically categorizing the processed meat consumed, explicitly distinguishing between those meats which are nitrite-containing and those which are not” [
66].
Vegetarians and vegans typically have lower risks of cancer than non-vegetarians [
67,
68,
69,
70]. Additionally, there is a lower risk of mortality from breast, lung, and prostate cancers compared to omnivores [
67]. Lacto-ovo vegetarian diets may also protect against precursors to cancer, such as gallbladder polyps in adults [
71], and regular fruit and vegetable intake is associated with a lower risk of breast fibroadenomas [
72]. Plant foods are sources of cancer-protective nutrients and compounds, such as vitamin E, beta-carotene, fiber, and phytochemicals. As an antioxidant, various naturally-occurring forms of vitamin E, including gamma and alpha-tocopherols and tocotrienols, have demonstrated benefits in cancer prevention as well as the potential to be an adjunct therapy for cancer control [
73]. Insoluble fibers contained in many fruits and vegetables accelerate the digestive process and facilitate the elimination of carcinogens that come in contact with the intestines [
74]. Butyrate, produced through fermentation by the gut microflora, may offer additional protection [
74].
Plant foods are also abundant in phytochemicals, some of which are thought to stop carcinogenesis through their antioxidant properties by interfering with oxidative stress signaling pathways and suppressing DNA damage [
75]. For example, resveratrol seems to reduce estrogen toxicity to protect against breast cancer, and may also reduce the risk of skin, stomach, and pancreatic cancers [
76]. β-sitosterol, a phytosterol found in wheat germ, peanuts, and corn oils, may reduce the growth of colon, breast, and prostate cancer cells [
77]. The effective phytosterol dosage used in some studies (16 µM) is within typical dietary limits, although the potential effects based on genetic variations and dietary habits of Blacks is not known [
77]. Phytosterols may reduce prostate cancer risk by inhibiting tumor growth and stimulating apoptosis [
76,
77]. Sulforaphane, an antioxidant found in broccoli, has reduced breast tumor size and growth by 75%, and prostate tumor volume by 50% [
78]. A 2019 review demonstrated that consuming 500 mg/day of total phytosterols from foods could reduce cancer development risk [
79], and a 37% risk reduction was seen for highest dietary phytosterol intake compared to the lowest intake [
79]. Phytosterols are highest in vegetable oils (150.4–1230.9 mg/100 g), legumes (129.6–275.6 mg/100 g), and nuts (18.9–255.2 mg/100 g) [
80].
2.3. Diabetes
Type 2 Diabetes (T2D), a chronic disease caused by a reduction in the effectiveness of insulin’s control over blood sugar, is increasing in the United States and globally. Currently, more than 100 million Americans are living with either prediabetes or T2D [
81]. Diagnoses for T2D are currently at a rate of 1.5 million per year, and it accounts for close to 80,000 deaths in the United States annually [
81]. US Blacks are twice as likely to be diagnosed with T2D as Whites and are also more likely to suffer complications from the disease, such as lower limb amputations and end-stage renal disease [
82].
Numerous studies have highlighted the role that vegetarian and vegan diets play in reducing the risk of T2D [
83,
84,
85]. Epidemiological studies have documented that vegetarians and vegans have higher insulin sensitivity than omnivores and lower diabetes risk, even after adjusting for BMI [
85]. In part, plant-based diets tend to be higher in fiber and lower in saturated fats [
29]. High fiber diets increase satiety and lessen a blood glucose spike, thereby decreasing insulin requirements [
36], A recent systematic review and meta-analysis noted that dietary cereals, whole grains, and fiber are inversely related to T2D incidence [
86]. Recommendations for preventing the disease include weight reduction by diet change and increasing consumption of whole grains, fruits, vegetables, and fiber [
86]. These recommendations could provide practical guidelines for patients who are at risk. In addition, reducing intake of animal proteins and increasing intake of plant proteins may play a role. In one study, researchers found that when 5% of energy intake from animal proteins was substituted for plant proteins (peanuts, peanut butter, other nuts, legumes, and whole grains), there was a 23% reduction in risk for T2D [
87]. Intervention studies have also confirmed these findings. For example, Lee et al. recorded that A1C improvements were greater for diabetic participants who followed a vegan diet over 12 weeks than for those who did not [
88].
Diabetes patients are at risk for various neuropathies, which may be influenced in part by an excess of free radicals and oxidative stress [
89]. Oxidative stress in other parts of the body can worsen endothelial function and insulin resistance, and may also lead to cellular dysfunction and damage [
89]. Various antioxidants found in plant foods have been shown to help to reduce oxidative stress, which can improve nerve and heart function. For example, Vitamins C and E, found abundantly in citrus fruits (vitamin C) and vegetable oils (vitamin E) have both been associated with lower coronary complications, due to their antioxidant properties [
90,
91]. However, both of these vitamins in large supplemental doses could have adverse effects. The greatest benefit has been observed from dietary doses [
90,
91]. In a 20 week pilot intervention, participants were randomized to follow either a B12-supplemented low-fat, plant-based (vegan) diet with weekly classes, or their customary diet supplemented with B12. Dietary and clinical data were collected at baseline, midpoint, and at the end of the study. Participants following the vegan diet reported reduced neuropathic pain compared with the control group, as measured by the McGill pain questionnaire (−8.2 point score change,
p = 0.04), and also reported less neuropathy, as measured by the Michigan Neuropathy Screening Instrument (−1.6 point score change,
p = 0.03) [
92].
2.4. Obesity
As an independent risk factor for heart disease, cancer, and diabetes, obesity disproportionately affects Blacks, with the highest rates among those living in the southeastern region of the United States (Alabama, 41.8%; Mississippi, 42.9%), and Black women (57.9%) [
6,
7,
8]. In a systematic review and meta-analysis of 86 cross-sectional and 10 prospective studies, the cross-sectional data showed that vegetarians and vegans had significantly lower BMI values than omnivores [
67], while another study demonstrated a reduction in waist circumference and fat mass [
93]. Evidence suggests that one potential mechanism may be through lowering the concentration of appetite-stimulating hormones, such as leptin. An observational study of healthy volunteers in Poland found that plant-based diets not only lowered body fat storage, but also circulating leptin levels [
94]. Since leptin triggers the appetite, lower levels may reduce total caloric intake to benefit weight management. Race was associated with leptin levels in a longitudinal analysis of 62 European American and 58 African-American women, with African-American women having higher circulating leptin levels both at baseline and after weight loss than European American women (baseline: 24.7 versus 19.9 ng/dl, respectively; after weight loss: 11.7 versus 8.48 ng, respectively) [
95]. Consistent evidence for the relationship between leptin and obesity is warranted, however, especially in Black populations [
95]. In addition to reducing body fat, a plant-based diet may also reduce chronic inflammation associated with the progression of obesity. Following a healthy plant-based diet was shown to reduce chronic inflammation in overweight and obese Iranian women, and even obese vegetarians may have lower adipose tissue inflammation than obese omnivores [
96,
97].
Overweight and obese women have more difficulty conceiving compared with women of normal weight [
98]. Obesity increases inflammation and may have a toxic effect on reproductive tissues by promoting cellular damage [
98]. Further, changes in hormone regulation that are triggered by obesity can disrupt the ovulatory cycle [
99]. Because of this, overweight and obese women are more likely to develop polycystic ovary syndrome (PCOS), which can lead to anovulation [
100]. Obese women with PCOS also have higher levels of free testosterone than women of normal weight, which may disrupt fertility [
100]. For overweight/obese women and those with PCOS who are trying to conceive, lifestyle interventions that include shifts in dietary patterns are often recommended as the first line of management to promote weight loss [
100]. In a clinical trial, women with PCOS who were placed on a vegan diet (39% African-Americans) lost significantly more weight than participants on a regular diet [
101]. A cohort study in 711 Indian women found that the shift from a vegetarian dietary pattern to a meat-based pattern with urbanization increased a woman’s risk for PCOS [
102]. However, more research is needed on the effects of plant-based diets on PCOS. Riley et al. noted that while there is still little evidence to confirm the role of diet in PCOS, epidemiological and clinical studies point to a possible protective role [
103]. Further, since vegetarians tend to weigh less than omnivores, this may reduce some challenges of conceiving.