1. Introduction

Legumes consist a part of the botanical family of Fabaceae (Leguminosae) and are categorized into oilseed legumes (soybeans and peanuts) and non-oilseed legumes. The non-oilseed are further categorized into dried legumes which are the pulses (chickpeas, cowpeas, dry beans, dry peas, lentils) and undried legumes which are harvested before drying and may be consumed with or without their pods (snap beans and snap peas) ^[1]. The nutritional profile of legumes varies between the different categories ^[2]; oilseed legumes have a higher protein and total fat content, as well as a lower carbohydrate content in comparison to non-oilseed legumes. Table 1 depicts the main nutritional features of the most widely consumed legumes.

Table 1. Nutritional composition of common legumes.

All values are referring to 100 g of mature seeds, cooked—boiled, without salt—except for peanuts, which are referring to a 100 g of raw peanuts. DFE: Dietary folate equivalent ^[2].

Low energy density, low glycemic index (GI), high fiber, and high protein content are characteristic properties of pulses. They are also low in total fat, high in monounsaturated fatty acids (MUFA), polyunsaturated fatty acids (PUFA), plant sterols, vitamins, and minerals ^[3]. Their metabolic effects are mainly attributed to their protein, fiber, mineral and phytochemical contents ^[3][4][5]. However, most of their characteristics are affected by the specific type of legume and the cultivar. Processing methods such as soaking, boiling, microwaving, and autoclaving can increase protein and starch digestibility and, in some instances, increase the lipid and fiber content as well. The process of germination has been found to increase legumes’ mineral, vitamin B complex, and antioxidant contents ^[6].

However, leguminous seeds have a high content of anti-nutrients and non-nutritive compounds, which obstruct, to some extent, the absorption of vitamins and other nutrients. Phytic acid, saponins, polyphenols, and alpha-amylase inhibitors are some of the anti-nutrients which are found in legumes ^[7]. Τhe content of anti-nutrients differs depending on the specific type of legume and the various methods of cooking and processing ^[7]. A review by Petroski and Minich ^[8] describes in detail the amounts of anti-nutrients in specific legumes, as well as the effects of different preparations on their content.

On the other hand, these same compounds exert beneficial effects for human health. For instance, isoflavones, which are phytoestrogens mainly in soybeans and chickpeas contribute to the prevention of cardiovascular diseases through their antioxidant and estrogenic properties ^[4][9]. Phytic acid has an antioxidant capacity and protects against DNA damage ^[8] while tannins possess a variety of beneficial properties such as antioxidant, anticarcinogenic, immunomodulatory, and cardioprotective ^[8]. Bioactive peptides contribute to the management of insulin-resistant and type 2 diabetes mellitus (T2DM) patients via obstruction of carbohydrate digestion due to the inhibition of enzymatic functions such as those of glucosidase and α-amylase ^[10]. By and large, daily amounts of 150 g of cooked pulses seem to be sufficient to reap their benefits in all areas of health ^[3].

2. Oxidative Stress and Inflammation

Reductions in oxidative stress and pro-inflammatory markers are thought to contribute to the overall health benefits resulting from a legume-rich diet. In a review paper released in 2023 by Grdeń et al. ^[10], it was reported that non-soy legumes can lower C-reactive protein (CRP) blood levels while multiple anti-radical activities were described. A cross-over randomized controlled trial (RCT) in 31 overweight T2DM patients reported significant reductions in the levels of CRP, interleukine-6 (IL-6), and tumor necrosis factor-a (TNF-a). These anti-inflammatory and antioxidant properties are believed to stem from some of the legumes’ aforementioned bioactive compounds (namely phytic acid and phenolic compounds i.e., tannins, anthocyanins, and other flavonoids), high magnesium and dietary fiber content, as well as them substituting animal protein sources ^[11][12].

3. Overweight and Obesity

Current literature confirms that legumes can contribute to weight management and obesity prevention. A study using data from the National Health and Nutrition Examination Survey (NHANES) indicated that participants who consumed beans had lower body weight, smaller waist circumference, and lower body mass index (BMI) in comparison to non-consumers ^[13]. According to a systematic review and meta-analysis of 21 RCTs, dietary interventions with pulses led to greater weight loss compared to diets without pulses. This effect was statistically vital as regards both hypocaloric diets and diets without any energy restrictions ^[14]. Significantly larger reductions both in body weight and waist circumference for obese and overweight individuals were reported when comparing hypocaloric diets rich in non-oilseed legumes with control hypocaloric diets, such as high-protein or low-carb ones. This was mainly attributed to the high fiber content in legume-rich diets. An increase in mitochondrial oxidation markers which has been noticed in such diets implies another possible physiological mechanism for the greater weight loss observed ^[3]. The high fiber content of legumes has been widely acknowledged as one of the main contributor to weight management, either by reducing energy intake or by increasing satiety ^[5][13]. Increased feelings of satiety have been very well documented in the literature when it comes to legume consumption, and have also been attributed to their high protein content, apart from fiber ^{[5][13][15][16][17]}. One more possible reason for the increased satiation reported in the literature could be legumes’ low GI ^{[13][14][15][16][17]}. Enhanced satiety can help the individual to better control their eating habits, and therefore comply more easily with nutrition recommendations benefiting both their weight and health status.

4. Cardiovascular Diseases (CVD)

Research focusing on the effect of legume consumption on the cardiovascular system includes many studies on primary prevention by means of regulating CVD risk factors. These mainly entail hypertension, hyperlipidemia, diabetes, and obesity. Data from 20 RCTs, analyzed in a systematic review by Ferreira et al. ^[3], indicated significant ameliorations in both blood pressure (BP) and the lipid profile. In particular, decreases in systolic (SBP) and diastolic (DBP) blood pressure, total cholesterol (TC), LDL-cholesterol (LDL-C), HDL-cholesterol (HDL-C), and triglycerides (TG) were statistically significant when following a hypocaloric diet rich in non-oilseed legumes, but were not significant or absent in other types of hypocaloric diets—e.g., low-carb diets. The majority of the included studies consistently displayed an improvement of the plasma lipid profile with a daily inclusion of legumes in the diet independent of weight loss, with most data indicating a reduction in TC and LDL-C ^[3]. The aforementioned alterations in lipid profiles have been attributed to the high content of MUFA, PUFA, and plant sterols in legumes ^[5]. BP is one of the most important risk factors for CVD ^[18]. The most recent meta-analysis and systematic reviews concerning BP and non-oilseed legume intake found no significant general effects on either SBP or DBP. Subgroup analyses revealed a significant reduction in SBP in overweight and obese subjects ^[19]. Regarding the mechanism via which legumes reduce BP, most publications agree that it involves dietary fiber and plant proteins ^[5][19]. An earlier publication has proposed that the high content of potassium in legumes might also play a role ^[13]. Furthermore, bioactive peptides with angiotensin-converting enzyme (ACE) inhibitory properties have been found in fermented legumes ^[20]. In terms of the overall CVD risk, the protective effect of legumes as a part of diet patterns—such as the Mediterranean, DASH and plant-based diets—has been very well documented ^[21]. An earlier review by Marventano et al. ^[22] concluded that three to four servings of legumes per week significantly reduces the risk of both CVD and coronary heart disease by close to 10%. An analysis on the Greek sub-cohort of the European Prospective Investigation into Cancer and Nutrition (EPIC) study indicated an inverse relationship between legume intake and incidence of cerebrovascular disease, such as a stroke ^[23]. A more recent meta-analysis of prospective cohort studies documented a reduced CVD and coronary heart disease risk with higher amounts of soy and non-soy legume intake ^[15]. The same conclusion was reached by an umbrella review of observational studies regarding oilseed and non-oilseed legumes ^[24].

5. Type II Diabetes Mellitus (Τ2DM)

Extensive research has been performed as to the effect of legume consumption in glucose homeostasis and T2DM patients. Reports on the association of legume intake and the risk of developing T2DM do not provide conclusive evidence. Some systematic reviews ^[25][26] and meta-analyses ^[25][26][27] showed no significant effect of legume consumption on T2DM incidence. Becerra-Tomás et al. ^[15] highlighted the inconsistencies through the literature and concluded that there is not enough consistent evidence pointing to a reduced risk of T2DM when consuming more legumes. It was also proposed that there was heterogeneity across the different studies which could be attributed to the different types of legumes consumed (soy and non-soy) and the wider dietary pattern they were a part of (Mediterranean, high fiber, low GI, etc.). A recent study in a European cohort found no significant reduction in T2DM incidence rate when increasing legume intake by substituting red and processed meat in the diet ^[28]. On the other hand, data from the prospective observational study Prevention with Mediterranean Diet (PREDIMED) ^[29] have indicated that non-soy legumes can decrease the risk of developing T2DM when consumed in high quantities. This is in accordance with a review published by Ramdath et al. ^[30], which stated that pulses as part of a Mediterranean diet pattern can lower the risk of developing T2DM. As far as the effect of legumes in glycemic control is concerned, the available literature also produces mixed results. An older systematic review and meta-analysis of 41 RCTs examined the effect of non-oilseed legumes on glycemic control in adults with and without diabetes. Regarding the former, legume intake had a significant inverse relationship with fasting blood glucose and insulin. When studies with non-oilseed legumes as part of a low GI diet or a high fiber diet were meta-analyzed, a mean decrease in glycosylated hemoglobin (HbA1c) of 0.48% was noted. This reduction is significantly larger than the threshold of 0.3% considered clinically relevant by the United States Food and Drug Administration (FDA). However, in normoglycemic subjects, results were conflicting. Specifically, fasting blood glucose was reduced in studies concerning non-oilseed legumes alone, but was increased in studies with them as part of a low GI diet ^[31]. Another publication also reported an increase in fasting glucose when 64 insulin-resistant men consumed a bean-enriched low GI diet, corroborating these results ^[32]. A paper by Pittaway et al. ^[33] regarding the impact of chickpea intake on glucose homeostasis in 45 healthy adults resulted in a decrease in fasting insulin and Homeostatic Model Assessment for Insulin Resistance (HOMA-IR). Contrary to these findings, an intervention with a hypocaloric diet rich in non-soy legumes in 42 premenopausal women with central obesity reported no beneficial effects in fasting glucose and insulin, as well as HOMA-IR ^[34]. Concerning patients with T2DM, a study by Jenkins et al. ^[35] documented a decrease of 0.5% in mean HbA1c when consuming beans, chickpeas, or lentils as a part of a low GI diet with at least 190 g of legumes per day versus a control of high whole wheat fiber diet. One cross-over RCT on 31 patients with T2DM resulted in a reduction in fasting blood glucose and insulin when replacing two portions of red meat with a cup of non-oil seed legumes ^[12]. A recent review concluded that a mean intake of 5 cups of pulses per week steadily resulted in better long-term glycemic control, as indicated by reductions of HOMA-IR, fasting blood glucose, and insulin in patients with T2DM. Moreover, it is estimated that in acute studies involving ¾ to 1 cup of lentils or black beans, the attenuation of postprandial glucose persisted to the following meal and even the day after ^[30]. As regards mechanism, the literature is in agreement and suggests that high fiber and resistant starch, low GI, plant-proteins, minerals, bioactive peptides, and phenolic compounds all contribute to antidiabetic effects ^[29][36].

6. Cancer

Recommendations regarding cancer prevention from WCRFI give emphasis on the importance of including legumes—e.g., lentils and beans—in most meals, making them a core part of ones’ diet. That is mainly due to the fact that they are rich in fiber, and evidence points to reducing the risk of certain tumors such as colorectal cancer ^[37]. Moreover, the WCFRI together with the American Institute for Cancer Research (AICR) concluded after a review of the globally available literature that there is convincing evidence that foods rich in dietary fiber, such as legumes, lower the risk of colorectal cancer, overweight, and obesity, with the latter two increasing the risk for at least 12 different tumors ^[38]. A meta-analysis of 12 cohort studies noted a 13% lower risk of breast cancer with a higher adherence to the vegetable-fruit-soybean dietary pattern. This negative correlation was in part ascribed to soy isoflavones and a decrease in reabsorption of estrogen from the gastrointestinal tract. Furthermore, in a publication using data from the Nurses Health Study II, a reduction of 24% in the incidence of breast cancer was reported when consuming beans or lentils at least two times per week ^[39]. The relationship between soy and non-soy legume intake and risk of prostate cancer was examined in a meta-analysis of prospective cohort trials. The dose-response analysis concluded that for every 20 g/day increase in total legume consumption, the risk of prostate cancer declined by 3.7%. In the stratified analysis, the same inverse association was observed for non-soy legumes, whereas an inverse trend was noted for soy, albeit without it reaching statistical significance ^[40].

Concerning the mechanism of anti-cancerogenic actions, the literature suggests that it is a result of flavonoids, mainly soy isoflavones, other antioxidants, fiber, resistant starch, phytochemicals, anti-inflammatory and anti-nutritional factors, and bioactive compounds—examples of the latter being phytic acid, tannins, saponins and protease inhibitors ^[40][41].

7. Gastrointestinal System

Due to the fact that legumes are rich in dietary fiber, an interest has been expressed in the literature concerning potential beneficial effects on the gastrointestinal system. The latest systematic review and meta-analysis showed that there are significant effects of legumes and their derivatives on the abundance, diversity, and richness of gut flora. However, there is a lack of in vivo human trials in this area ^[42]. A review by Knez et al. ^[43] focusing on the effect of fermented legumes on health and disease underlined their probiotic and antioxidant benefits. Fermented lentils, beans, peas, chickpeas, and fava beans were associated with a rise in lactic acid producing bacteria such as lactobacilli, which have been found to increase the bioavailability of nutrients, improve satiety, and assist in weight management. Additionally, fermented peas and beans were found to have larger amounts of polyphenols ^[43]. Legumes also help towards the improvement of gastrointestinal function by increasing stool volume, regulating bowel movements, and serving as a prebiotic ^[44]. Nonetheless, adverse effects have also been reported such as upset stomach, flatulence, bloating, heartburn, and diarrhea ^[35][45][46]. The high fiber content of legumes could be held responsible for the observed gastrointestinal adverse events ^[35] and the presence of FODMAPS (Fermentable Oligosaccharides, Disaccharides, Monosaccharides, and Polyols) as well ^[10].

8. Depression and Anxiety Disorders

Prevention of depression can be succeeded following a diet pattern such as the Mediterranean diet and having a high intake of legumes among other food groups, as suggested by Opie et al. ^[47]. This effect is mediated through antioxidant and anti-inflammatory mechanisms in combination with endothelial protective properties. One study has also reported that remission of major depressive disorder is possible when adhering to a healthy diet, including legumes, based on the Mediterranean diet ^[48]. One RCT on adults with diagnosed or self-reported depression focused on the outcomes of adopting a Mediterranean diet supplemented with fish oil versus a control following a regular diet. The analysis of the data showed a decrease of 45% in the Depression Anxiety Stress Scale (DASS), and an amelioration in all the categories of the quality-of-life assessment questionnaire (AQoL) regarding the Mediterranean diet group in the 3-month follow up. These results were sustained up to the 6-month mark. Specifically, higher legume intake was associated with a reduction in anxiety, stress, and negative effects, as well as an increased quality of life, coping, and psychosocial skills ^[49]. Multiple researchers stress that for normal brain structure and function, multiple nutrients are required; thus, the quality of the whole diet rather than specific nutrients should be the focus ^[47][49]. A systematic review by Bayes et al. ^[50] suggested that the polyphenol content of the Mediterranean diet could be the key factor rendered responsible for the lower risk of depression and the amelioration of depressive symptoms.

It should be noted that the exact mechanism of action is not known and the factors that have been proposed to mediate their effect, in the context of a healthy diet, are anti-inflammatory and antioxidant properties, along with the regulation of the endothelial function. References regarding the regulation of the microbiome-gut-brain axis have also been made ^[50][51][52].

9. Mortality

In regard to mortality, a study involving 7216 subjects from the PREDIMED cohort found an inverse association between cancer mortality and total legume and lentil consumption ^[53]. All-cause mortality was lower in the highest legume intake subgroup in a meta-analysis of prospective cohort studies but no statistically significant association was made with CVD-related mortality ^[54]. In another systematic review and meta-analysis of prospective studies which examined the relationship between total protein, animal-protein, and plant-protein with mortality, showed that plant-protein intake was associated with a significantly lower risk of all-cause mortality and CVD-related mortality. Legumes were regarded as a component of plant-protein. In the dose-response analysis, it was revealed that for every additional 3% of energy from plant-protein, the risk of all-cause mortality dropped by 5%. The researchers concluded that substituting animal-protein with plant-protein increases longevity ^[55].

This entry is adapted from the peer-reviewed paper 10.3390/nu16010098