Legumes are the pods of plants from the Fabaceae (leguminosae) family and include alfalfa, clover, lupin, peas, peanuts, green beans, chickpeas, broad beans and soybeans and lentils. Pulses are legumes that are harvested for their dry grain, hence the common name grain legumes or pulse grains. Because the peanut and soybean are harvested for their oil, they are legumes but not pulses. Pulses are useful in weight loss because they possess a number of nutritional properties that favours physiological and metabolic shifts towards weight control. In particular, the high protein, high fibre content of pulses may increase satiety by slowing digestion and increasing the bulk of the food respectively. In addition soluble fibre may produce short chain fatty acids (SFCA) which increase satiety and inhibit hepatic glucose production. The starch amylose to amylopectin ratio as well as the phytochemical content in pulses may also favour weight control by slowing glucose absorption.
Around 20 to 45% of the dry weight of a pulse grain is made up of starch which is composed of both amylose and amylopectin. Amylose is composed of glucose units with α-1,4 bonds, and has a molecular weight around 105-106, with few branch points. In contrast, amylopectin is composed of glucose units with α-1,4 bonds and α-1,6 branch points that increase the molecular weigh (and surface area) considerably to around 107 to 109. Amylose and the outer linear branches of amylopectin are able to gelatinise on heating forming a viscous solution in water, and then re-associate on cooling in a process called retrogradation. Retrograde cooling causes rearrangement of the molecule such that the cooled starch has a more crystalline structure. This capacity for retrograde cooling as well as the relatively high amylose to amylopectin ratio causes the digestion rate of the pulses to be slowed compared to other starch sources such as cereals.
Pulses also contains slowly digestible starch that causes a considerable delay in digestion and absorption throughout the small intestine. The exact reason for this is not clear, but it may result from cellular structures (cotyledons) that impede the activity of the human α-amylase enzymes or may be related to the high amylose content. Resistant starch is also present in high concentrations in pulses and this refers to the fibre fraction of carbohydrate not able to be digested by human starch digesting enzymes. The fibre content of pulses is equal to around 15 to 30% of the total dry weight, made up of half to three quarters insoluble fibre and the rest as soluble fibre. Insoluble fibre adds bulk to the contents of the gut, and may cause satiety. Soluble fibre are fermented in the colon by bacterial causing the formation of SCFA that are absorbed and cause satiety. SCFA also as decrease colonic pH and favourably change bacterial colonies.
The protein content of pulses is around 15 to 35% by dry weight which is high for plant material. High protein diets have been shown to produce favourable effects on weight loss and a number of pulses are particularly high in protein. Protein is thought to reduce weight in obese and over weight individuals because it delays gastric emptying and therefore has a satiety effect when in high enough amounts in the diet. Pulses contain the storage proteins legumin and vicillin which are categorised as salt-soluble globulin proteins and also a number of enzymes, amylase inhibitors and lectins which fall into the category of water soluble albumins. Pulses also contain a number of enzyme inhibitors including trypsin and amylase inhibitors. Tyrpsin (and chymotrypsin) inhibitors do not appear to be beneficial to weight loss, but the amylase inhibitors reduce starch digestibility and therefore can reduce energy intake. However, enzyme inhibitors can be inactivated by cooking.
A number of nutritionally important phytochemicals are also present in pulses in reasonable amounts. Phenolic compounds in pulses are generally present in higher concentrations in the darker grains such as black beans and red kidney beans. The phenolic compounds include polyphenols, tannins, phenolic acids and flavonoids. Phenolic compounds may interfere with glucose absorption in the gut, although their role in weight control is not established. Pulses also contain phytic acid (myo-inositol hexaphosphate) which is a storage form of phosphate for some plants. Phytic acid is important nutritionally because evidence suggests that it slows the rate of carbohydrate digestion which may therefore slow the absorption and help control blood glucose levels. With the high protein and fibre content, pulses are therefore a good way to provide energy but at the same time lower the postprandial glucose concentrations, which may improve satiety and aid weight control.
RdB
