Soybeans are legumes and as such they share many of the health benefits with seeds (pulses) from other leguminous plants. In particular, soybeans have attracted attention because of their ability to modulate plasma lipoproteins favourably. This effect is not unique to soybeans however, as many studies attest to the plasma lipoprotein modifying effects of legumes. Substituting legumes for animal proteins does have favourable effects on plasma lipoproteins, and this has lead some to speculate that animal proteins are detrimental to plasma lipoprotein levels. However, a high meat intake is associated with poor dietary habits in the typical Western diet and so the inference that animal proteins are a bad heathy choice is disingenuous. That being said, many legumes do have unique effects of plasma lipoproteins and seem to be able to favourably improve lipoprotein profiles particularly in those with elevated baseline levels as might occur in those with the metabolic syndrome and abdominal obesity.
However, the plasma lipoprotein modulating effects of soybeans and other legumes are not fully understood. Studies have investigated the effects of soybean fibre on plasma lipoprotein levels and found that isolated fibre is not able to favourably modulate plasma concentrations in healthy humans (here). Researchers have also investigated the effects of isolated soybean protein on plasma lipoprotein levels in order to ascertain if it is the vegetable protein that is the cause of the beneficial effects of soybeans. For example in one study1, researchers fed healthy human subjects either soybean protein or casein protein for one month each. The diets of the subjects in both periods were identical with 30 % of calories coming from fat, 55 % from carbohydrate and 15 % as either soy protein or casein protein. The period of soybean protein consumption produced no changes to the plasma lipoprotein levels of the subjects except for a significant reduction in the plasma triglyceride levels of 3 of the 4 subjects with initially high levels.
These results suggest that the vegetable protein may not be the reason for the beneficial plasma lipoprotein modifying effects of soybeans. This therefore begs the question as to why soybeans are beneficial at lowering plasma lipoprotein levels if the isolated fibre and isolated protein does not have this effect? This question is difficult to answer, but based on other studies and the epidemiological data it may be that soybeans and other legumes are beneficial at lowering plasma lipoproteins when in their whole food form because they have beneficial glycaemic effects that improve blood sugar control. This may reduce nutrient overload on the liver and prevent the formation of fatty acids and triglycerides in the de novo lipogenesis pathway which in turn reverses the development of insulin resistance. The improved insulin sensitivity then lowers fasting plasma levels of glucose and this has beneficial weight loss effects. Whole soybeans might therefore be necessary to obtain the beneficial lipoprotein effects.
Dr Robert Barrington’s Nutritional Recommendation: Isolating plant food components is interesting from a research point of view, but often the isolated plant components do not have the expected metabolic effects. Whole soybeans and other whole legumes have beneficial metabolic effects because they contain a number of substances that have various effects, and it is the synergism between these components that is likely a requirement for their beneficial health effects. For this reason a high quality diet should be based on whole foods.
RdB
