Sunday, January 15, 2012

DGA 2011 - Fruit, Vegetables and Legumes

2.2 Plenty of vegetables, including different types and colours, and legumes/beans, and eat fruit

Mature Legumes

“As a group, these foods are nutrient dense, relatively low in energy (kilojoules) and are good sources of minerals and vitamins (such as magnesium, vitamin C and folate), dietary fibre and a range of phytochemicals including carotenoids.”

Vegetables, fruit and legumes (beans are legumes) have been grouped together in the dietary guidelines because they are said to share many of the nutrients and other compounds listed above.  This is the case with immature legumes (green beans, snow peas, and green peas), but not true of mature legumes (black beans, chickpeas, kidney beans, lentils, peanuts and soy beans).  This can be illustrated by following graphs, plotted from data in [1].

The values on the x-axis are 23 different essential micronutrients.  The y-axis value is the median value of a food group based on the percentage of the NRV amount of a given nutrient in 8700 kJ worth of a food, divided by the NRV (for men) for the nutrient.  Nutrient dense foods should have values greater than 100 in most nutrients.

1. Calcium
2. Iron
3. Magnesium
4. Phosphorus
5. Potassium
6. Sodium
7. Zinc
8. Copper
9. Manganese
10. Selenium
11. Vitamin C
12. Thiamine
13. Riboflavin
14. Niacin
15. Vitamin B5
16. Vitamin B6
17. Folate
18. Choline
19. Vitamin B12
20. Vitamin A
21. Vitamin E
22. Vitamin D
23. Vitamin K1



Non-starchy vegetables and immature legumes are very similar nutritionally.  Both food groups are low in energy density and are in many nutrients.  For the most part they have roughly similar quantities of each nutrient.  This may be because when we eat immature legumes we eat more of the plant (vegetable) and less of the seed.  Immature legumes definitely belong in this category alongside of vegetables.


Mature legumes on the other hand are among the most energy dense foods and low in nutrient density relative to calories.  Many nutrients fall below the critical 100 line for mature legumes.  Unlike non-starchy vegetables and immature legumes, mature legumes don’t contain adequate calcium, riboflavin, niacin, pantothenic acid, choline, and vitamins A, C, E and K1.  Seeing as mature legumes are only adequate sources in 3 of the 13 vitamins, whereas non-starchy vegetables and immature legumes are adequate in 11 of the 13, it is misleading to describe mature legumes as being a good source of vitamins (sugary fruits are adequate sources in 7 of the 13 vitamins).  This is because when we eat mature legumes we are eating the seed and seeds foods (grains, mature legumes, nuts and seeds) tend to have high energy density, low energy density and deficits in the vitamins listed above.  Many minerals in seed foods have reduced bioavailability due to a number of antinutrients such as phytic acid.  This was discussed in the previous dietary guidelines.

“…non-haem iron absorption is inhibited by phytates, polyphenols (for example, tannins) and calcium.” [2]

“Zinc found in animal products, crustaceans and molluscs is more readily absorbed than zinc found in plant foods. In contrast, legumes and unrefined cereals contain phytates that reduce zinc absorption” [2]

The previous dietary guidelines showed that Australians on average, particularly women, had low to marginal intakes of iron and zinc (the minerals most affected by phytic acid) and some had deficiencies in these nutrients [2].  Despite this, the current dietary guidelines recommend consumption of phytate rich foods.

Phytic acid alone strongly chelates calcium [3], iron [4], zinc [4], magnesium [5], is a matrix of phosphorus (representing ~80% of the total phosphorus) [6], prevents absorption of all those minerals and blocks the digestive enzymes pepsin, amylase and trypsin [7].  The combination of phytic acid reducing the absorption and retention of nitrogen (protein), calcium and phosphorus results in weaker bones [3].  High fibre diets prevent vitamin D reabsorption [8], it’s possible that fibre is a proxy for phytic acid, which is responsible for this effect too.

All plants have a number of antinutrients and other toxics but seed foods such as matured legumes tend to have more and what they have tend to be more potent.  The lectins in mature legumes, such as phytohaemagglutinin in the kidney bean family, soybean lectin and peanut lectin, influence the intestine structure and function negatively, leading to intestinal permeability, autoimmunity and poor nutrient absorption (other potent lectins include wheat germ agglutinin and corn lectin) [9].  Canavanine is an amino acid found in some legumes that is structurally similar to arginine and induces toxicity when incorporated into proteins in place of arginine.  Saponins are found in some legumes (also nightshades) and bind to the intestine wall, increasing intestinal permeability and poor nutrient absorption, especially fats, fat soluble vitamins and cholesterol.  (Saponins are revered for their cholesterol lowering effect, but they do more harm than good).  Cyanogenic glycosides are found in some legumes (also some root vegetables and other seed foods) and are a storage form of cyanide, which can be metabolised into hydrogen cyanide, which is toxic at low doses and can cause dysfunction in the central nervous system, respiratory failure and cardiac arrest.  Tannins are found in legumes (also the skins of some fruit) and increase intestinal permeability, inhibit trypsin and amylase and reduce the absorption of iron and vitamin B12 [10].  When plants don’t want their seeds to be eaten, to ensure reproductive success, we should expect a number of toxic antipredation mechanisms.

More evidence that matured legumes shouldn’t be in this category comes from the lack of health benefits associated with them.  The guidelines have cited a number of studies that have found fruit and vegetables tend to promote health and are associated with better health.  The research concerning legumes only amounts to soy reducing LDL cholesterol and a possible association with less colorectal cancer.

“Recent evidence suggests that consumption of soy foods is associated with reduced total cholesterol and LDL cholesterol levels, as markers for coronary heart disease risk (Grade C, Section 7.4 in Evidence Report [14]) [180].”

Just because soy reduces LDL-C doesn’t necessarily mean it would reduce coronary heart disease.  If a study found such a result, it would have been included.

“Evidence suggests that consuming legumes is associated with reduced risk of colorectal cancer (Grade C, Section 7.3 in Evidence Report [14]) [184-188]. However, in one study the effect was only significant for women [187], as also seen in the recent analysis of the European Prospective Investigation into Cancer and Nutrition (EPIC) database [189]. However no evidence of an association between consumption of legumes and colorectal cancer was described in the WCRF report [42].”

The association between legumes and lower rates of colorectal cancer doesn’t appear very strong.  In the event that legumes do reduce colorectal cancer, this would likely be from soluble fibre being fermented by bacteria, which then release butyric acid and butyric acid nourishes the epithelial cells of the colon.  If this is the case soluble fibre can also be sourced from fruit and vegetables, both of which have a greater nutrient density and more positive health outcomes.

Some of the mechanisms used to explain the good health outcomes of fruit, vegetables and legumes do not apply to mature legumes.  The components listed include:-

Vitamins (especially vitamins A, C, E and folate): Mature legumes are mediocre sources of many vitamins and poor sources of, vitamin A, C and E [1].

Minerals (especially potassium and magnesium): Mature legumes are good sources of potassium, manganese and magnesium though whether they are a good source of other minerals, such as phosphorus and zinc, is questionable due to the high concentrations of phytic acid

Carotenoids: Mature legumes are poor sources of carotenoids such as vitamin A precursors, lycopene, lutein and zeaxanthin [1].

Bioflavonoids (anthocyanins and flavonols): Anthocyanins are used in nature as an antioxidant during photosynthesis and also fruits use them to attract pollinators.  Seeds don’t undergo photosynthesis nor do they encourage predation (quite the opposite), so it’s unlikely that as seeds, for mature legumes to be rich in anthocyanins.

Dietary fibre and resistant starch: Mature legumes are source of dietary fibre and resistant starch.  Yet overall legumes do not seem to reduce colorectal cancer.

Low energy density: This doesn’t apply to mature legumes as they tend to have an energy density of 1350-1550 kJ per 100 grams, which is among the highest of all whole food groups.  Soybeans and peanuts have an even higher energy density at 1,866 and 2,374 kJ per 100 grams respectively [1].

Low in sodium: All whole foods except dairy, eggs, shellfish and offal are low in sodium [1].  This is not a unique quality.

Low in saturated fat: Saturated fat is not associated with cardiovascular disease [11].  A study cited in the guidelines found that fats improved the TC:HDL ratio relative to carbohydrates [12].  Mature legumes are high in carbohydrates and replacing vegetables with mature legumes will increase the carbohydrate content of the diet.  So consumption of mature legumes will likely result in an increased TC:HDL ratio, triglycerides and a higher proportion of atherogenic small, dense LDL particles.  Carbohydrate intake is also associated with atherosclerosis [13].

Phytoestrogens and isoflavones (see below):
 
Isoflavones

“These reviews suggest the isoflavone in soy foods may have a role in cholesterol reduction, improved vascular health, preservation of bone-mineral density [209] and anti-oestrogenic, anti-proliferative, pro-apoptotic, anti-oxidative and anti-inflammatory processes [210].”

Both studies report that isoflavones in soy:

“…have stimulatory effects in low-estrogen environments, and that in high-estrogen environments, they block the effects of estrogen.”

The guidelines only mention the anti-oestrogenic effect, but not the pro-oestrogenic effect in low-oestrogen environments.  As a male I would be cautious to eat soy foods because one low-oestrogen environment is the male body.  The isoflavones in the soybeans are antipredation mechanisms, designed to make the males of the species sterile [14] and succeed in making a variety of animals sterile [15].  Not only are we bombarded by xenoestrogens (substances that have estrogenic activity) in the environment such as bisphenol A (plastics) [16], but the dietary guidelines recommend an even greater oestrogen load for men.

Men who consumed 2 serves of soy food had 41 million sperm/ml less than men who did not consume soy foods [17].  56g of soy protein decreased testosterone in men by 19% after 28 days [18].  Female monkeys on soy protein had a lower adrenal weight suggestive of androgen deficiency and hypercortisolemia [19].  All isoflavones are associated with breast cancer in women [20].  Soy isoflavones are goitrogenic, extra iodine is needed to compensate [21].  Early exposure to genistein (an isoflavones in soy) reduces testosterone long term in male rats, which may overarouse the immune system, increasing the risk of autoimmune diseases [22].  Testosterone:oestriadol was 13% lower and the free androgen index was 7% lower in the tofu group than the lean meat group.  These factors can increase the risk of prostate cancer [23].  Bioflavanoids such as genistein cleave DNA strands resulting in childhood leukimia [24].

That being said, all foods seem to have some isoflavones, but it is the concentration in soy foods, roughly 1,000 times greater, that is responsible for those effects and makes me cautious.  Surely those studies and that number is concerning.  See the table below, sourced from data in [25].

Food
Total isoflavones (mg/100g)
Fruit
<0.10
Vegetables
<0.10
Non-soy breakfast cereals
<0.10
Nuts (except pistachio nuts)
<0.10
Extra virgin olive oil
0.04
Whole eggs
0.05
Other legumes
<1.00
Pistachio nuts
3.63
Soybeans (depending on preparation)
12.50-48.95
Red clover
21.00
Natto
82.29

Before soy products and isoflavones are recommended to the public the science should be conclusive that these ‘foods’ will cause no health problems.  This investigation should be carried out by independent scientists who do not have a vested commercial or political interest in the result.  You wouldn’t recommend that men should take oestrogen tablets, so you shouldn’t recommend soy and isoflavones either with the current evidence.

Conclusion

Fruit and vegetables are widely regarded to be healthy as they are nutrient dense foods.  The inclusion of mature legumes into this category is somewhat misplaced.  Immature legumes are very nutritionally similar to non-starchy vegetables and should be included, but it’s likely that most Australians consider these foods – green beans, snow peas and green peas – to be vegetables anyway.  The legumes that don’t belong in this category are the mature ones.  Mature legumes are energy dense, low in nutrients and have little to no health benefits to speak of.  Mature legumes (especially soy) also contain many potent and toxic antinutrients.  One of the antinutrients is the isoflavones in soy (and clover).  Isoflavones have pro-estrogenic effects in low oestrogen environments such as men.  They cause sex hormone imbalances, thyroid suppression and may cause certain cancers.  All plant foods have antinutrients and other toxins, it is just that mature legumes have more and what they do have is more potent.  The recommendation to eat plenty of mature legumes will displace the more nutrient dense foods in the diet such as vegetables and immature legumes.

Instead of recommending: eat plenty of vegetables, including different types and colours, and legumes/beans, and eat fruit.  What should be recommended instead is: to eat plenty of vegetables, including different types and colours, and eat fruit.

2 comments:

  1. Steven, is this sentence correct? -

    "This is because when we eat mature legumes we are eating the seed and seeds foods (grains, mature legumes, nuts and seeds) tend to have high energy density, low energy density and deficits in the vitamins listed above".

    Just under the mature legume chart - "high energy density, low energy density"?

    Other than that it is very good, cheers Peter

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    Replies
    1. That should be 'low nutrient density'. Thanks Peter.

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