N° 21 | March 2008

Impact of dietary alkalinization on skeleton : bone sparing effects of cations and anions from fruits and vegetables

The nutritional prevention of bone disorders (ricketts, osteopenia and osteoporis) has long focused on the provision of calcium and related factors such as vitamine D. During the last decade, there was an increasing awareness that the western diet leads to profound alterations of the nutritional value of the diet: higher energy density in general with a lowered density of minerals (except sodium), vitamins and various other micronutriments. It is also well-recognized that modern dieting leads to an almost permanent situation of latent metabolic acidosis, resulting in mobilization of bone minerals (chiefly Ca) and of muscle amino acids for glutamine production, as well as renal disturbances. However, the above processes will be limited if foods provide alkalinizing compounds and there is a growing interest for the ‘alkalinizing functionality’ of foods which are sources of KHCO3 precursors.

The alkalinizing functionality, an almost exclusive feature of fruits and vegetables, is based on their ionic composition: (i) a cationic profile in which potassium prevails, together with some magnesium and calcium; (ii) an anionic profile in which polycarboxylic anions prevail, especially citrate and malate; (iii) a relative paucity of inorganic anions such as Cl, PO4 or SO4. With the exception of legumes and cereal products, most plant foods contain substantial amounts of organic anions, with levels varying from around 100 up to 4000 mg/100 g fresh weight for the richest products such as Citrus fruits. Generally, organic anion concentrations are consistently greater in fruits than in vegetables: a comparison of panels of fruits or of vegetables usually consumed indicates that average levels of (citrate + malate) will be around 1400 mg/100 g for fresh fruits and only 300 mg/100 g for fresh vegetables.

The potassium content of fruits is in the range of 100 to 400 mg/100 g fresh weight and that of vegetables is generally higher, exceeding 600 mg/100 g fresh weight in some cases. In fact, one of the most salient points distinguishing fruits from vegetables is the [K]/(organic anions) ratio (in mEq): this ratio is generally less than 0.5 for most fruits but is greater than 1 for vegetables (up to 2.3 for pumpkins). Basically, from the point of view of alkalinizing functionality, it appears that potassium may be limiting in fruits whereas the [K]/(organic) anions ratio is relatively well-balanced in vegetables.

The daily supply of organic anions is closely connected to intake of fruits and vegetables. Data in this domain are still scarce, but calculations using composition tables and food intake data suggest that organic anion intake may be in the range of 1-2 g/d in low plant food consumers, and around 3-4 g/d in well-diversified omnivores. It must be noted that these values are close to those observed for potassium, which reflects to a certain extent the fact that ingested organic anions are chiefly present as potassium salts in foods.

Epidemiological evidence of a favourable effect of fruit and vegetable consumption on bone status is relatively recent, and they used with different criteria (fractures incidence, bone mineral density or markers of bone cells activity) and populations (older people at risk of osteopenia/osteoporosis, younger people for bone peak mass). Nevertheless, a survey of the 1995-2007 epidemiological studies published in this domain (New et al 2004, updated to 2007) support the view that a substantial fruit and vegetable intake (in the ‘5-10/d’ range) may exert a protective effect against the risk of bone alterations. Potassium has been identified as a protective factor in most of them and this likely reflects the alkalinizing functionality of fruits and vegetables due to organic anions in potassium salts. There is little doubt that the observed effects are multi-factorial and that some other compounds are also protective for bone in fruits and vegetables, such as vitamins C or K, as well as various phytonutrients (possibly through antioxidant or pseudo-hormonal effects). In turn, potassium may also have other protective effects, for example against cardiovascular diseases (He & McGregor, 2003).

He FJ, McGregor GA (2003) Potassium: more beneficial effects. Climacteric 6: 36-48.
New SA (2004) Do vegetarians have a normal bone mass? 15: 679-688.
Demigné C, Sabboh H, Rémésy C, Meneton P (2004) Protective effects of high dietary potassium: nutritional and metabolic aspects. J Nutr 134:
2903-2906.

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