Fruit and Vegetables and Bone Health

Susan A. New, BA, MSc, PhD

Disclosures

September 05, 2003

Question

What is the link between fruit and vegetable intake and bone health?

Response From the Expert

Susan A. New, BA, MSc, PhD, RPHNutr
Center for Nutrition & Food Safety, School of Biomedical & Life Sciences, University of Surrey, Guildford, Surrey, United Kingdom

 

The health-related benefit of a high consumption of fruit and vegetables on a variety of diseases has been gaining increasing prominence in the literature over a number of years. A number of observational, experimental, clinical, and intervention studies over the past decade have suggested a positive link between fruit and vegetable consumption and the skeleton.

Acid-base homeostasis is critical to health, and it is well documented that extracellular fluid pH remains between 7.35 and 7.45. A major requirement therefore of our metabolic system is to ensure that hydrogen ion concentrations are maintained between 0.035 and 0.045 mEq/L.[1] The body's adaptive response involves 3 specific mechanisms: (1) buffer systems; (2) exhalation of CO2; and (3) kidney excretion. On a daily basis, humans eat substances that both generate and consume protons and, as a net result, adult humans on a normal Western diet generate approximately 1 mEq per kg body weight of acid per day. Of course, the more acid precursors a diet contains, the greater the degree of systemic acidity.[2] We know that as humans become older, their overall renal function declines, which includes their ability to excrete acid.[3] Hence, with increasing age, humans become slightly but significantly more acidic.[4]

Three key points should be considered when examining the possible link between acid-base homeostasis and the skeleton:

 

  1. The theoretical considerations of the role that alkaline bone mineral may play in the defense against acidosis date back as far as the late 1880s/early 1900s, with the fundamental concepts being established in the late 1960s/early 1970s. A number of studies published during this period provided evidence that in natural (eg, starvation), pathological (eg, diabetic acidosis), and experimental (eg, ammonium chloride ingestion) states of acid loading and acidosis an association exists with both hypercalciuria and negative calcium balance.[5,6]

  2. There are clear mechanisms for a deleterious effect of acid on bone. A direct enhancement of osteoclastic activity has been demonstrated following a reduction in extracellular pH. This effect was shown to be independent of the influence of parathyroid hormone (PTH).[7] Furthermore, osteoclasts and osteoblasts appear to respond independently to small changes in pH in the culture media in which they are growing. There is evidence that a small drop in pH, close to the physiologic range, causes a tremendous burst in bone resorption.[8,9]

  3. The effect of dietary acidity on the skeleton needs only to be relatively small for there to be a large impact over time. When one considers the extent of loss, if 2 mEq/kg per body weight of Ca per day is required to buffer about 1 mEq/kg per body weight of fixed acid per day, over 10 years (and assuming a total body Ca of approximately 1 kg), this would account for 15% loss of inorganic bone mass in an average individual.[10]

A variety of population-based studies published in the latter part of the 1900s, and more recently between 2001 and 2003, have demonstrated a beneficial effect of fruit and vegetable/potassium intake on indices of bone health in young boys and girls, premenopausal women, perimenopausal women, postmenopausal women, and elderly men and women.[11]

From a clinical point of view, the study by Sebastian and colleagues[12] in 1994, which demonstrated that potassium bicarbonate administration resulted in a decrease in urinary calcium and phosphorus, with overall calcium balance becoming less negative (or more positive) is a very important study in the investigation as to whether alkali is important for bone health. Changes were also seen in markers of bone metabolism, with a reduction in urinary excretion of hydroxyproline (bone resorption) and an increased excretion of serum osteocalcin (bone formation). Longer-term studies looking at alkali administration on indices of bone health throughout the lifecycle are now urgently required.

Further support for a positive link between fruit and vegetable intake and bone health can be found in the results of the DASH (Dietary Approaches to Stopping Hypertension) and DASH-Sodium intervention trials. In DASH, diets rich in fruit and vegetables were associated with a significant fall in blood pressure compared with baseline measurements. However, of particular interest to the bone field were findings that increasing fruit and vegetable intake from 3.6 to 9.5 daily servings decreased the urinary calcium excretion from 157 mm/d to 110 mg/d.[13] The authors suggested that this was due to the "high fibre content of the diet possibly impeding calcium absorption." However, a more likely explanation put forward by Barzel[14] was a reduction in the "acid load" with the fruit and vegetable diet compared with the control diet. This study is the first population-based fruit and vegetable intervention trial showing a positive effect on calcium economy (albeit a secondary finding).

More recently, Lin and colleagues[15] reported the findings of the DASH-Sodium trial. The impact of 2 dietary patterns on indices of bone metabolism were examined. The DASH diet emphasises fruits, vegetables, and low-fat dairy products and is reduced in red meats. In this second DASH II trial, 3 levels of sodium intake were investigated (50, 100, and 150 mmol/L). Subjects consumed the control diet at the 150-mmol/L sodium intake/d levels for 2 weeks and were then randomly assigned to eat either the DASH diet or the control diet at all 3 sodium levels for a further 4 weeks in random order. The DASH diet, compared with the control diet, was found to significantly reduce bone turnover -- both bone formation (by measurement of the marker osteocalcin) by 8% to 10% and bone resorption (by measurement of the marker CTx) by 16% to 18% (personal communication, Dr F. Ginty). This is an important intervention study that shows a clear benefit of the high intake of fruit and vegetables on markers of bone metabolism.

In summing up the evidence currently available, there is a positive link between alkali administration (either in the form of clinical studies using potassium bicarbonate or potassium citrate, or observational/experimental studies looking at dietary potassium intake or fruit and vegetable consumption) and markers of bone health. Research is now required to determine the long-term clinical impact of alkalinity on bone health and fracture risk as well as clarification of the exact mechanisms involved with respect to this diet on skeletal protection.

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