Nutrition
Alkaline diet and osteoporosis: acid load, pH and bone health
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MPH, MSc
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Although diet can modify the acid load of the diet, urinary pH, and some aspects of mineral metabolism, the body maintains blood pH within very narrow limits. We review which mechanisms are physiologically plausible and what the evidence actually shows regarding acidic diets, alkaline diets, and bone health.
ALKALINE DIET
The existence of an "alkaline diet" that can improve bone health is a controversial issue (Frasseto et al., 2018). Diet does not substantially modify blood pH in a person with normal renal and respiratory function, although it can modify the net production of endogenous acid and the amount of acid that the kidneys must eliminate. An acidifying diet can be seen as one whose metabolism generates a greater net production of non-volatile acids that must be neutralized and eliminated, primarily by the kidneys. This acid load arises primarily from three processes: the net release of protons from organic phosphate compounds, the oxidation of sulfur-containing amino acids to sulfates, and the endogenous formation of incompletely metabolized organic acids. In contrast, a more alkaline diet, rich in fruits and vegetables, for example, provides organic anions such as citrate and malate, which, when metabolized, generate bicarbonate and reduce the net acid load. This primarily modifies renal acid excretion, which can lead to changes in urinary pH. However, blood pH remains tightly regulated.ACID-BASE BALANCE AND pH
Acid-base balance is essential for overall organ function, which is why blood pH is always within a very narrow range, approximately 7.35-7.45. The body produces acids, especially from the metabolism of proteins, sulfur-containing amino acids, and other normal cellular processes. The kidneys are responsible for eliminating this acid load and regenerating bicarbonate, helping to maintain the physiological range of blood pH. If the kidneys fail and a certain degree of true metabolic acidosis develops, H+ ions are buffered by mobilizing carbonate, phosphate, and calcium from bone mineral. This acidosis can promote protein catabolism, loss of muscle mass, bone loss, inflammation, and general metabolic deterioration. It has been theorized that there is an acidic diet, high in protein, phosphates, and other acidic dietary elements typical of the Western diet, that would negatively impact bone health, and another diet with alkaline components provided by vegetables that would reduce this effect. However, these mechanisms associated with a state of metabolic acidosis do not correspond to the small variations produced by a typical diet in a healthy person, since the pH is maintained within a very narrow range.ACIDITY AND CALCIUM LOSSES
On the other hand, the existence of acute urinary calcium losses associated with diets high in protein, phosphorus, acid load, or other dietary changes has been taken as evidence. However, acute calcium losses do not necessarily imply a loss of calcium from bone. Calciuria also depends on intestinal absorption, acute changes in renal filtration and reabsorption, and acute changes in hormones such as PTH, vitamin D, sodium, and other minerals. Therefore, increased urinary calcium excretion and subsequent retention can occur without implying a negative body balance. In fact, diets higher in protein or phosphorus can increase calciuria while simultaneously increasing intestinal calcium absorption, and epidemiologically, they may even be associated with greater bone density than diets lacking these elements that do not produce such acute calcium losses. Protein, phosphorus, and sodium can increase certain components of acid load or calciuria, but this does not automatically equate to bone loss: protein provides amino acids for the matrix and muscle, increases IGF-1, and can promote calcium absorption. Phosphorus, for its part, is essential for hydroxyapatite formation, and the sodium-induced increase in calciuria can activate compensatory responses in calcium metabolism, including, in some studies, increases in PTH, 1,25(OH)2D, and intestinal calcium absorption, although the magnitude and sufficiency of this compensation depend on the context, especially calcium intake. The final effect, therefore, depends on the mineral and energy context, particularly on the availability of sufficient calcium. With low calcium intake, a high sodium or protein load can make it more difficult to maintain calcium balance, while with sufficient calcium, this same exposure can be neutral or even coexist with improved bone mineral density. Therefore, acute changes in urinary pH, calciuria, or resorption markers should not be interpreted in isolation as bone deterioration, because the body adjusts intestinal absorption, PTH, vitamin D, renal function, and remodeling through various mechanisms.FOOD ACIDITY AND ACID LOAD
There are other points of confusion. The pH of a food does not determine its metabolic acid load. A lemon is acidic in the mouth and stomach, but its organic anions can be metabolized to generate bicarbonate. This is why fruits and vegetables often contribute to a lower renal acid load. Conversely, other foods can generate acid after metabolism, especially through the metabolism of sulfur-containing amino acids and certain phosphorus compounds. Acid load is the amount of non-volatile acids that the body generates from metabolism. Foods rich in animal protein, some grains, and certain Western dietary patterns tend to generate a higher net acid load, while fruits and vegetables provide alkaline precursors. Animal proteins, especially due to their content of sulfur-containing amino acids such as methionine and cysteine, tend to increase net acid production. Some cereals also contribute to the acid load. In contrast, fruits and vegetables contain organic anions such as citrate and malate, which, when metabolized, can generate bicarbonate or alkaline equivalents.DOES SUBCLINICAL ACIDOSIS EXIST?
Having shown the main weaknesses of the model, however, subclinical acidosis may be more significant with aging, which tends to acidify the blood to some extent, or with some degree of renal impairment, when the capacity to eliminate the acid load decreases, without overt acidosis being present. With age, there may be a slight tendency toward a higher systemic acid load, with an increase in H+ and a decrease in plasma bicarbonate. This has been interpreted as a very low-grade chronic metabolic acidosis, probably related in part to the reduced renal capacity to excrete acid (Frassetto, L., & Sebastian, A. 1996). On the other hand, studies in people with stage 1-3 CKD show that H+ retention tends to increase progressively as glomerular filtration rate decreases, before overt metabolic acidosis develops. Serum bicarbonate may thus be a relatively late and imperfect marker of the entire phenomenon. Another question is whether subclinical acidosis can exist even with healthy kidneys. Furthermore, relatively low bicarbonate concentrations, even within near-normal ranges, have been associated with lower bone mineral density.BONE AS A BUFFER
Bone also contains significant amounts of carbonate. In response to a proton load, the dissolution of minerals that releases bases and minerals can contribute to buffering. If this process were small but sustained over years, it could have cumulative importance. The osteoclast is specifically designed to work in an acidic environment: it generates H⁺ itself to perform its function of dissolving hydroxyapatite. Osteoclasts increased their resorptive activity approximately 6.7-fold when the pH changed from 7.4 to 7.0, with rapid changes in morphology and gene expression (Davies et al., 2025). This shows that pH is a regulatory signal for osteoclasts, although this experimental pH represents a much larger experimental change than what might occur in subclinical acid retention. However, osteoblasts and osteoclasts have mechanisms capable of detecting H+ as a biological signal, so, theoretically, it is plausible that a diet can increase the net production of endogenous acid and increase resorptive activity without producing clinical acidosis. For example, in a metabolic assay, modifying the acid-base load of the diet changed urinary pH by approximately 1 unit, but blood pH only changed by 0.014 (Buclin et al., 2001). This shows that small shifts in bicarbonate or pH can occur, but not systemic acidosis, given the kidney's acid excretion system into the urine and the activation of organic buffering systems.ACIDIFICATION OF LOCAL TISSUE MICROENVIRONMENT
On the other hand, blood pH can be perfectly normal while certain extracellular microenvironments have a lower pH. Tissues continuously produce CO2, lactate, and protons. In bone, this is especially important because the osteoclast already works physiologically by creating an extremely acidic microenvironment under its rough border to dissolve hydroxyapatite. Blood pH doesn't tell us exactly what the interstitial pH is in each tissue. There aren't many dietary experiments that show consistent evidence of this type of acidification, not even localized acidification. Some controlled studies with 2 g/kg/day of protein don't show systemic acidosis, and other studies with increased protein intake haven't shown decreases in skeletal muscle pH with muscle microdialysis (Boschmann et al., 2020). In fact, in human muscle, much greater variations in interstitial pH have been measured than the simultaneous variations in venous pH during exercise. And exercise and its associated acidification are linked to good overall health, even in people who exercise intensively throughout their lives. A diet high in protein and with a high potential renal acid load can increase urinary acidity and calciuria without increasing bone resorption. There is also no epidemiological relationship that consistently shows that protein consumption is associated with poorer bone health. Additionally, protein increases intestinal absorption of calcium and IGF-1.EPIDEMIOLOGY AND INTERVENTION STUDIES
At the epidemiological level, systematic reviews of observational studies tend to show an increased risk of fracture with higher dietary acid loads, although generally small (Mirzababaei et al., 2025). However, vegan populations tend to show lower bone density and a higher prevalence of fractures despite lower protein intake and consumption of acidifying foods (Ogilvie et al., 2022). Another interesting line of evidence is provided by studies that have directly attempted to produce greater alkalization through supplementation. For example, some studies with potassium bicarbonate show a reduction in urinary calcium excretion and some resorption markers in older adults, although there is insufficient evidence for hard outcomes to show that the intervention translates into a final reduction in fractures. Other relevant, longer-term clinical trials have not found a clear benefit. One relevant controlled clinical trial with 276 women over 2 years found no differences in CTX, P1NP, or lumbar or hip bone density (Macdonald et al., 2008). A 2026 meta-analysis shows no consistent effects on resorption markers, with no other relevant outcomes (AlSejari et al., 2026).SUBCLINICAL ACIDIFICATION AS A PHYSIOPATHOLOGICAL PROCESS
On the other hand, subclinical acidification can also exist in a state of inflammatory catabolism due to poor energy-protein intake and metabolic conditions associated with poorer health. That is, diet can contribute to the net acid load, but the development of subclinical acidosis also depends on the body's ability to manage it. Therefore, age and metabolic and renal health could represent a greater "acid load" than diet itself. With age, renal acid excretion, available bicarbonate, and the functional reserve of tissues such as muscle and bone can decrease, while chronic diseases, inflammation, and metabolic disorders can increase endogenous acid production or reduce its compensation. Additionally, low protein intake in older adults poses a greater problem for general health, muscle mass, functionality, and the risk of falls and fractures than the role that some subclinical acidification might play.FINAL COMMENTS
In summary, the hypothesis of an "alkaline diet," understood in absolute terms as a significant modification of blood pH, is a difficult interpretation to sustain in individuals with normal acid-base regulation. However, dietary acid load can indeed modify kidney function, acid excretion, and certain parameters of mineral metabolism, and this effect could become more significant when the physiological capacity to compensate for it decreases, as occurs with aging or kidney disease. A diet rich in plant-based foods is reasonable for multiple reasons, including a lower acid load, but without sacrificing sufficient protein, energy, or mineral intake, which could pose a greater risk to bone health than reducing bone health to a mere attempt at alkalization.
#alkalinediet #pH #osteoporosis
References:
AlSejari, N., Alkhulaifi, N. S., Alenezi, R. S., Alharran, A. M., & Alsubaiei, A. A. (2026). Effects of potassium citrate supplementation on bone turnover markers in postmenopausal women: A GRADE-assessed systematic review, meta-analysis and trial sequential analysis. Calcified Tissue International, 117(1), 111. https://doi.org/10.1007/s00223-026-01569-9
Boschmann, M., Kaiser, N., Klasen, A., Klug, L., Mähler, A., Michalsen, A., Vormann, J., Werner, T., & Stange, R. (2020). Effects of dietary protein-load and alkaline supplementation on acid-base balance and glucose metabolism in healthy elderly. European Journal of Clinical Nutrition, 74(Suppl 1), 48-56. https://doi.org/10.1038/s41430-020-0695-3
Buclin, T., Cosma, M., Appenzeller, M., Jacquet, A. F., Décosterd, L. A., Biollaz, J., & Burckhardt, P. (2001). Diet acids and alkalis influence calcium retention in bone. Osteoporosis international : a journal established as result of cooperation between the European Foundation for Osteoporosis and the National Osteoporosis Foundation of the USA, 12(6), 493-499. https://doi.org/10.1007/s001980170095
Davies, B. K., Skelton, A. J., Hopkinson, M., Lumb, S., Holdsworth, G., Arnett, T. R., & Orriss, I. R. (2025). Extracellular pH is a critical regulator of osteoclast fusion, size and activation. Bone, 195, 117466. https://doi.org/10.1016/j.bone.2025.117466
Frassetto, L., & Sebastian, A. (1996). Age and systemic acid-base equilibrium: analysis of published data. The journals of gerontology. Series A, Biological sciences and medical sciences, 51(1), B91-B99. https://doi.org/10.1093/gerona/51a.1.b91
Frassetto, L., Banerjee, T., Powe, N., & Sebastian, A. (2018). Acid Balance, Dietary Acid Load, and Bone Effects-A Controversial Subject. Nutrients, 10(4), 517. https://doi.org/10.3390/nu10040517
Macdonald, H. M., Black, A. J., Aucott, L., Duthie, G., Duthie, S., Sandison, R., Hardcastle, A. C., Lanham-New, S. A., Fraser, W. D., & Reid, D. M. (2008). Effect of potassium citrate supplementation or increased fruit and vegetable intake on bone metabolism in healthy postmenopausal women: A randomized controlled trial. The American Journal of Clinical Nutrition, 88(2), 465-474. https://doi.org/10.1093/ajcn/88.2.465
Mirzababaei, A., Daneshvar, M., Basirat, V., Asbaghi, O., & Daneshzad, E. (2025). Association between dietary acid load and risk of osteoporotic fractures in adults: a systematic review and meta-analysis of observational studies. BMC musculoskeletal disorders, 26(1), 266. https://doi.org/10.1186/s12891-025-08495-1
Ogilvie, A. R., McGuire, B. D., Meng, L., & Shapses, S. A. (2022). Fracture Risk in Vegetarians and Vegans: the Role of Diet and Metabolic Factors. Current osteoporosis reports, 20(6), 442-452. https://doi.org/10.1007/s11914-022-00754-7
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