Ask most recreational athletes whether a high-protein diet is bad for their bones and you'll hear confident warnings about acid load, calcium leaching, and "demineralisation." The concern sounds biochemically plausible. Protein catabolism generates ammonium and sulphate ions that require renal buffering. Neutralising acids requires calcium, which is abundant in bone. Therefore, eating more protein must be eroding your skeleton.
There is just one problem: the definitive meta-analysis on this question, published by Fenton et al. in 2011, found no evidence that dietary acid load causes bone loss — and that the hypothesis itself was based on a fundamental misreading of the calcium excretion data that appeared to support it.
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The acid-ash hypothesis originated in the 1960s and 1970s from observations that high-protein diets increased urinary calcium excretion. Sherman and Gershoff, and later Linkswiler, reported that as protein intake rose from 50 g/day to 150 g/day, urinary calcium output increased significantly. The logical inference: if calcium is appearing in the urine, it must be coming from somewhere, and the most plausible reservoir is bone.
The mechanism proposed was net endogenous acid production (NEAP): protein generates acid that the kidney buffers using calcium released from the skeleton. Measure the PRAL score (Potential Renal Acid Load) of a diet — which is high for meat, eggs, and cheese, and negative (alkaline) for most fruits and vegetables — and you supposedly predict bone resorption.
This became the basis for decades of dietary advice: eat less protein to protect your bones, eat more alkaline foods, avoid acid-forming diets. Some commercial programmes still sell "alkalising" supplements on this premise.
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Fenton et al. (2009, BMC Musculoskeletal Disorders) conducted a systematic review and meta-regression examining 28 studies of the NEAP-bone relationship. Their finding: the correlation between acid load and bone mineral density was statistically negligible. When controlled properly, greater dietary acid load did not predict greater bone loss, lower BMD, or fracture risk.
Their follow-up work (Fenton 2011, Journal of Bone and Mineral Research) addressed the calcium excretion paradox directly. The increased urinary calcium seen with high protein diets does not originate from bone resorption. It reflects two separate phenomena: (1) increased intestinal calcium absorption — protein stimulates calcium absorption in the duodenum via IGF-1 and calbindin upregulation, and (2) reduced tubular reabsorption — more calcium filtered and excreted in the final urine pool. Net skeletal calcium balance remains neutral or positive.
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The subsequent literature has consistently supported this reversal. Bonjour (2011, British Journal of Nutrition) reviewed 14 cohort studies and found that higher protein intake was associated with greater bone mineral density across all age groups, not lower. The proposed mechanism is direct: amino acids — particularly leucine — stimulate osteoblast activity and collagen cross-link formation through mTORC1 and IGF-1 signalling pathways. Bone is not merely a calcium reservoir; its collagen matrix requires continuous protein substrate for remodelling, and that supply diminishes with low protein intake.
For athletes specifically, the implications are opposite to what the alkaline diet myth would suggest. Athletes with high training loads experience elevated bone turnover — both formation and resorption — driven by mechanical loading stimuli. They require adequate protein to support osteoblast collagen synthesis alongside calcium for mineralisation. In female athletes at risk for low energy availability and RED-S, protein restriction compounds the bone density risk rather than mitigating it.
The only population where caution around protein-associated calcium excretion remains clinically relevant is individuals with pre-existing hypercalciuria or calcium oxalate kidney stones, where the increased urinary calcium filtration from high protein intake may potentiate stone formation. In otherwise healthy athletes, this mechanism does not apply.
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The practical summary: eat adequate protein to support muscle protein synthesis, osteoblast activity, and connective tissue remodelling. Prioritise calcium from dairy, fortified plant milks, or leafy greens alongside protein, not instead of it. The alkalising supplement industry exists to solve a problem that, in healthy athletes, is not real.
For athletes optimising protein targets across training blocks — and ensuring intake supports both muscle and skeletal remodelling simultaneously — the free calculator at winsport.uk/tools/nutrition/protein-calculator estimates your optimal daily protein target based on body weight, training frequency, and goal, grounded in current evidence rather than acid-ash mythology.
Has anyone in your training or clinical environment recommended a lower-protein "alkaline" diet for bone health — and how did you respond to that advice?