The theory of dietary acid load has been controversial in nutrition, partly because of its association with alkaline diet marketing. But the underlying physiology is sound and the evidence is growing. The kidneys maintain blood pH within a narrow range of 7.35 to 7.45. When dietary acid load exceeds the kidneys' excretion capacity, the body uses mineral phosphates from bone and glutamine from muscle to buffer the excess acid. Over years, this buffering has measurable effects on bone mineral density and muscle mass.
Which Foods Are Acid-Forming
Acid-forming foods are those that produce a net acid load on the body when metabolised. Meat, fish, eggs, cheese, and grains are the primary acid-forming foods. Vegetables and fruits are alkaline-forming despite containing organic acids, because their mineral content produces a net alkaline ash when metabolised. A diet heavily dominated by meat, processed foods, and refined grains without adequate vegetables and fruit generates a higher net endogenous acid production than a diet with substantial vegetable and fruit intake.
The Evidence for Fruit and Vegetables in Bone Protection
Multiple prospective cohort studies have found that fruit and vegetable intake is positively associated with bone mineral density in older adults. A study of over 3,000 older adults found that those with the highest fruit and vegetable intake had significantly higher bone mineral density at the hip and lumbar spine. The alkaline-forming nature of plant foods is one of the proposed mechanisms, alongside their provision of calcium, magnesium, vitamin K1, and potassium.