The idea that certain foods can alter the acidity or alkalinity of your body, and this can then go on to cause or treat all sorts of health problems is a popular concept in alternative nutrition circles. To see if there is any scientific evidence to support following an ‘alkaline diet’, Master of Human Nutrition student Melissa Ferguson digs deeper.
Alkaline diet principles
At first glance, alkaline diets have similarities to the Australian Dietary Guidelines. They recommend eating lots of vegetables, fruits, seeds and nuts, exercising daily and avoiding or limiting fats, sugars, refined foods, salt, alcohol, tobacco, coffee and tea. However, while the Australian Dietary Guidelines advocate modest consumption of meat and dairy and quite a large portion of whole grains, alkaline diets instruct that these foods make up 20% or less of the diet combined.
Alkaline diets also distinguish between low sugar fruits as being alkaline (for example avocado and pumpkin) and high sugar fruits as being acidic (for example pineapples and oranges).
Acidity and alkalinity in the human body
The pH of human blood serum and fluid around the cells needs to be tightly regulated in the slightly alkaline range for normal, healthy function. Small deviations can be dangerous, even deadly. The human body regulates pH with chemical buffers, respiration and excretion through the kidneys.
When the body has to work hard, using the mechanisms listed above, to maintain slightly alkaline pH this is known as metabolic acidosis. It doesn’t mean that the pH has become more acidic, just that buffering and excretion of acids are required to maintain balance. Often this is assessed by measuring urine pH. Advocates of alkaline diets claim that the chronic stress our bodies are put under to maintain slightly alkaline body pH can have long-term health effects.
The link between diet and metabolic acidosis
Metabolic acidosis can be caused by illnesses, such as renal disease and diarrhoea. A 2010 literature review concluded that there is a compelling case that diet-induced acidosis is a real phenomenon. However, this is controversial and not universally accepted. Alkaline diet advocates claim that modern diets are more acid producing than the stone age (pre-agricultural) diets humans are evolutionary adapted to.
Classifying foods as acid or alkaline producers
The basic premise for classifying foods as acid or alkaline is that animal proteins and cereal grains contain sulphur-containing amino acids which are major contributors to acid load. Several methods have been used since way back in 1912 to quantify the metabolic acid loads of foods. All of these methods have their limitations and critics. Remer et al in 1995 devised a formula for predicting the production of acids upon digestion. They assessed 114 frequently consumed foods and beverages and reported negative values (more alkaline or base-forming) in vegetables and fruits and the highest acid loads in cheese followed by meat, fish and grain products. Using these values, the research team were able to predict a person’s urinary pH fairly accurately when looking at total dietary intake.
Evidence for benefits of alkaline diets
A 2011 scientific literature review concluded that there was evidence that low urine pH predisposes to uric acid stone formation and that high dietary acid load is more likely to result in diabetes, hypertension and increased cardiovascular risk.
Similarly, a 2012 review concluded that alkaline diets result in a more alkaline urine pH and may result in a number of health benefits including an improvement in potassium/sodium ratio (important for cardiovascular health), increased bone health and a reduction in muscle wasting.
Evidence refuting alkaline diet claims
Two separate reviews concluded that milk and dairy products are not acid producing and do not cause metabolic acidosis and that there was no evidence that an alkaline diet is protective of bone health.
Other criticisms of alkaline diets
Many question the reliability of methods used to calculate acidity of foods and claim that the physiological environment in which digestion and metabolism take place is more nuanced and complex than can be reflected in a mathematical formula. The relationship between calculated acids and reported health effects is also somewhat murky.
Alkaline diet websites are fairly heterogeneous in their recommendations and information. Most seem to be selling merchandise such as water alkalinising equipment, tablets or drops; cookbooks and guides and even chlorophyll supplements. Many sites list far more foods than have been scientifically analysed and many sites cite no reference for the data. Often the lists differ slightly from site to site. Some sites even claim that decreased pH levels in the urine indicate lowered body pH.
So far, research on the effect of diet on metabolic acidosis and long term health effects is limited. Some nutritious and fantastic foods appear on the acidic lists, such as oats, brown rice and bananas. I think it would be damaging to the promotion of good nutrition to dissuade people from including a wide range of fruit, vegetables and whole grains in their diets. Healthy eating, for the average person, shouldn’t involve complicated lists, calculations and avoidance of nutritious fresh foods.