Do food combinations affect the digestive system?

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Many years ago it was suggested to dieters that their weight problems weren’t to do with how much they ate, but rather lay in the combination of foods they ate at each meal. Is there any truth to this? Let’s look into it further.

Food-combining diets such as the Hay diet recommend that protein and carbohydrate foods should not be eaten together, because each hindered the digestion of the other and this poor digestion of food somehow caused weight gain or prevented weight loss. Foods were divided into three categories: protein foods, such as meat, fish and dairy; neutral foods, such as fruit and veg; and starchy foods, such as cereals and grains.

The rationale behind the advice to not combine protein with starch is a belief that when protein-rich foods are consumed, the body produces too much acid to digest starch, for which an alkaline condition is needed. Conversely, the digestion of starch results in there being too little acid present to kick-start the digestion of protein. To date there has been no scientific evidence to support this theory. More recently, food-combining has taken on a new dimension, focusing on acidic and alkaline foods. The alkaline diet is one of the breakout diets of the last few years, promising to improve energy levels and memory as well as help prevent headaches, bloating, heart disease, muscle pain and insomnia.

Supporters of the diet claim that almost all foods we eat break down into either an acidic or alkaline base. Fresh fruit, vegetables, roots, nuts and legumes are all good. Dates, figs, grapefruit, lemon, lime, fennel, broccoli, artichoke, asparagus, beetroot, kale, spinach, watercress and cauliflower are considered the most alkaline. But pasta, wheat, all dairy products, meat, fish and shellfish, coffee, tea, sugar, fizzy drinks and alcohol are seen as acidic and therefore out. Considering the health benefits and low energy density (i.e. calories per gram) of the “good” alkaline foods, it is unsurprising that such a diet would aid weight loss.

There are also bold claims that cutting out acidic foods and sticking to mainly alkaline-based diet can help prevent a range of medical problems including osteoporosis, arthritis, diabetes and cancer, as well as slowing the ageing process. One popular message among the trendy bloggers is that dairy is actually bad for you and bad for your bones because it is acidic. Certainly it is clear the the pH (scale of acidity) and the acid load in the human diet has changed considerably from hunter-gatherer civilisation to the present. With the agricultural revolution and industrialisation, our diets have seen a decrease in potassium intake compared to sodium and an increase in chloride intake compared to bicarbonate.



The bicarbonate buffer system is a regulatory system that balances the pH of the body. The sodium carbonate and carbonic acid ratio is maintained by the kidneys and the lungs.

The ratio of potassium to sodium has reversed: previously it was 10 to 1, whereas the modern diet has a ratio of 1 to 3. It is generally accepted that today we have a diet poor in magnesium, potassium and fibre, and too high in saturated fat, simple sugars, sodium and chloride. However, the claims that such changes can result in metabolic acidosis are unfounded. Instead, the human body has an amazing ability to maintain a steady pH in the blood through the compensatory mechanisms that occur in the kidneys and through respiration. This process is known as the bicarbonate buffer system.

With aging, there is a grdual loss of this regulatory function and popular low-carbohydrate, high-protein diets can increase the acid load, but even in such circumstances there is very little change in blood pH. Instead, changes occur in the urine where the lower pH may be a risk factor for kidney stones. Our pH levels vary considerably from one area of the body to another, with the highest acidity in the stomach (pH of 1.35 to 3.5) in order to aid in digestion and protect against opportunistic bacteria. The skin is also quite acidic (pH 4-6.5) in order to provide an acid mantle as a protective barrier against microbial overgrowth.

Kidney stones are caused by a build-up of crystallized waste products from the blood that collect in the kidneys

Urine has a variable pH, from acid to alkaline, reflecting what the body has been doing during digestion to balance itself. On this basis, foods can be categorised by their potential acid loads and so alkaline diets will result in a more alkaline urine pH and may result in reduced calcium in the urine, but this does not impact on bone health and there is no evidence that an alkaline diet improves bone health or protects from osteoporosis.

Scientific study: – https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3195546/

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