Food Intolerance

Food intolerance, also referred to as food hypersensitivity or non-allergic food hypersensitivity, is a biochemical or metabolic issue caused by the body's inability to properly digest a specific food or food ingredient. It is important to distinguish food intolerance from food allergy, which involves an abnormal immune response where the body creates antibodies to combat the perceived threat.

Food intolerance can be categorized based on different mechanisms such as metabolic, pharmacologic, or gastrointestinal responses to foods. The individual's reaction to certain foods can vary depending on their level of sensitivity. Symptoms of food intolerance typically manifest slowly and can appear anywhere from half an hour to 48 hours after consuming the food.

Common symptoms of food intolerance can impact various parts of the body, including the skin, respiratory tract, and gastrointestinal tract. Skin symptoms may include rash, hives, and swelling. Respiratory symptoms can manifest as nasal congestion, sinusitis, and asthma. Gastrointestinal symptoms may include abdominal cramps, bloating, nausea, gas, heartburn, diarrhoea, constipation, and irritable bowel syndrome. Other symptoms like headache, migraine, fatigue, and behavioural issues may also be present.

Diagnosing food intolerance can be challenging due to the absence of a definitive and reliable test. While tests for lactose intolerance and celiac disease are well established, the most effective method for identifying intolerance to specific foods is through elimination diet protocols.

Intolerance due to Metabolic disorders

Metabolic disorders result from genetic or acquired abnormalities in nutrient metabolism. The most common type is carbohydrate intolerance, which is caused by a deficiency of enzymes needed for carbohydrate digestion. Some common food intolerances include:

Lactose Intolerance: Lactose, the sugar found in milk, is broken down by the enzyme lactase into glucose and galactose. People who lack this enzyme due to genetic conditions are unable to digest milk properly. The undigested lactose travels to the colon where it is fermented by bacteria, producing propionic, acetic, formic, and lactic acid. Bacteria further break down formic acid to create hydrogen and carbon dioxide, which can then be converted into methane gas. These gases can cause symptoms such as flatulence and stomach rumbling, which are common in individuals with lactose intolerance. While uncomfortable, this condition is not life-threatening and can be managed by avoiding dairy products or taking galactosidase supplements.

Galactosaemia: Galactosaemia is a rare genetic condition caused by a deficiency in the enzyme galactose phosphate uridyl/transferase, which is needed to metabolize galactose into glucose. Galactose is found in foods like milk, chickpeas, legumes, fermented soybean products, and organ meats. If left undetected, galactosaemia can be life-threatening, as galactose in the blood can lead to symptoms such as vomiting, diarrhoea, low blood sugar, jaundice, liver enlargement, brain damage, and increased susceptibility to infections. The only treatment for galactosaemia is to follow a strict diet that avoids foods containing galactose.

Fructose intolerance: Fructose is commonly consumed in the form of free sugar, sucrose, or fructans (a polymer of fructose) from various dietary sources. Foods high in fructose include fruits (such as apples, pears, guava, melons, mangoes, papayas, etc.), honey, and corn syrup. The absorption of fructose is facilitated by the absorption of glucose in the small intestine through solvent drag and passive diffusion. Fructose intolerance can be due to either fructose malabsorption or hereditary fructose intolerance, an autosomal recessive disorder that can lead to hypoglycaemia, liver, and kidney damage. The only treatment is to completely avoid foods containing fructose.

Phenylketonuria (PKU): PKU is an autosomal recessive genetic inborn disorder caused by a deficiency of the enzyme phenylalanine hydroxylase which leads to the inability to metabolise phenylalanine.  Phenylalanine is an essential amino acid that is obtained from the diet and is necessary for protein synthesis. The accumulation of phenylalanine in the blood can be toxic and can result in growth failure, poor skin pigmentation, microcephaly (abnormally small head size), and severe intellectual impairment. PKU is a rare condition, affecting approximately 1 in 10-15,000 newborns, and its prevalence varies among different ethnic and geographic populations. The only treatment for PKU is a lifelong low-protein diet.

Deficiency of Glucose 6-phosphate Dehydrogenase (G6PD): G6PD is a crucial enzyme that shields red blood cells (RBCs) from reactive oxygen species. A lack of G6PD can make RBCs vulnerable to oxidative stress, resulting in haemolysis (the rupture or destruction of RBCs). This genetic disorder is inherited through the X chromosome, making it more common in men than women. The prevalence of diabetes mellitus (type 2) and hypertension may be directly linked to G6PD deficiency. Gene mutations causing G6PD deficiency are more common in Africans, Afro-Americans, and individuals in Asia. While there is no cure for G6PD deficiency, the condition can be managed by avoiding triggers, and in severe cases, blood transfusions may be necessary.

Intolerance due to Malabsorption

Fructose Malabsorption:  Fructose malabsorption is a digestive disorder where the body has difficulty absorbing fructose due to a lack of fructose carriers in the small intestine. Consuming too much fructose can lead to its accumulation in the gut, drawing water into the small intestine and causing loose stools to move into the colon. Colonic bacteria ferment the excess fructose, producing short-chain carboxylic acids and gases that can result in symptoms like those seen in lactose intolerance, such as abdominal discomfort, bloating, and changes in bowel movements. These symptoms are often seen in individuals with irritable bowel syndrome. Treatment for fructose malabsorption typically involves following a low-fructose diet, and in severe cases, a low FODMAP diet may be recommended. This diet eliminates certain carbohydrates, including lactose (milk sugar), fructose (fruit sugar), fructans (wheat and some vegetables), galactans (beans and lentils), and polyols (sorbitol, xylitol, and manni

Galactose Oligosaccharides: Short-chain polymers of galactose monosaccharides are not broken down in the intestine due to the absence of the galactosidase enzyme. These oligomers pass through the large intestine without being digested and are fermented by bacteria, leading to the production of intestinal gas. Legumes, lentils, grains, nuts, and human milk are typical dietary sources of galacto-oligosaccharides.

Polyols: Mannitol, sorbitol, and xylitol are naturally present in various fruits and vegetables like apricots, apples, peaches, pears, cherries, mushrooms, and cauliflowers, and are absorbed passively. The absorption of these polyols depends on their molecular size and transit time in the small intestine, and they are recognized for their laxative properties.

Salicylates and biogenic amines: Foods containing natural salicylates and biogenic amines can trigger symptoms of food intolerance in some people. Salicylates, found in fruits (such as apples, avocadoes, berries, grapefruit, figs, and peaches), vegetables (like cauliflower, broccoli, chilli peppers, spinach, turmeric), and other foods, can lead to rhinitis and asthma. Biogenic amines, nitrogen-containing organic bases produced by microbial, vegetable, and animal metabolisms, include histamine, tyramine, phenyl ethyl amine, and tryptamine. These amines can cause skin reactions, gastrointestinal issues, respiratory problems, and headaches. High histamine levels are toxic, while moderate levels can trigger food intolerance symptoms. A lack of the enzyme amine oxidases can result in the accumulation of biogenic amines in the body.

Food additives: Food additives are natural or synthetic chemicals added to foods for various purposes such as anti-caking agents, antioxidants, artificial sweeteners, emulsifiers, colours, flavour enhancers, preservatives, thickeners, stabilizers, glazing, gelling, and bulking agents. These additives are used to improve the colour, taste, flavour, texture, and shelf life of foods. However, they have been linked to adverse reactions in some individuals, including skin problems (such as hives, itching, rashes, and swelling), gastrointestinal disorders (like diarrhoea and colicky pain), nervous disorders (such as hyperactivity, insomnia, and irritability), and respiratory problems (such as asthma, rhinitis, and sinusitis). Common additives that may cause issues include the flavour enhancer monosodium glutamate (MSG), the food colour tartrazine, preservatives like benzoates, nitrates, and sulphites, and the artificial sweetener aspartame.

Identifying food malabsorption intolerance to specific foods can be challenging. Common symptoms of intolerance include irritable bowel syndrome, flatulence, abdominal cramps, nausea, gas, and intermittent diarrhoea. Currently, there is no cure for food intolerances, so the best approach is to avoid the foods that cause symptoms.

I have extensively discussed food intolerance in Chapter 13 of my book "The Science of Food Nutrition and Health" published by Austin Macauley Publishers. In addition, I will address food-induced allergic reactions and food poisoning caused by various contaminants such as bacteria, viruses, parasites, and toxic chemicals in a separate post.

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