When Food Helps, When Food Harms
Functional Foods, Therapeutic Diets, Contraindications, and the Limits of “Let Food Be Thy Medicine”
by:
lim ju boo - Chinese name lin ru wu (林 如 武)
Note:
This article is a companion piece to the earlier discussion I wrote here:
"Let Food Be Thy Medicine": Wisdom, Misquotation, and the Limits of Turning Medicine into Foods.
The first article examines the quotation and the limits, while this one explains, in practical medical detail, why food can be therapeutic in one setting and dangerous in another .
The diabetes and kidney sections especially require individual medical and dietetic supervision, because medicines, blood results, and disease stage can change the advice.
The familiar saying, “Let food be thy medicine and medicine be thy food,” contains an attractive truth, but it can easily be misunderstood. Food is not merely fuel. It supplies proteins, fats, carbohydrates, vitamins, minerals, fibre, and thousands of plant chemicals that influence metabolism, inflammation, blood vessels, immunity, intestinal bacteria, and cellular repair. In this sense, food can certainly contribute to prevention and treatment.
Yet food is not automatically medicine, and a food that is beneficial for one person may be dangerous for another. A banana is nutritious for a healthy child, but may be hazardous for a patient with advanced kidney failure and dangerously high blood potassium. Milk is a valuable food for most infants, but can be life-threatening for a baby with galactosaemia. Broad beans are wholesome for many people, but may trigger severe haemolytic anaemia in someone with glucose-6-phosphate dehydrogenase deficiency.
The wiser principle is therefore not that all food is medicine, but that food is biologically active. It can help, harm, protect, aggravate, or occasionally become essential treatment when selected according to the individual’s disease, medicines, metabolism, age, and nutritional needs.
This brings us to the subject on nutritional deficiency. However, we shall not go too deeply into these diseases below as they are clinically very lengthy, and technical to describe. It is a subject that requires a 4-year university course in nutrition to learn. A very brief mention of these diseases will do to emphasize that food containing all these nutrients is the only medicine that can cure these diseases - in short “let food be thy medicine”
Macronutrient Deficiencies (Protein-Energy Malnutrition)
Kwashiorkor: Caused by a severe deficiency in dietary protein. It is characterized by a swollen abdomen (edema), fluid retention, and a fatty liver.
Marasmus: Caused by a severe deficiency in overall calories and protein. It leads to extreme wasting of muscles, subcutaneous fat loss, and severe weight
Mineral Deficiencies
Iron Deficiency Anemia: Caused by a lack of iron. It leads to a reduction in red blood cells, causing fatigue, pale skin, shortness of breath, and dizziness.
Goiter and Hypothyroidism: Caused by a lack of iodine. Iodine is vital for thyroid function; its absence causes the thyroid gland to swell.
Hypocalcemia & Osteoporosis: Caused by a lack of calcium. This leads to weak, brittle bones, muscle cramps, and dental issues.
Hypomagnesemia: Caused by a lack of magnesium. Symptoms include muscle spasms, numbness, and irregular heart rhythms.
Vitamin Deficiencies
Scurvy: Caused by a lack of Vitamin C (ascorbic acid). It results in bleeding gums, slow wound healing, and easy bruising.
Rickets & Osteomalacia: Caused by a lack of Vitamin D. Rickets causes weak, soft, and deformed bones in children, while osteomalacia is the adult equivalent.
Xerophthalmia & Night Blindness: Caused by a lack of Vitamin A. It leads to severe dry eyes, corneal damage, and impaired vision in low light.
Bleeding Diathesis (Coagulation Disorder): Caused by a lack of Vitamin K. It causes impaired blood clotting, leading to excessive bruising and bleeding.
Vitamin B-Complex Deficiencies
Beriberi: Caused by a lack of Vitamin B1 (Thiamine). It impacts the nervous and cardiovascular systems, leading to muscle wasting or heart failure.
Ariboflavinosis: Caused by a lack of Vitamin B2 (Riboflavin). Symptoms include painful cracks at the corners of the mouth (angular cheilitis) and an inflamed tongue.
Pellagra: Caused by a lack of Vitamin B3 (Niacin). It is characterized by the "4 Ds": Dermatitis, Diarrhea, Dementia, and ultimately Death if left untreated.
Megaloblastic Anemia: Caused by a lack of Vitamin B12 (Cobalamin) or Vitamin B9 (Folate). It results in abnormally large, poorly functioning red blood cells, causing fatigue and nerve damage.
Impact of Over Nutrition
On the other end of malnutrition is over nutrition or bad nutrition, driven by excessive caloric, fat, and sugar intake. The prefix ‘mal’ in Latin means "bad," "wrong," or "evil". Thus the word ‘malnutrition’ does not mean under nutrition only as most people - unfortunately including some nutritionists and doctors think, but malnutrition is also over-nutrition.
Over or excessive nutrition is a major public health crisis in Malaysia and also in affluent countries. It primarily leads to overweight, obesity, and Non-Communicable Diseases (NCDs).
According to national health data, over 54% of Malaysian adults are either overweight or obese, fueling high rates of "metabolic syndrome".
The primary diseases and health conditions associated with over-nutrition in Malaysia include:
1. Obesity and Overweight
Caused by prolonged energy imbalance (consuming more calories than burned), combined with sedentary lifestyles and diets high in refined carbohydrates and sugars. This affects over half of the adult population, putting heavy strain on joints and organs, and acting as the gateway condition to other chronic metabolic diseases.
2. Type 2 Diabetes
Excess body fat, particularly visceral fat (belly fat), causes the body to become resistant to insulin. Its impact causes approximately 15% to 16% of Malaysian adults live with diabetes. It is a leading cause of kidney failure and nerve damage in the country.
3. Hypertension (High Blood Pressure)
Diets high in sodium (salt), processed foods, and saturated fats, coupled with obesity, force the heart to work harder to circulate blood. As a result nearly 30% of Malaysian adults have hypertension. It significantly elevates the risk of heart attacks and strokes.
Hypercholesterolaemia (High Blood Cholesterol)
High consumption of saturated fats and ultra-processed foods causes a buildup of LDL (bad) cholesterol in the arteries, restricting blood flow. Its impact affects about 33% of the adult population and is a primary driver for coronary artery disease.
Cardiovascular Disease (CVD)
The combined long-term effects of obesity, hypertension, and high cholesterol result in narrowed or blocked blood vessels. Thus heart disease remains one of the leading causes of mortality in Malaysia.
Metabolic Dysfunction-Associated Fatty Liver Disease (MAFLD)
Excessive caloric intake and obesity lead to the accumulation of fat in the liver cells, which can progress to inflammation and liver damage. It is becoming increasingly prevalent among Malaysian adults alongside the rise in obesity rates. There are also liver diseases due to food toxicities
1. Food as a Biological Signal, Not Only as Calories
However, modern nutrition science has moved beyond the old idea that food merely provides calories and nutrients for the prevention of nutritional deficiency diseases or the impact of over-nutrition.
Food molecules interact with enzymes, hormones, receptors, genes, intestinal microbes, and inflammatory pathways. Fibre can slow glucose absorption and feed beneficial gut bacteria. Omega-3 fatty acids can alter inflammatory signalling. Plant polyphenols may influence oxidative stress and vascular function. Sodium affects fluid balance and blood pressure. Certain amino acids can become toxic in children born without the enzymes needed to metabolise them.
This is why diet is central to the management of diabetes, hypertension, kidney disease, coeliac disease, cardiovascular disease, inherited metabolic disorders, and many gastrointestinal conditions. However, it is also why dietary advice must be individualised. A “healthy food list” without clinical context can be misleading.
2. Foods That May Worsen Diabetes and Cardiovascular Disease
In diabetes, sugary drinks, sweets, candy, desserts, and other foods rich in rapidly absorbed sugars can produce sharp rises in blood glucose. Their problem is not simply that they taste sweet, but that they deliver a large carbohydrate load with little fibre, protein, or fat to slow absorption. Sweetened beverages are particularly undesirable because liquid sugar is consumed quickly and produces relatively little fullness.
White bread, refined rice, many sweet biscuits, cakes, and highly refined breakfast cereals may also raise blood glucose rapidly. It is more accurate to say that they can behave metabolically like rapidly absorbed carbohydrate, rather than that they are literally sugar. Their effect varies with portion size, cooking method, fibre content, and what is eaten with them. A plate of white rice eaten with vegetables, legumes, fish, and protein will generally have a different glycaemic effect from white rice eaten alone.
For people with diabetes, the preferable direction is toward high-fibre carbohydrate sources: vegetables, pulses, oats, intact wholegrains, nuts, seeds, and fruits eaten whole rather than taken as juice. Whole fruit contains fructose, but it also contains water, fibre, micronutrients, and plant compounds; it should not be confused with fruit juice or sugar-sweetened drinks.
The late Professor Dr John Yudkin, MA (Cambridge), MD (Cambridge), PhD (Cambridge), FRCP (London), FRIC, FIBiol. of Queen Elizabeth College, University of London, was among the early researchers who drew attention to the possible relationship between high sugar intake, raised triglycerides, obesity, diabetes, and coronary heart disease. His work was important historically because it challenged the assumption that dietary fat alone explained cardiovascular risk. Modern evidence supports concern about excess added sugar, especially from sugary beverages and highly processed foods, although coronary disease is multifactorial and cannot be attributed to sugar alone. I was extremely fortunate to study for my postgraduate in nutrition under Professor John Yudkin.
Excess added sugar may promote weight gain, fatty liver, raised triglycerides, and poorer glucose control. When large amounts of fructose-containing sugars are consumed, especially in drinks and highly processed foods, the liver may convert some of the excess energy into fat. This can contribute to elevated triglycerides and accumulation of fat in the liver and around internal organs. Such metabolic disturbances are associated with insulin resistance and increased cardiovascular risk.
Highly processed foods may also combine several harmful features: refined starch, added sugar, excessive salt, poor fibre content, unhealthy fats, and high energy density. Their long-term effect is not merely “artery inflammation” in a simple direct sense, but a complex interaction involving obesity, insulin resistance, abnormal blood lipids, hypertension, endothelial dysfunction, and chronic low-grade inflammation. Together these processes accelerate atherosclerosis—the formation of plaques within arteries.
3. Salt, Processed Meat, and High Blood Pressure
For people with hypertension, heart failure, or fluid retention, excessive sodium is a major dietary concern. Canned soups, instant noodles, processed sauces, salty snacks, fast foods, cured meats, bacon, sausages, hot dogs, ham, and many packaged foods can contain surprisingly large amounts of salt.
Sodium causes the body to retain water. In susceptible individuals, this increases circulating blood volume and raises blood pressure. Over years, persistent hypertension damages arteries, the heart, kidneys, brain, and eyes. Salt reduction is therefore not a minor lifestyle suggestion; for many patients it is an important part of treatment.
Processed meats deserve special caution. They are often high in sodium and may contain saturated fat, preservatives, and nitrites or nitrates. Frequent consumption is associated with poorer cardiovascular health and with increased risk of colorectal cancer. The problem is not that a single piece of sausage instantly damages an artery, but that habitual intake can form part of a long-term dietary pattern that promotes hypertension, adverse lipid levels, obesity, and vascular disease.
A more protective dietary pattern includes vegetables, fruits where appropriate, beans, whole grains, nuts, fish, and unsaturated fats such as olive oil, while reducing processed meats, sugary drinks, refined carbohydrates, and excess salt.
4. Kidney Disease: When Nutritious Foods Need Restriction
Kidney disease provides one of the clearest examples of why food cannot be judged simply as “good” or “bad.” The kidneys regulate water, sodium, potassium, phosphorus, acid-base balance, and the removal of metabolic waste products. When kidney function declines, foods that are normally wholesome may require restriction.
Dark colas and many packaged or processed foods may contain phosphate additives. These additives are readily absorbed and can raise blood phosphorus levels in people with advanced chronic kidney disease. Persistent phosphorus excess contributes to bone disease, itching, vascular calcification, and cardiovascular complications.
Bananas, oranges, coconut water, tomatoes, potatoes, dried fruits, and certain fruit juices may be high in potassium. Potassium is essential for normal nerve and muscle function, including the heartbeat. But when the kidneys cannot excrete potassium adequately, blood potassium can rise to dangerous levels and may cause serious heart rhythm disturbances.
Nevertheless, potassium restriction should never be applied indiscriminately to every person with kidney disease. Some patients, particularly those in earlier stages of kidney disease or those taking certain medicines, may have normal or even low potassium. Dietary potassium must therefore be guided by kidney function, blood tests, medicines, dialysis status, and professional dietary advice.
Protein is another example. Some people with non-dialysis chronic kidney disease may benefit from avoiding excessive protein intake, because protein metabolism produces nitrogenous waste that diseased kidneys must clear. But patients receiving dialysis often need more protein to prevent muscle wasting and malnutrition. Thus, a diet that is suitable for one kidney patient may be harmful for another.
5. Coeliac Disease and Gluten-Related Disorders
In coeliac disease, gluten from wheat, barley, and rye triggers an abnormal immune reaction in genetically susceptible individuals. The immune system damages the lining of the small intestine, especially the villi—the tiny finger-like structures responsible for nutrient absorption. Untreated coeliac disease can lead to diarrhoea, weight loss, anaemia, osteoporosis, fatigue, infertility, poor growth in children, and nutritional deficiencies.
For confirmed coeliac disease, strict lifelong avoidance of gluten is the established treatment. Rice, corn, millet, buckwheat, quinoa, potatoes, legumes, meat, fish, eggs, fruits, and vegetables are naturally gluten-free, although processed foods must be checked for contamination.
It is important, however, to distinguish coeliac disease from wheat allergy and from non-coeliac gluten sensitivity. These conditions may produce overlapping symptoms but differ in their immune mechanisms, diagnosis, and management. A person should ideally be tested for coeliac disease before beginning a gluten-free diet, because removing gluten beforehand can make diagnostic tests less reliable.
6. Broad Beans and G6PD Deficiency
Broad beans, also called fava beans, are nutritious legumes for most people. But they can be dangerous for individuals with glucose-6-phosphate dehydrogenase, or G6PD, deficiency. This inherited enzyme deficiency is relatively common in parts of Asia, the Mediterranean, Africa, and the Middle East.
G6PD helps red blood cells defend themselves against oxidative damage. Fava beans contain oxidant compounds, including vicine and convicine. In a susceptible person, these compounds can trigger rapid destruction of red blood cells, a condition known as favism.
The result may be sudden pallor, weakness, fever, jaundice, dark urine, shortness of breath, and severe haemolytic anaemia. In serious cases, kidney injury can occur. The same individual may also need to avoid certain medicines and chemicals that produce oxidative stress. This is a powerful reminder that a food praised for its protein and fibre can still be medically unsafe for a genetically susceptible person.
7. Inborn Errors of Metabolism: When Diet Is Lifesaving Treatment
Inborn errors of metabolism are inherited disorders in which a child lacks, or has very little of, an enzyme needed to process a particular nutrient. In these conditions, ordinary foods may produce toxic metabolic substances. Diet is not merely supportive; it may be the principal lifesaving treatment.
Phenylketonuria
In phenylketonuria, or PKU, the body cannot adequately convert the amino acid phenylalanine into tyrosine. Phenylalanine is found in protein-rich foods such as meat, fish, eggs, milk, cheese, nuts, beans, and many ordinary cereals. Without early dietary treatment, phenylalanine accumulates and can damage the developing brain.
Children with PKU require a carefully controlled low-phenylalanine diet, special medical formulas, and lifelong monitoring. The purpose is not to eliminate all protein indiscriminately, but to provide enough protein and nutrients for growth while keeping phenylalanine within a safe range.
Maple Syrup Urine Disease
In maple syrup urine disease, or MSUD, the body cannot properly metabolise the branched-chain amino acids leucine, isoleucine, and valine. These are abundant in protein-containing foods. If they accumulate, particularly leucine, they can cause poor feeding, vomiting, lethargy, seizures, brain swelling, coma, and death.
Children with MSUD need specialised formulas and strict dietary control. During illness, fever, fasting, or infection, metabolic control can deteriorate rapidly, and emergency medical management may be required.
Galactosaemia
In classic galactosaemia, the body cannot properly metabolise galactose, a sugar derived largely from lactose in milk. Breast milk, cow’s milk, standard infant formulas, and many dairy products therefore become dangerous. In an affected newborn, continued milk feeding can rapidly lead to jaundice, liver dysfunction, poor growth, cataracts, serious infection, and potentially fatal illness.
The treatment is immediate removal of lactose and galactose-containing foods, with specialised infant feeding under medical supervision. Even with careful treatment, long-term follow-up is important because some complications may still occur.
Hereditary Fructose Intolerance
In hereditary fructose intolerance, the body lacks the enzyme aldolase B needed to metabolise fructose properly. Fructose is found in fruits, fruit juices, honey, table sugar or sucrose, and many sweetened foods. Once fructose-containing foods are introduced, the child may develop vomiting, sweating, lethargy, low blood glucose, liver enlargement, and liver or kidney injury.
The treatment is strict avoidance of fructose, sucrose, and sorbitol. Such children often develop a natural dislike of sweet foods because they associate sweetness with illness. Early diagnosis and dietary exclusion can prevent serious damage.
These inherited disorders show most clearly that dietary advice must never be based on fashionable slogans. For some children, a carefully designed diet is as essential as any drug.
8. Functional Foods: Foods That May Support Health
There are foods that may be described as functional foods because, beyond basic nutrition, they contain biologically active compounds with potential health and medicinal effects as the saying goes "let food be thy medicine" But “functional” does not mean miraculous, and it does not mean that a food can replace insulin, blood-pressure medicine, antibiotics, chemotherapy, or specialist medical care.
Bitter Gourd or Bitter Melon
Bitter gourd, Momordica charantia, has long been used in Asian traditional medicine for diabetes. It contains several compounds that have attracted scientific interest, including charantin, vicine, and peptides sometimes described as insulin-like, including polypeptide-p.
Laboratory and small clinical studies suggest that bitter gourd may influence glucose metabolism by improving glucose uptake in tissues, affecting intestinal carbohydrate digestion, and possibly influencing insulin secretion or insulin sensitivity. However, the human evidence remains inconsistent. Bitter gourd should therefore be regarded as a possible dietary adjunct, not as a substitute for prescribed diabetes treatment.
It also deserves caution. Taken in large amounts, especially as concentrated extracts, bitter gourd may contribute to low blood glucose in people using insulin or sulfonylurea medicines. Pregnant women, young children, and people with G6PD deficiency should seek medical advice before using concentrated preparations. The food itself, used sensibly in cooking, is different from taking large-dose supplements.
Turmeric and Curcumin
Turmeric contains curcuminoids, of which curcumin is the best known. Curcumin has anti-inflammatory and antioxidant activity in laboratory studies and may influence signalling pathways such as nuclear factor kappa-B, commonly called NF-kB. NF-kB helps regulate genes involved in inflammation.
This gives a scientific basis for the traditional use of turmeric in inflammatory conditions. Some clinical studies suggest modest improvement in pain and function in osteoarthritis, although the quality and formulations of studies vary greatly. Curcumin is poorly absorbed unless prepared with fat, piperine from black pepper, or specialised formulations.
Turmeric in ordinary food is generally safe for most people. Concentrated supplements, however, can interact with anticoagulant drugs, aggravate gallbladder problems in susceptible individuals, and occasionally cause liver injury. Therefore, culinary turmeric should not be confused with high-dose curcumin capsules.
Garlic
When garlic is crushed or chopped, the enzyme alliinase converts alliin into allicin and related sulphur compounds. These compounds may influence vascular tone, platelet activity, oxidative stress, and lipid metabolism. Some studies suggest that garlic preparations can produce a small reduction in blood pressure, particularly in people with hypertension, and may have modest effects on cholesterol.
Garlic is not a natural equivalent of an ACE inhibitor. Its effects are smaller, less predictable, and dependent on the preparation and dose. Nevertheless, garlic can be a useful part of a heart-healthy diet because it adds flavour and may help reduce the need for excessive salt.
People taking warfarin, aspirin, clopidogrel, or other blood-thinning medicines should be cautious with concentrated garlic supplements, which may increase bleeding tendency. Culinary garlic is generally much less problematic.
Blueberries, Blackberries, and Other Dark-Coloured Fruits
Blueberries, blackberries, purple grapes, blackcurrants, and other deeply coloured fruits contain anthocyanins and other polyphenols. These compounds have antioxidant and anti-inflammatory properties and may support vascular function. Observational studies and some trials suggest possible benefits for blood pressure, insulin sensitivity, and aspects of cognitive function.
It is reasonable to say that berries may support brain and vascular health as part of a balanced dietary pattern. It is not yet justified to say that they prevent or cure Alzheimer’s disease, or that they directly remove toxic plaques from the brain in humans. The biology is promising, but the clinical evidence remains incomplete.
Broccoli, Brussels Sprouts, Cabbage, and Other Cruciferous Vegetables
Broccoli, Brussels sprouts, cabbage, cauliflower, kale, and related vegetables contain glucosinolates. When the vegetables are chopped and chewed, these compounds can be converted into isothiocyanates, including sulforaphane. Sulforaphane has been studied for its ability to activate protective cellular pathways, including enzymes involved in detoxification and antioxidant defence.
These vegetables may help the body handle certain reactive chemicals and may reduce inflammation. Diets rich in cruciferous vegetables are associated with lower risk of several chronic diseases. However, no vegetable can guarantee protection from cancer. Cancer prevention depends on a broad pattern: avoiding tobacco, maintaining healthy weight, exercising, limiting alcohol, eating fibre-rich foods, reducing processed meat, and participating in appropriate screening.
People taking warfarin should keep their intake of vitamin-K-rich green vegetables reasonably consistent rather than suddenly consuming very large or very small amounts. Those with hypothyroidism do not usually need to avoid cruciferous vegetables unless intake is extreme and iodine intake is poor.
Salmon, Sardines, and Omega-3 Fatty Acids
Salmon, sardines, mackerel, herring, and other oily fish provide eicosapentaenoic acid, EPA, and docosahexaenoic acid, DHA. These omega-3 fatty acids are incorporated into cell membranes and can influence the production of inflammatory mediators. They may also lower triglycerides and contribute to cardiovascular health.
For autoimmune diseases such as rheumatoid arthritis, omega-3 fatty acids may modestly reduce inflammation, morning stiffness, and the need for some pain medicines in selected patients. They do not cure lupus, psoriasis, rheumatoid arthritis, or other autoimmune diseases, and they should not lead patients to stop prescribed immunological treatment.
Fish is generally preferable to high-dose fish-oil capsules because it provides protein, vitamin D, selenium, and other nutrients. People using anticoagulant medicines or preparing for surgery should discuss high-dose omega-3 supplements with their doctors. Unfortunately doctors too may not know much about nutrition or about therapeutic diets. A dietician would be a better qualified person to consult. A better expertise would be a medical doctor with a postgraduate qualification in nutrition or in dietetics. But it may be very rare to find such an expertise - a doctor and a nutritionist in one for a final advice.
Fish choices should also consider mercury exposure; sardines are generally low in mercury and rich in omega-3 fats.
9. The Difference Between Food, Supplement, and Drug
A useful distinction must be made between eating a food and taking an extract. A clove of garlic in cooking is not the same as a concentrated garlic capsule. Turmeric in curry is not the same as high-dose curcumin. Bitter gourd as a vegetable is not the same as an unregulated bitter-melon supplement.
Drugs are standardised, tested for dose, purity, pharmacokinetics, interactions, and adverse effects. Foods are more variable. Their active compounds differ according to species, soil, ripeness, storage, cooking, preparation, and portion size. A food may be safe as part of a meal but unsafe when concentrated into capsules or powders.
For this reason, the phrase “natural medicine” should never be interpreted as “automatically safe medicine.”
10. A More Accurate Version of the Ancient Wisdom
The enduring wisdom behind the famous saying is not that food can replace medicine. Rather, it is that daily eating patterns can either support health or gradually undermine it. Good food can reduce risk, improve metabolic control, support treatment, and sometimes become essential therapy. Poor food choices can worsen disease, interfere with treatment, and in certain inherited disorders cause acute medical crises.
The most mature conclusion is therefore this:
Food may be preventive medicine, supportive medicine, and occasionally lifesaving medical treatment. But food must be chosen according to the disease, the person, the dose, and the evidence. What nourishes one patient may endanger another.
The task of nutrition science is not to turn every food into a drug, nor to reject the healing potential of diet. It is to understand food with enough humility and precision to know when it helps, when it harms, and when medical treatment remains indispensable.
In nutrition there is also a highly specialized area on food toxicology - an area I was conducting research on at the Massachusetts Institute of Technology (MIT) before I joined the Institute for Medical Research in Malaysia. But I shall write on these areas as well as the adverse interactions between certain foods with certain medicine and drugs. We shall write on these separately to continue this discussion in Part 3 and Part 4 to debunk this adage - 'let food be thy medicine and let medicine be thy foods' We shall discuss the details in depth later .
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