Improving Kidney Health with Grapes 

Introduction

The kidneys are a pair of essential organs, responsible for the filtration of waste products from the blood. They play a vital role in maintaining overall well-being, and issues with kidney function can lead to serious health problems.  Kidneys can be referred to as the renal system. Kidney disease not only affects the filtration of the blood, but can cause problems with other bodily organs, leading to cardiovascular disease, strokes, weak bones, neuropathy (damage to the nerves), or anaemia.  Unfortunately, kidney disease can often remain detected until advanced stages, necessitating dialysis to filter the blood or a kidney transplant.

Kidney health can be profoundly impacted by diet, so ensuring the food you eat is “renal-friendly” is paramount. Many foods are known to maintain kidney health, manage existing diseases, and reduce the risk of developing complications to existing disorders. Control of many factors, including salt and protein intake, as well as staying hydrated, can be beneficial to kidney function. One food that benefits kidney health is the grape which is widely consumed and rich in antioxidants and anti-inflammatory properties. Grapes, high in water content, aid hydration and are low in oxalates, reducing the risk of kidney stone formation. Additionally, they help regulate blood pressure and are low in calories and sugars, making them suitable for diets aimed at preventing obesity and diabetes-related kidney disease.

About kidney health

The kidneys filter blood and remove waste products and toxins,  including urea and drugs. Kidneys are comprised of many small units called nephrons, which collectively filter approximately 180 litres of blood daily through a specialised sieve-like structure called the glomerulus in the tubules. Here, nearly all of the filtered liquid is reabsorbed back into the blood, whilst waste products and excess fluid are passed to the bladder for excretion as urine. The kidneys also remove excess electrolytes like potassium and sodium during this filtration and regulate the acidity of the blood. Additionally, the kidneys produce various small molecules that regulate important body functions. These include:

• Renin: an enzyme that  controls blood pressure;
• Erythropoietin: a hormone which signals to the bone marrow to stimulate production of red blood cells;
• Calcitriol: the activated form of Vitamin D involved in calcium absorption.

If the kidneys become damaged, the ability of the renal system to function correctly may be inhibited, and waste products that would normally be filtered and removed can accumulate in the blood. Additionally, important proteins within the blood may leak from the nephron and be excreted within the urine, impairing essential body functions and causing complications such as cardiovascular disease and neurodegeneration.

Kidney problems can be short-term (e.g. Acute Kidney Injury or Urinary Tract Infections [UTIs]), where a sudden loss of kidney function occurs, often following infection or the use of certain medications. In most instances, this can be reversed and normal renal function will resume. Kidney diseases can also be chronic and often progressive (e.g. Diabetic Kidney Disease or Polycystic Kidney Disease).

Often, a person’s likelihood of developing kidney disease can be worsened by various risk factors. The most common risk factors for kidney disease are diabetes and high blood pressure. Consistent high blood sugar levels  can damage the nephrons within the kidney. High blood pressure, also called hypertension, causes chronic kidney disease. Other risk factors include age, medication usage, recurrent UTIs or kidney stones, autoimmune disorders such as Lupus, and chronic dehydration. 

Nutritional value of grapes

The grape is an extremely common fruit, which grows in clusters on the climbing vine Vitis. They can be eaten fresh, termed ‘table grapes’, or can be processed into various products, including wine, jam, grape seed extract, and raisins. Table grapes are usually larger and seedless, with thinner skins, whereas wine grapes are often smaller, sweeter, and seeded, with thicker skins. There are over 10,000 varieties of grapes worldwide. Different varieties of grapes possess differing qualities, including their taste; white grapes are often sour with a citrus undertone, whereas red grapes generally taste sweeter and have a richer flavour. 

The nutritional content of a grape renders it an ideal snack. Predominantly composed of water, grapes are very low in calories and salt, yet high in fibre. When eaten raw (as table grapes) they virtually are fat-free. Grapes are a rich source of many vitamins (including C and K) and minerals (such as copper, manganese, and magnesium) known to offer health benefits. The U.S. Department of Agriculture reports the nutritional content of grapes as :

Table 1 presents the nutritional facts of grapes

Grapes contain antioxidant and anti-inflammatory compounds, with darker varieties like red grapes containing greater amounts of the flavonoid compounds anthocyanins, whilst white grapes contain more flavonoid catechin. Additionally, the processing of grapes to produce various products (e.g. jam vs wine) leads to the extraction of varying proportions of these molecules. Key compounds in grapes known to be beneficial for kidney health are listed below.

Grapes and kidney health

Grapes are rich sources of various compounds beneficial to health, especially kidneys. Daily intake of whole grape powder has been shown to lower blood sugar, and prevent damage to kidney tissue in diabetes. Kidney function is improved, as evidenced by reduced leakage of essential proteins into the urine.¹ Their high water content is a great source of hydration, a crucial factor for maintaining renal function. Hydration supports the kidneys’ filtration capacity, and ensures constant flushing of pathogens and crystals, limiting the capacity for infections to take hold or kidney stones to form. 

High levels of salts such as potassium can contribute to kidney damage, both directly, and by their role in increasing blood pressure. Coupled with their hydrating properties, the low-potassium composition of grapes helps to maintain kidney function and health. 

Antioxidants protect against harmful molecules termed “free radicals”. Oxidative stress, caused by free radical damage, is a key contributor to renal injury and the progression of kidney disease. Consumption of antioxidants, like those found within grapes, can help protect the kidneys from such stress. Likewise, inflammation is often involved in renal diseases, and thus the anti-inflammatory compounds within grapes may be beneficial in reducing inflammation within the kidneys. 

Antioxidant properties of grapes

The rich antioxidant nature of grapes helps to protect cells from free radicals and their damaging properties. 

Flavonoids

Flavonoids are compounds found in plants, with known protective effects against cardiovascular disease and neurodegeneration.² Quercetin is a flavonoid often found in red grapes. It is known to have antioxidant and anti-inflammatory properties. It can suppress the expression of the sodium channel within the tubules, helping to regulate blood pressure.³ Supplementation with quercetin also helps with overall kidney function, and chronic treatment reduces protein leakage into the urine.⁴

Anthocyanins are another class of molecule within the flavonoid category. Again, these molecules are found with red grapes, and act against free radicals to support health.

Incorporating anthocyanin-rich food reduces diabetes-associated renal failure. Proanthocyanidin extract helps reduce renal inflammation, protects against protein leakage, and helps regulate blood pressure.⁵

Catechins are a further type of flavonoid, and research suggests administration of epichatechin protects against renal metabolic dysfunction, protects against protein leak, and tissue damage.⁶

Stilbenes

Resveratrol is a known stilbene compound, found predominantly within the skins of red grapes. It is known to combat free-radical damage, inflammation, and age-dependent mechanisms of injury.⁷

Vitamin C

As with other antioxidant compounds, Vitamin C is mostly found within red grape varieties. Vitamin C acts to neutralise free radicals and also has roles in supporting the immune system, especially in the protection of the kidney against COVID-19.⁸

Incorporating grapes into the diet

Fresh grapes are a great snack. They are convenient and tasty; a simple way to add a healthy option to your diet. If you’re looking for something a little different, try freezing them. These make for an ideal snack on a hot day. If you want something a little more extravagant, thread grapes along some kebab skewers with some strawberries, pineapple, and watermelon - a refreshing summer snack, hassle-free, delicious, and more importantly, good for your kidneys!

Snacking is easy, but adapting your diet to every single new piece of health information can seem like a chore. Instead, add them to meals you are already making. Mixing savoury and sweet doesn’t always appeal to all (thoughts on ham and pineapple pizza?), but grapes make the perfect accompaniment to a leafy green salad. Try adding walnuts and feta for a sweet and salty combination. In the same vein (or vine), add some grapes to your cheeseboard. Nothing says an elegant appetiser/ dessert/ mid-afternoon snack like a beautiful charcuterie board. If you’re looking for something a little less indulgent and slightly more guilt-free, blend grapes with yoghurt, spinach, and other fruits, for a refreshing smoothie.

Grapes can also be blended to make homemade grape juice. This is often much healthier than shop-bought versions, with no added sugars. However you must be wary, although juice may be a convenient way to incorporate grapes into your diet, it lacks the same fibre as raw grapes, and so sugars are more readily absorbed. 

Raisins are also another way of incorporating grapes into your diet. Again, however, be wary, as they contain a lower fibre content and a higher sugar content than raw grapes.

Grapes can be a great addition to your diet, as a fantastic source of antioxidants and anti-inflammatories. They also help to satisfy those sweet cravings, whilst helping to keep you hydrated! Consumption of grapes as part of a balanced diet is important, and a healthcare professional should be consulted for dietary guidance.

Precautions and considerations

Allergies and potential side effects

Although not common, allergies to grapes can occur. Be mindful of the symptoms of an allergic reactions . In cases of allergic reactions, seek medical attention immediately. Unless grown organically, grape skin may be coated with pesticide residues. In sensitive individuals, contact with residues may cause skin irritation, and ingestion can cause nausea and gastrointestinal upset. As with any fibre-rich foods, grapes should be eaten in moderation, as overconsumption can lead to gastrointestinal discomfort, including diarrhoea and gas. Although grapes are considered a low-calorie, low-carbohydrate food, they still contain sugars. Consumed in large quantities, grapes may cause tooth decay, especially in instances where good oral hygiene is not maintained.

Suitability for individuals with certain medical conditions

Although representing a renal-friendly option, grapes may not be a suitable option for all, especially in the case of detailed medical conditions:

• Gout: Grapes contain purines, compounds known to exacerbate the symptoms of gout. Large quantities of grapes, and therefore ingestion of foods high in purines, may worsen symptoms, including chronic pain;
• Gastro-oesophageal Reflux Disease: The acidic content of grapes may trigger the symptoms of acid reflux;
• Irritable Bowel Syndrome (IBS): Fructose is a type of sugar, often poorly digested in people with IBS. Grapes can contain large quantities of fructose, and so may cause irritation and discomfort if large portions are consumed.

It is always best to check with your healthcare provider in case of interactions between grapes (and products of grapes) and the medications you are taking.

Summary

Grapes are an attractive addition to a person’s diet to maximise kidney health. These small fruits are packed with water, antioxidants, and anti-inflammatory molecules, which all contribute to the maintenance of kidney function and protection against renal damage. Ensure to always consult a medical or nutritional professional for personalised guidance before attempting to alter your diet.