Influence of a low- and a high-oxalate vegetarian diet on intestinal oxalate absorption and urinary excretion

Optimization of Plant Oxalate Quantification and Generation of Low-Oxalate Maize (Zea mays L.) through O7 Overexpression

Oxalate, the simplest dicarboxylic acid, is a prevalent antinutrient that chelates with various metals and can lead to the formation of kidney stones in humans. The accurate detection of the oxalate concentration in food and the cultivation of low-oxalate crops are important for enhancing public health. In this study, we established a high-throughput and highly sensitive technique for oxalate detection using ultra-high-performance liquid chromatographic-triple quadrupole tandem mass spectrometry (UPLC-QqQ-MS/MS). Additionally, we overexpressed the gene O7, which encodes oxalyl-CoA synthetase in the maize oxalate degradation pathway, resulting in O7-OE lines. By employing the UPLC-QqQ-MS/MS method to measure oxalate levels in these transgenic lines, we observed that the oxalate content in the kernels of O7-OE lines was reduced by approximately 43%, with a concurrent increase in some micronutrients such as zinc. Importantly, the transgenic maize showed normal seed storage compound accumulation or other agronomic characteristics. In summary, we developed a high-throughput detection method that advances oxalate measurement. Furthermore, by generating new maize germplasm with diminished oxalate, our work offers potential health advantages to consumers.

The Relationship between Modern Fad Diets and Kidney Stone Disease: A Systematic Review of Literature

OBJECTIVES

Kidney stone disease (KSD) has a strong association with diet metabolic syndrome. This review aims at exploring the lithogenic risk posed by the current most popular diets. Our approach was to search for the effect of each diet type on the major urinary risk factors, to try to draw conclusions regarding the association of a specific diet type and KSD.

METHODS

This systematic review searched for the available literature exploring the association between the existing popular fad diets and KSD. Articles in English, French and Spanish were included, without restriction of the search period with the final search done in August 2021.

RESULTS

Total number of studies and studies for each diet type was as follows: 22 articles for the low carbohydrate diet, 20 articles for high protein diets, 26 articles for vegetarian and vegan diets. There exists a substantial variability in different low carbohydrate and high protein diets, and considerable overlap between modern popular fad diets. High carbohydrate intake might increase urine uric acid, calcium and oxalate levels. High protein diets increase urine calcium and uric acid and lower urine pH and citrate. Consumption of fruits and vegetables increases the urinary volume and urinary citrate. In vegan diets, sufficient daily calcium intake is important to avoid possible secondary hyperoxaluria.

CONCLUSIONS

Few studies evaluated the direct relationship between modern fad diets and KSD. In general, the reduction of carbohydrate in the diet, and counterbalancing protein rich diets with sufficient intake of fruits and vegetables, seem to play a protective role against KSD formation. Maintaining sufficient calcium intake in vegan and vegetarian diets is important. Additional research is needed to directly evaluate the link between KSD and each diet type.

Calcium Oxalate Nephrolithiasis and Gut Microbiota: Not just a Gut-Kidney Axis. A Nutritional Perspective

Recent studies have shown that patients with kidney stone disease, and particularly calcium oxalate nephrolithiasis, exhibit dysbiosis in their fecal and urinary microbiota compared with controls. The alterations of microbiota go far beyond the simple presence and representation of Oxalobacter formigenes, a well-known symbiont exhibiting a marked capacity of degrading dietary oxalate and stimulating oxalate secretion by the gut mucosa. Thus, alterations of the intestinal microbiota may be involved in the pathophysiology of calcium kidney stones. However, the role of nutrition in this gut-kidney axis is still unknown, even if nutritional imbalances, such as poor hydration, high salt, and animal protein intake and reduced fruit and vegetable intake, are well-known risk factors for kidney stones. In this narrative review, we provide an overview of the gut-kidney axis in nephrolithiasis from a nutritional perspective, summarizing the evidence supporting the role of nutrition in the modulation of microbiota composition, and their relevance for the modulation of lithogenic risk.

Epidemiology of stone disease across the world

Nephrolithiasis is a highly prevalent disease worldwide with rates ranging from 7 to 13% in North America, 5-9% in Europe, and 1-5% in Asia. Due to high rates of new and recurrent stones, management of stones is expensive and the disease has a high level of acute and chronic morbidity. The goal of this study is to review the epidemiology of stone disease in order to improve patient care. A review of the literature was conducted through a search on Pubmed^®, Medline^®, and Google Scholar^®. This review was presented and peer-reviewed at the 3rd International Consultation on Stone Disease during the 2014 Société Internationale d'Urologie Congress in Glasgow. It represents an update of the 2008 consensus document based on expert opinion of the most relevant studies. There has been a rising incidence in stone disease throughout the world with a narrowing of the gender gap. Increased stone prevalence has been attributed to population growth and increases in obesity and diabetes. General dietary recommendations of increased fluid, decreased salt, and moderate intake of protein have not changed. However, specific recommended values have either changed or are more frequently reported. Geography and environment influenced the likelihood of stone disease and more information is needed regarding stone disease in a large portion of the world including Asia and Africa. Randomized controlled studies are lacking but are necessary to improve recommendations regarding diet and fluid intake. Understanding the impact of associated conditions that are rapidly increasing will improve the prevention of stone disease.

Fad diets and their effect on urinary stone formation

The influence of unhealthy dietary habits on urinary stone formation has been widely recognized in literature. Dietary advice is indeed the cornerstone prescription for prevention of nephrolithiasis as well. However, only a small amount of medical literature has addressed the influence of popular or fad diets, often self-prescribed for the management of obesity and overweight or for cultural beliefs, on the risk of kidney stones. Thereby in this paper we analyze the current knowledge on the effects of some popular diets on overall lithogenic risk. High-protein diets, like Dukan diet, raise some concerns, since animal proteins are able to increase urinary calcium and to decrease urinary citrate excretion, thus leading to a high overall lithogenic risk. Low-carbohydrate diets, like Atkins diet or zone diet, may have a protective role against kidney stone formation, but there are also evidences stating that this dietary approach may rise calciuria and decrease citraturia, since it is generally associated to a relatively high intake of animal proteins. Vegan diet can be harmful for urinary stone disease, especially for the risk of hyperuricemia and micronutrient deficiencies, even if only few studies have addressed this specific matter. On the other side, the benefits of a lacto-ovo-vegetarian diet on kidney stone prevention have been largely emphasized, provided that the intake of calcium and oxalate is balanced. Traditional Mediterranean diet should exert a protective effect on nephrolithiasis as well, even if specific studies have not been carried out yet. High phytate and antioxidant content of this diet have however demonstrated to be beneficial in preventing the formation of new or recurrent calculi. Anyway, at the current state of knowledge, the most effective dietary approach to prevent kidney stone disease is a mild animal protein restriction, a balanced intake of carbohydrates and fats and a high intake of fruit and vegetables. Other fundamental aspects, which are often neglected in fad diets, are a normal intake of milk and dairy products and salt restriction. All these nutritional aspects should be greatly taken into account when patients who are willing to undergo fad or commercial diets ask for dietary advice.

Intestinal permeability in subjects from two different race groups with diverse stone-risk profiles

It is well established that calcium oxalate stones may be caused by colonic or ileum oxalate (Ox) hyperabsorption (secondary to intestinal dysfunction). Studies have reported that increased intestinal permeability (IP) can cause hyperabsorption of nutrients culminating in passive diffusion of Ox. In South Africa, renal stones occur in the white population (W) but are extremely rare in the black population (B). Previous studies have shown that despite B having a hyperoxalurogenic diet relative to W, urinary Ox in the former is not higher. It has been suggested that different Ox handling mechanisms in the groups are the cause of this disparity. The present study was undertaken to examine whether the IP index, a reliable and accurate measure of intestinal integrity, plays a role in this anomaly. Ten healthy males from each group ingested a dual-sugar isotonic solution containing 5 g lactulose (LA) and 2 g mannitol (MA). IP was assessed by comparing the LA:MA ratio in 5 h urine samples using high performance anion exchange chromatography coupled with pulse amperometric detection to measure the concentration of each sugar. 24 h dietary intake and urine composition were also determined. LA excretion was identical in both groups (0.03 %) while MA excretion was 8.3 % in B and 11.3 % in W. IP index was 0.004 for B and 0.003 for W. It is concluded that IP is not a contributory factor in the apparent different handling of dietary Ox in B and W South Africans. It is speculated that differences in renal transporters may play a role.

Oxalate in renal stone disease: the terminal metabolite that just won't go away

The incidence of calcium oxalate nephrolithiasis in the US has been increasing throughout the past three decades. Biopsy studies show that both calcium oxalate nephrolithiasis and nephrocalcinosis probably occur by different mechanisms in different subsets of patients. Before more-effective medical therapies can be developed for these conditions, we must understand the mechanisms governing the transport and excretion of oxalate and the interactions of the ion in general and renal physiology. Blood oxalate derives from diet, degradation of ascorbate, and production by the liver and erythrocytes. In mammals, oxalate is a terminal metabolite that must be excreted or sequestered. The kidneys are the primary route of excretion and the site of oxalate's only known function. Oxalate stimulates the uptake of chloride, water, and sodium by the proximal tubule through the exchange of oxalate for sulfate or chloride via the solute carrier SLC26A6. Fecal excretion of oxalate is stimulated by hyperoxalemia in rodents, but no similar phenomenon has been observed in humans. Studies in which rats were treated with (14)C-oxalate have shown that less than 2% of a chronic oxalate load accumulates in the internal organs, plasma, and skeleton. These studies have also demonstrated that there is interindividual variability in the accumulation of oxalate, especially by the kidney. This Review summarizes the transport and function of oxalate in mammalian physiology and the ion's potential roles in nephrolithiasis and nephrocalcinosis.