Review of the plant-based nutrition news April 2023: Chronic kidney disease

To mark the release of our first of 3 factsheets on plant-based diets and chronic kidney disease, I summarise recent data on the benefits of eating more plants.

Until recently the dogma has been that plant-based diets are not suitable for patients with kidney disease. This is now changing. Evidence suggests that predominately plant-based diets are not only associated with a lower risk of developing chronic kidney disease (CKD), but also may slow the progression of already established kidney failure and delay the need for dialysis. The National Kidney Foundation in the US have been promoting plant-based diets for quite a while, so it is great that the UK is now catching up with these recommendations.

Prospective cohort studies have convincingly show that diets centred around whole plant foods, which also minimise meat, are associated with a lower risk of developing CKD. A large observational study published in the Clinical Journal of American Society of Nephrology provides high quality data supporting plant-based diets for the prevention of CKD. The study of more than 14,000 adults enrolled in the Atherosclerosis Risk in Communities study followed for 24 years examined the association between adherence to the plant-based diet index and the risk of CKD. The results showed that adherence to a healthy plant-based diet was associated with better kidney function and prevented a decline in kidney function over time. A high plant-based diet score was associated with a 14% reduction in the risk of developing CKD. It should be noted that an unhealthy plant-based diet, defined as being high in refined grains and sugar and other processed foods, increased the risk of CKD.

There are a number of reasons why plant-based diets may benefit kidney health. The lower protein content puts less of a strain on the kidneys and reduces the production of nitrogenous waste. Plant-based diets prevent the risk factors for developing kidney failure, including type 2 diabetes, hypertension and cardiovascular disease, partly due to a higher intake of potassium and fibre and of course all the other beneficial phytonutrients. Plant-based diets also support the health of the gut microbiota, considered to be essential in maintaining overall health, including kidney health. Thus the consensus for dietary recommendations for preserving kidney function and improving kidney disease outcomes is a plant-dominant, low protein diet or PLADO. The acronym PLADO (Plant-Dominant Low Protein Diet) is being used to describe the optimal diet for preserving kidney health.

Check out our new factsheet which summarises the evidence-based dietary recommendations for people with CKD stage 1–5. Further factsheets for dialysis and kidney transplant will be published very soon.

RISKS AND BENEFITS OF DIETARY PATTERNS: This review summarises what we know about diet and kidney health. Evidence for various diet patterns is reviewed, including Mediterranean, low-carbohydrate and ketogenic diets, Western diets, plant-based diets and intermittent fasting. Overall, the weight of evidence supports a diet pattern that is high in unprocessed whole plant foods, whilst minimising/avoiding animal foods. This is because plant-based diets are high in fibre and unsaturated fatty acids, lower in protein and low in saturated fat. Although low carbohydrate/ketogenic diets can be associated with weight loss and better glucose regulation in the short term, the concern is that the higher protein and saturated fat intake can increase the work of the kidneys, increase the acid load, elevate blood cholesterol levels and in the longer term increase the risk of cardiovascular disease, cancer and early death. There are currently no data on low carbohydrate plant-based diets and CKD. There is currently insufficient evidence for recommending intermittent fasting in the setting of CKD.

The authors conclude ‘a recurring theme of these healthy diets is the consumption of unprocessed, whole, plant-based foods, with a growing body of literature showing the potential benefits for patients with CKD’.

NUTS CONSUMPTION AND CKD: Regular consumption of nuts is associated with a positive impact for several health outcomes. They are nutrient-dense foods that are rich in minerals, vitamins, unsaturated fatty acids, and fibre. The high mineral content of nuts has traditionally led to unsubstantiated recommendations to avoid nuts in CKD. Nuts have high phosphorus, potassium and magnesium content, which are nutrients that are often restricted in advanced CKD. However, for a number of reasons, the high content of these nutrients is not of clinical concern and may in fact be beneficial.

The current cross-sectional study provides reassurance for the continued promotion of nut consumption in CKD. Nut consumption was assessed in 6,072 adults from the US National Health and Nutrition Examination Study (NHANES) cohort surveyed from 2003–2006. There were 1,203 participants with CKD and 4,869 participants without CKD. Frequency of nut consumption in the preceding 12 months was categorised as (1) never, (2) 1–11 times per year, (3) 1–3 times per month, (4) 1–6 times per week, and (5) more than once a day. Those who have never consumed nuts were used as the reference group. Consuming nuts 1–6 times per week was associated with a 33% lower prevalence of CKD after adjusting for confounding factors, including socio-demographics, lifestyle factors (smoking, alcohol use) and presence of other chronic conditions. Higher nut consumption was significantly associated with lower all-cause (40% reduction) and CVD mortality (57% reduction) in non-CKD participants. For mortality risk there was a greater benefit in those consuming nuts more than once per day, but this was not the case for prevention of CKD where 1–6 times per week appeared to be the optimal dose. In participants with CKD, nut consumption 1–6 times per week was associated with a 37% lower risk of all-cause mortality although there did not appear to be a benefit for CVD mortality.

Although there are limitations to this type of observational study, it does seem that consumption of nuts in line with healthy eating recommendations, such as those outlined in the planetary health diet, is a good choice for prevention of CKD and for those with established CKD.

POTASSIUM AVAILABILITY FROM PLANT FOODS: The high level of potassium in many plant foods has always been a concern for people with CKD and traditional recommendations have included restricting the consumption of high potassium plant foods. However, studies have not shown an adverse effect on blood potassium levels in people with CKD who are consuming a predominately plant-based diet. The reason is thought to be due to the alkalinising effects of plant-based diets. In addition, potassium from plants may promote more intracellular distribution and faecal excretion of potassium, the latter being related to the high fibre content of plant foods. If high potassium remains a problem, reducing the content of potassium in plant foods can be achieved through adequate cooking techniques, such as blanching and boiling. Concerns about potassium content should not be a reason to restrict plant foods in people with CKD.

The study highlighted supports this viewpoint. The aim of this study was to assess the bioaccessibility (the amount of an ingested nutrient that is available for absorption in the gut after digestion) of potassium from fruits and vegetables and assessing all aspects of the plant (fruit, flower, root, tuber, leaf and seed). The researchers also investigated the impact of different boiling techniques on potassium content and bioaccessibility of these plant foods. Bioaccessibility was evaluated by an in vitro digestion methodology, resembling human gastro-intestinal tract.

The results showed that potassium content was higher in the seeds and leaves of the plant. Boiling reduced potassium content in all vegetables excluding carrot, zucchini, and cauliflower. Boiling starting from cold water resulted a greater reduction of the potassium content in potato, peas, and beans. Bioaccessibility after in vitro digestion ranged from 12 (peas) to 93% (tomato). Higher bioaccessibility was found in spinach, chicory, zucchini, tomato, kiwi, and cauliflower, and lower bioaccessibility in peas. Overall, bioaccessibility in the fruits and vegetables studied was 67% on average with boiling further reducing this. The authors conclude that, ‘This supports the clinical advice to maintain a wide use of plant-based food in the management of renal patients’.

DIETARY PHOSPHATE IN ADULTS ON HAEMODIALYSIS: As CKD advances, the kidneys are less able to excrete phosphorus, which leads to secondary compensatory effects, including hyperparathyroidism. High level of blood phosphorus is an independent predictor for mortality in patients with advanced renal failure. It is recommended that dietary phosphate consumption is restricted.

Phosphorus in food products exists in two forms: organic and inorganic. Organic phosphorus, or naturally occurring phosphorus, can be found in animal and plant foods such as seeds, nuts, and legumes. A concern about plant-based diets is that they are higher in phosphate. However, this has not been shown in clinical studies to be a genuine concern. The phosphate in plant foods is less bioavailable as it is mostly in the form of phytate, which humans are unable to digest, thereby decreasing the bioavailability of phosphorus in these foods. Inorganic phosphorus, such as phosphates added to foods during processing, has an absorption rate of greater than 90%. Therefore, most of the phosphate consumed in a Western-style diet pattern is from dairy and animal protein and also from the preservatives used in processed and fast foods. When sources of phosphate in the diet have been studied, plant-based sources are in fact associated with a reduction in mortality in people with kidney failure.

This study specifically looked at dietary phosphorus and its sources in relation to risk of death in 8110 adults on dialysis. During the median of 3.8 years of follow-up there were 2953 deaths, 1160 cardiovascular-related. The median phosphorus intake was 1388 mg/day. There was a dose relationship noted, in that the higher the phosphorus intake from animal sources and processed foods, the greater the risk of death. However, the higher the intake of phosphorus from plant sources the lower the risk of death. A higher total phosphorus intake was associated with increased all-cause and cardiovascular death, but this was driven by the phosphorus intake from processed food (such as sauces, cakes, and drinks). Plant based phosphorus was associated with lower all-cause mortality. The authors conclude ‘our findings suggest a survival benefit of controlling the total phosphorus intake by limiting the intake from processed and other sources in favour of the intake from plant sources’.

ULTRA-PROCESSED FOODS (UPFs). Ultra-processed food consumption is associated with a number of health concerns, including increased risks of overweight/obesity, cardiovascular disease, type 2 diabetes and possibly even cancer. This study specifically examines the association between UPF consumption and risk of death in people with a kidney transplant. Even though it is a relatively small cohort study involving just 632 patients, there was a clear relationship between the consumption of UPFs and increased risk of death during the 5.4 years of follow up. It’s worth noting that this was despite the fact that compared to the US and UK population who consume more than 50% of calories from UFPs, this Swedish population consumed only 28% of calories from UPFs. The increased risk of death was particularly related to the consumption of sugar-sweetened beverages, desserts, and processed meats and this was independent of the overall diet quality. Intake of UFPs was associated with a higher risk of renal function decline, but not with graft failure or post-transplant diabetes mellitus.

There is no doubt that UPFs are best minimised in any chosen diet pattern. Along with this study in people with kidney transplants, prior studies show that UPFs are associated with a greater and faster decline in kidney function and thus increased risk of chronic kidney disease in a pretty much dose-dependent fashion. Learn everything you need to know about UFPs from this factsheet.

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