Over the last few decades, we have seen the accumulation of evidence that supports the health benefits of plant-based diets. Vegetarian diets have been shown to be associated with a lower incidence of obesity, hypertension, diabetes, and coronary artery disease. Since all these entities are risk factors for chronic kidney disease (CKD), it might be reasonable to assume that these diets might actively reduce the risk of CKD development and progression. However, at the very outset, let me emphasize that any diet, vegetarian or not, comes with the same restrictions that are advised for patients with CKD. For instance, potassium intake might need to be restricted in advanced CKD. Fruits, a significant portion of the vegetarian diet, are an important source of potassium, and will need to be appropriately restricted. So you have to watch what you eat regardless. More importantly, you owe it to yourself to know what exactly is there in the food that you eat.
Given the observational nature of the above studies, it is technically hard to delineate the exact reasons for this association, and the extent to which the results could be confounded by lifestyle. In my personal experience, I have often observed patients switch to vegetarianism in parallel with other positive lifestyle changes like exercising, giving up smoking, or just being proactive about their health. The studies are often non-standardized, which makes it hard to determine the role of varying levels of restriction of animal sourced foods in the diet. For instance, is a vegan diet (when all animal products including milk, eggs, and sometimes even honey, are excluded from the diet) necessarily better than a lacto-vegetarian (milk allowed) or lacto-ovovegetarian (milk and eggs allowed) diet? Finally, is the beneficial effect on kidney disease a result of the source of the protein (plant vs animal), or just a function of the amount of protein contained in these sources? If two patients consumed the same amount of protein, one from a plant and the other from an animal source, who would see a more favorable outcome for their kidney disease? These are hard questions to answer, but lets look at the evidence, with the focus being on kidney disease.
ARE HUMANS SUPPOSED TO EAT MEAT ANYWAY?
Lets segue briefly in to this anthropological evolutionary question. I remember a medical school professor who firmly believed that eating meat was an unnatural, relatively modern concept, and that we as humans are just not programmed to be able to process the animal protein that we so often eat. He would show us the human teeth sets; much closer in appearance to plant eating animals like cows, than to meat eaters like tigers. However, this assumption has been changing over the last decade. Anthropologists at the University of Berkeley found that not only were humans eating meat as early as 2 million years ago, they were actually doing so on a regular basis and not just occasionally. Whats more, this food preference could have played a big rule in transforming early humans in to the "sociable, intelligent creatures of today". Vegetables are not as calorie dense as meat, and have much less fat. This sounds good today, but for the early cave dweller it meant expending most of his energy trying to digest the huge amount of plant material in his gut, energy that could perhaps be better used by the brain. With meat consumption, the volume of food and the gut size shrank, and so did the energy required to complete the digestive process. This energy windfall benefited the brain which purportedly uses 20 times energy as an equivalent amount of muscle, as per this article! So meat, with its densely packed nutrients was one of many catalysts of human evolution, and apparently made our ancestors smarter. In fact, lack of meat lead to well recognized nutritional deficiencies even a million years ago. Humans and meat go back a long way, and it is hard to argue that humans have lost more than they have gained.
|Are plant based diets necessarily better than non-vegetarian diets?|
Image courtesy of BrianHolm/ FreeDigitalPhotos.net
IS IT THE PROTEIN AMOUNT, OR ITS SOURCE, THAT INFLUENCES THE PROGRESSION OF CKD? THE CONCEPT OF "BIOLOGICAL VALUE"
Traditionally, and before the availability of dialysis, protein restriction in the diet was the core of management of advanced kidney disease/kidney failure. Protein restriction may have a protective effect on kidney function by reducing glomerular hyperfiltration (a phenomenon that happens in patients with kidney disease), and by reducing the production of hormones that are produced in excess as a result of that (these include insulin-like growth factor, glucagon, angiotensin II, etc). However, these protective benefits have never quite panned out as expected, with results from human trials on low protein diet being pretty conflicting. These studies have variously demonstrated a statistically significant reduction in progression of CKD, a modest non-significant reduction, and small to no benefit on progression of CKD. A study from Taiwan showed no differences in the renal function of vegetarians vs omnivores. I came across a review that vouched for the benefits of a low protein diet, while a recent meta-analysis of relevant clinical trials actually showed no reduction in mortality or need for dialysis.
My take home message from this would be: a low protein diet (about 0.6-0.8 g/day of protein intake) may benefit some patients with CKD, although it could be a modest/insignificant benefit. However, with low protein comes great responsibility. The responsibility to ensure that the protein is of a high biological value (the fraction of absorbed protein that is eventually incorporated into the body; for instance, the value for eggs is a perfect 100, while kidney beans score a low 49). In other words, the benefit from eating eggs vs. beans, even if the ingested face value of the protein content in these two foods is similar, might not be the same! Finally, severe protein restriction could actually do more harm than good. A very low protein diet (about 0.3 g/day of protein intake) is associated with increased risk of death!
VEGETARIAN AND MEAT DIETS COMPARED: THE PHOSPHORUS ANGLE
The components of food are not found in isolation in nature. The protein that you eat often comes packaged with phosphorus; and typically, animal protein and dairy has more phosphorus content. Elevated phosphorus levels in the blood can be seen in advanced CKD. High phosphorus load in diet can lead to increased production of a hormone called FGF-23 which acts to lower phosphorus levels in the blood by increasing phosphorus excretion from the kidneys. FGF-23 itself has been postulated to play a role in the progression of CKD. Could the beneficial effects of vegetarian diet be linked, at least in part, to the lower phosphorus load that comes with the ingested plant-protein? Perhaps. In a major study published in 2010, researchers showed that just one week of vegetarian diet lead to lower phosphorus and FGF-23 levels in the blood. Again, this underscores the importance of not only how much protein you eat, but also where it comes from, that has an influence on CKD progression.
Stricter levels of restriction of animal protein in the diet may come with its own set of drawbacks, and is debatable. For instance, there is evidence to indicate that a lactovegetarian diet where milk is part of the diet, may be more beneficial than being a vegan. Without vitamin D/calcium supplementation, vegans could risk low bone density and increased risk of fractures. It is for these reasons that I make sure that my patients are regularly followed by a renal dietitian who can assess the diet plan to prevent these nutritional deficiencies. If you have this support, I don't see why anyone couldn't go vegan if they wished to, without exposing themselves to the expected nutritional pitfalls.
Keeping the available evidence in mind, you could possibly conclude that vegetarian diets, with the appropriate nutritional supplementation to prevent known deficiencies, may slow down the progression of CKD. However, playing the devil's advocate, one could also argue that the health benefits of vegetarianism are not unique, and one could possibly gain the same benefit by following a prudent plant based diet while still allowing for the occasional intake of meat, eggs, and dairy, aka the DASH diet. The jury is still out on this. In the mean time, I will recommend that let common sense rule. Don't eat a high salt/high fat diet, exercise, don't smoke, and stay out of trouble.
Veeraish Chauhan, MD, FACP, FASN