I often tell my patients that a significant aspect of kidney disease treatment
really boils down to what they eat (or what they don't eat). Everything
that a nephrologist would do in terms of drugs and testing can get
negated in one swoop if you do not pay attention to your diet. The
typical example is the diabetic who eats too many carbs, or the
hypertensive loading up on salt. I have tried to answer questions like,
"how much salt or protein a patient with kidney disease should eat?", here and here.
Vegetarianism seems to an increasing trend in the US, with about 5% of American adults now
considering themselves vegetarian. But is it just a fad? Or is a
vegetarian (or vegan) diet really going to work wonders for you and your
kidneys?
WHY MIGHT A VEGETARIAN DIET IMPROVE KIDNEY FUNCTION
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.
Other unanswered issues include questions
like, 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-ovo-vegetarian (milk
and eggs allowed) diet? And, is the beneficial effect on kidney disease a
result of the source of the protein (plant vs animal), or does it
depend on 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.
I EAT MEAT, THEREFORE I AM
Whats more, this
food preference could have played a big rule in transforming early
humans in to "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.
MEAT AND PROTEIN INTAKE IN KIDNEY DISEASE: THE EVIDENCE IS CONFLICTING
Once
upon a time, and before the availability of dialysis, protein
restriction by cutting out meat in the diet was the core management of
advanced kidney disease/kidney failure. Protein restriction may have a
protective effect on kidney function via various mechanisms that are
beyond the scope of this article (if you really want to know, read the
role of glomerular hyperfiltration, a phenomenon that happens in
patients with kidney disease, and insulin-like growth factor, glucagon, angiotensin II, etc).
However, in practical terms, the protective benefits of cutting out meat 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 even a small to no benefit on progression of CKD. A study from Taiwan showed no differences in the kidney function of vegetarians vs others. I came across a review that vouched for the benefits of a low protein diet, even while a recent meta-analysis of relevant clinical trials actually showed no reduction in mortality or need for dialysis. Confused? So am I!
VEGETARIAN AND MEAT DIETS COMPARED: THE PHOSPHORUS & FGF-23 STORY
The components of food are not found in isolation in nature. The protein that you eat comes packaged with phosphorus;
and typically, animal protein and dairy has a higher 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
then, the beneficial effects of vegetarian diet on the kidney 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. This in turn might benefit
the kidney function. Again, this underscores the importance of not only
how much protein you eat, but also where it comes from, and how it could
all influence your risk of developing kidney disease.
COULD VEGETARIANISM BE BAD IN SPITE OF THE ABOVE?
Stricter
levels of restriction of animal protein in the diet may come with its
own set of drawbacks, and is therefore still debatable. For instance,
there is evidence to
indicate that a lacto-vegetarian diet where milk remains 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.
CONCLUSION
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. The
mechanisms are complicated but include differing ways in which
vegetarian diet influences phosphorus or uric acid metabolism. 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.
By Veeraish Chauhan, MD
Kidney Disease Expert
