Kidneys, kidney disease and protein
Most
people have two bean-shaped kidneys, each about the size of a clenched
fist, in the rear left and right sides of their torso, just below their
ribs. They have many essential functions (e.g., regulating blood
pressure, producing hormones and activating vitamin D), but most
importantly they filter our blood and remove excess body fluids and
wastes for elimination in the urine. They are able to balance out body
fluid, electrolyte (salts) and acid (pH) levels so that all of our
organs function optimally, despite our consumption of a variety of
fluids from foods and beverages, and fluid losses from physical activity
and sweating. Here’s how they do it.
NORMAL KIDNEY FUNCTION
Blood flows from the aorta
(the main blood vessel from the heart) into the kidneys via the renal
artery. Within the kidney, the parts that filter the blood are called
nephrons. Within each nephron is a glomerulus, a bulb-like capsule which
contains tiny blood vessels that look a bit like a loosely wound ball
of wool.
When the kidney is functioning normally, these
tiny blood vessels have a large number of fine holes. They work a bit
like sieves, allowing water, some salts (e.g., sodium, potassium,
calcium, phosphorus) and waste products to pass through, but they are
too small to allow red and white blood cells, and most blood proteins,
to leak out. The exact composition of the filtered fluids (filtrate)
depends on the body’s requirements for essential minerals like sodium
and potassium, the acid-base balance (pH), and concentration of wastes
from general metabolism.
Eventually, the filtrate from
each nephron flows together and enters collecting ducts within the
kidney, where the concentration of the final urine product is
determined. The urine then travels through the ureters to the urinary
bladder for temporary storage before voluntary urination.
Kidney
function is evaluated based on the glomerular filtration rate (the rate
at which the kidneys form filtrate) and the amount of the protein
albumin lost in the urine. In health, glomerular filtration rates are
greater than 90mL/minute, and there is essentially no albumin in the
urine.
KIDNEY DISEASES
Nephrotic syndrome
occurs when the glomeruli are damaged, increasing the size of the
filtration holes, and therefore decreasing their ability to prevent
proteins (e.g., albumin, lipoproteins, clotting factors and
immunoglobulins) from escaping into the urine. The loss of these
proteins can lead to serious health problems including oedema, high
cholesterol levels, blood clotting and immune issues. Nephrotic syndrome
can be caused by infections, immune disorders, chemical damage (from
medications or illicit drugs), or poorly managed diabetes.
Chronic kidney disease
is characterised by gradual, irreversible deterioration of the
nephrons. Because we have a lot of nephrons, chronic kidney disease
develops over many years without causing any symptoms. People are
therefore often diagnosed late in the course of the illness, after most
kidney function has been lost. Like the nephrotic syndrome, damage to
the blood vessels in the nephrons leads to excessive loss of proteins in
the urine, which can lead to serious health problems including oedema,
high cholesterol, blood clotting and immune issues. Additionally, the
nephrons lose their ability to maintain electrolyte (e.g., sodium and
potassium) levels, acid-base balance (e.g., uric acid and phosphorus
levels) and uraemia (the level of urea – the waste product of protein
metabolism – in the blood). The more common causes of chronic kidney
disease include diabetes and hypertension (chronically high blood
pressure).
PROTEIN AND KIDNEY FUNCTION
The
Recommended Dietary Intake (RDI) of protein for women is 0.75 g per kg
body weight, and for men it is 0.84g per kg. High protein diets provide
more than 1.2g per kg body weight and lower protein diets less than 0.6 g
per kg body weight.
Unlike carbohydrate which is
stored as glycogen in our muscles and liver, and fat which is stored as
triglycerides in our fat cells, we have a limited capacity to store
protein. If we eat more protein than our body’s require, some of it can
be converted to glucose in the process of gluconeogenesis (see July 2019
edition of GI News). The metabolic process of converting proteins (or
more specifically amino acids – the building blocks of proteins) to
glucose leads to the production of ammonia, which in turn is converted
to urea and excreted in the urine.
HIGH PROTEIN DIETS AND KIDNEY FUNCTION
High
protein diets increase glomerular filtration rates, which increases the
pressure within the glomerulus and may cause damage in susceptible
people. High protein diets also increase urea production. On the other
hand, lower protein diets decrease the pressure within the glomerulus
and produce less urea.
The highest quality evidence
available to date suggests that high protein diets do not have a
negative effect on the glomerulus in people with healthy kidneys as the
glomerulus is able to cope with the increased pressure and higher urea
content.
However, in people with nephropathy/chronic
kidney disease, the increased glomerular pressure caused by a high
protein diet may lead to increased protein (e.g., albumin) loss in the
urine and consequently increase the rate of progression of kidney
disease. Therefore, a lower protein diet of 0.6–0.8 per kg body weight
is generally recommended to people with existing nephropathy/chronic
kidney disease. The problem is, chronic kidney disease develops over
many years without causing any symptoms, so some high-risk people (e.g.,
people with diabetes) may have the condition but not know it.
Finally,
for people with nephropathy/chronic kidney disease, approximately half
of the proteins should come from plant sources (e.g., beans, lentils,
chickpeas, etc...), as there is some evidence that they place less
stress on the kidneys than animal sources.
Read more:
- PROTEIN QandA: OUR EXPERTS ANSWER YOUR QUESTIONS
- Nutrient Reference Values: Protein
- High-protein diet is bad for kidney health: unleashing the taboo
- Changes in Kidney Function Do Not Differ between Healthy Adults Consuming Higher- Compared with Lower- or Normal-Protein Diets: A Systematic Review and Meta-Analysis.
- Efficacy of low-protein diet in diabetic nephropathy: a meta-analysis of randomized controlled trials
- Effect of whole soy and purified isoflavone daidzein on renal function--a 6-month randomized controlled trial in equol-producing postmenopausal women with prehypertension.
Alan Barclay, PhD is a consultant dietitian and chef (Cert III). He worked for Diabetes Australia (NSW) from 1998–2014 . He is author/co-author of more than 30 scientific publications, and author/co-author of The good Carbs Cookbook (Murdoch Books), Reversing Diabetes (Murdoch Books), The Low GI Diet: Managing Type 2 Diabetes (Hachette Australia) and The Ultimate Guide to Sugars and Sweeteners (The Experiment, New York).
Contact: You can follow him on Twitter or check out his website.
