Low back pain is unlikely to be associated with the kidneys. They're located higher up, above the waist .. kidney pain typically radiates out to the sides, the so-called flank region.
Carbs (and ketones for that matter) don't have any effect on the kidneys EXCEPT ... chronically high levels of sugar in the blood will cause damage. Glucose molecules are large and can damage the kidneys' filtering cells as they push through.
The only diet-related thing I can think of that might trigger low back pain is constipation or gas or something intestinal.
Doreen
from ..
http://health.yahoo.com/health/ency...a/003108/0.html
Low Back Pain: Common Causes
Muscle strain, pulled muscle in the back or similar injury (the most common cause)
Muscle aches related to a viral infection
Menstruation or premenstrual syndrome (PMS)
Prolonged sitting in a person who is susceptible to back problems
Arthritis
Poor posture
Nonspecific back pain
In children, causes of back pain include the following:
Scoliosis
Scheuermann's kyphosis
Spondylolysis
Less common but serious causes of back pain include the following:
Bladder infection
Endometriosis
Ovarian cancer
Ovarian cysts
Pressure on a nerve root (for example, spinal stenosis or herniated nucleus pulposus)
Ruptured vertebral disk