I'm waiting to hear about the mineral content of the high protein low carbohydrate diet used to generate arteriosclerosis in mice. Personally I feel the whole study is totally irrelevant but I'm interested because (a) it is a major achievement in its own right and (b) it is being used politically to specifically warn against low carbohydrate diets. The press release is very cleverly done, anecdote and personal testimony is, I am very well aware, powerful stuff. Just like blogging really!
OK, it's time to talk about renal disease in rats and mice. I'm afraid quite a lot of it involves "models" but, well, that's just how it goes. Much of the information here comes from this paper. I think it took Malcolm Kendrick, to whom I am greatly indebted, about 5 minutes to find it on PubMed.
We're going to talk about CKD-MBD, that is Chronic Kidney Disease-Mineral Bone Disorder. CKD-MBD has three components: 1) abnormal serum biochemistries, 2) abnormal bone remodeling, and 3) vascular calcification.
Especially interesting is the last of these three, vascular calcification.
Using acute models of renal failure, particularly the 5/6ths nephrectomy, it is very very hard to demonstrate arterial calcification within the the lifetime of the animal unless you include a genetic modification, typically apoE-/- or LDLR-/- knockouts. So, if you wish to get calcification of arteries, choose an apoE-/- mouse as it speeds up the process. Adding a high content of phosphorus to the diet is also normal as this accelerates the kidney failure and is a known trigger for the conversion of a smooth muscle cell in to an osteoblast.
The phosphorus in the diet has to be bioavailable. That provided by grains is mostly there as phytic acid and, although rodents do have a phytase (humans don't), the amount of phosphate absorbed from grain based diets is much lower than from casein based diets.
In the face of early renal damage it seems like 0.7% phosphate in a casein diet (0.7g/100g) progresses the renal failure where as 0.2% keeps renal damaged animals similar to controls for many weeks. The control groups being given non bioavailable phosphate from grains.
So the next question you have to ask is whether rats or mice on lab chow have renal damage. Well, I blogged about that here.
Enough to say that nephrocalcinosis was absolutely routine until the NTP-2000 diet was introduced. This improved diet still does this:
"The NTP-2000 diet prevented nephrocalcinosis and decreased the severity of nephropathy and cardiomyopathy, the common lesions of F344 rats in 13-week studies"
Note the time scale for nephropathy. Less than 13 weeks. And this is a grain based diet with its phosphate mostly inaccessible to the rat/mouse. Whether you could get changes in the week between weaning and starting on a specialist diet is an interesting point.
But a casein based diet will allow phosphate uptake far better than a grain based diet. Two question then come to mind. Normal rat/mouse chow usually runs at just under 20% of calories from casein. Does increasing this to 45% increase the phosphate uptake? I don't know.
And perhaps you would still need to add supplementary phosphorus? Again I don't know.
So let us summarise: Casein based diets markedly facilitate the toxic action of phosphate on mildly damaged kidneys. Renal failure causes arterial calcification. Arterial calcification will occur in apoE knockout mice with renal failure under circumstances where it is impossible to get it with normal lipid metabolism. With enough tweaks it is possible to get arteriosclerosis in mice by careful manipulation of the model.
Epithelial progenitor cells are produced in the bone marrow under the influence of erythropoietin. Erythropoietin is produced by normal kidneys but only in reduced amounts by nephrotic kidneys. Vascular damage and regeneration appear to be heavily influenced by these cells.
If you wanted to make a diet for a mouse which triggered both arterial calcification and depressed EPC numbers a reasonable stratagem might be to feed a casein based high phosphate diet. To really make things happen choose an apoE knockout mouse. A normal mouse might not oblige.
You really need to know exactly how to manipulate renal function, genetics and EPC numbers in lab mice. This might not happen by accident and would require a great deal of knowledge about renal function and arterial damage.
I think that, for the time being, we will have to await the composition of both the mineral supplement used in the diets and the phosphate content of the calcium salts used as a partial replacement for that supplement in the high protein arm of the study.
But the thought train is interesting.
I was wishing for histopath on the kidneys but, if this hypothesis is correct, all that would have come out would have been that high protein diets damage kidneys.