On 09/17/2008 07:00 PM, Lynn Martin wrote:
> I feel embarrassed to ask this question because for all I know it's
> already been answered somewhere on the MoreLife site. But I've been
> so pressed with so many urgent tasks to do, including other things
> to research, that month after month I never go hunting for this on
> MoreLife. If it *is* answered there, I'll ask for your indulgence,
> and maybe someone will post the link. If it isn't, maybe my asking
> here will spark a discussion. So here goes:
>
> Spinach contains oxalates, which bind to calcium and keep the human
> body from assimilating the calcium.
>
> Do these oxalates make only the calcium in the spinach inaccessible
> to the body, or do they bind to calcium from other foods/supplements
> in the meal and make that calcium inaccessible as well?

I had to do a little research on this one to find out for sure. Here
are my results.
1) Spinach does contain a large amount of both soluble and insoluble
oxalates, of the order of 1% by weight of fresh spinach with the
soluble being about 4 times more than the insoluble. Most other greens
(eg kale) do not contain oxalates.
2) The soluble oxalates are the ones that are absorbed and can combine
with minerals. The insoluble oxalates are mostly not absorbed.
3) About 75% of the calcium in spinach is in the form of insoluble
oxalates and so is not absorbed.
4) The soluble oxalates (which are not attached to calcium) are
available in the digestive tract and after digestion to bind with
calcium and some other minerals. However, I found one study that
showed iron absorption reduction by oxalates is negligible and that
boiled spinach is a good source of magnesium. In spite of the latter,
spinach can diminish absorption of magnesium from other foods.
5) The soluble oxalates can be caused to leach from the spinach best
by boiling it and discarding the water (too bad because I have always
loved the taste of spinach water).
6) Another way to reduce the absorption of oxalates from spinach is to
eat calcium containing milk products with it. This will cause the
soluble oxalates to become insoluble by being bound to the calcium in
the milk products, and of course this will also reduce the amount of
calcium absorbed from the milk products.

So the answer to your question is yes to the second part. Oxalates do
bind calcium in other foods and presumably supplements if the
supplement calcium is not strongly bound to something else which
compound is also absorbable. The solution is to do one or more of the
following:
1) Eat mostly other leafy greens instead of spinach.
2) When eating spinach boil it most of the time.
3) Do not take calcium supplements or foods with a spinach meal.
4) Take sufficient extra calcium that the amount absorbed will average
out okay.
5) Eat spinach with milk products (creamed spinach is good) and
realize that much of the calcium in the milk products will be lost at
that meal.
6) Take calcium supplements in chelated or other hard bound forms, at
least when you eat spinach.

OR

Just take lots of calcium supplements and forget about the oxalates in
spinach unless you have a tendency to form kidney stones. This is the
solution of me and Kitty - Kitty's kidney stone was *not* an oxalate
type.

Here are some abstracts which relate:

Am J Clin Nutr. 1988 Apr;47(4):707-9.
Calcium absorbability from spinach.
Heaney RP, Weaver CM, Recker RR.
Department of Medicine, Creighton University, Omaha, NE 68178.

The absorbability of calcium from spinach was compared with the
absorbability of Ca from milk in 13 healthy adults in a randomized
cross-over design in which the test meal of either milk or spinach had
200 mg of Ca labeled with 45Ca. Absorption was measured by the
standard double-isotope method in which both the test food and the
miscible Ca pool are labeled with different Ca tracers. Measurement of
both Ca and oxalate in our test spinach revealed a very slight
stoichiometric excess of oxalate; hence it is likely that all of the
spinach Ca was effectively bound. Absorption was higher from milk in
every case, with the mean absorption from milk averaging 27.6% and
from spinach, 5.1%. The mean within-subject difference between Ca
absorption from milk and from spinach was 22.5 +/- 9.5% (P less than
0.0001). These results conclusively establish that spinach Ca is much
less readily available than milk Ca.
PMID: 3354496


Am J Clin Nutr. 1989 Oct;50(4):830-2.
Oxalate: effect on calcium absorbability.
Heaney RP, Weaver CM.
Creighton University, Omaha, NE 68178.

Absorption of calcium from intrinsically labeled Ca oxalate was
measured in 18 normal women and compared with absorption of Ca from
milk in these same subjects, both when the test substances were
ingested in separate meals and when ingested together. Fractional Ca
absorption from oxalate averaged 0.100 +/- 0.043 when ingested alone
and 0.140 +/- 0.063 when ingested together with milk. Absorption
was, as expected, substantially lower than absorption from milk
(0.358 +/-0.113). Nevertheless Ca oxalate absorbability in these women
was higher than we had previously found for spinach Ca. When milk and
Ca oxalate were ingested together, there was no interference of
oxalate in milk Ca absorption and no evidence of tracer exchange
between the two labeled Ca species.
PMID: 2801588


Am J Clin Nutr. 1990 Apr;51(4):656-7.
Calcium absorption from kale.
Heaney RP, Weaver CM.
Creighton University, Omaha, NE 68178.

Absorption of calcium from intrinsically labeled kale was measured in
11 normal women and compared in these same subjects with absorption of
calcium from labeled milk. The average test load was 300 mg.
Fractional calcium absorption from kale averaged 0.409 +/- 0.101
(means +/- SD) and from milk, 0.321 +/- 0.089 (P less than 0.025). In
contrast with the poor absorption previously reported for spinach
calcium, kale, a low-oxalate vegetable, exhibits excellent
absorbability for its calcium.
PMID: 2321572

Br J Nutr. 2004 Apr;91(4):601-6.

Fractional magnesium absorption is significantly lower in human
subjects from a meal served with an oxalate-rich vegetable, spinach,
as compared with a meal served with kale, a vegetable with a low
oxalate content.
Bohn T, Davidsson L, Walczyk T, Hurrell RF.
Laboratory for Human Nutrition, Institute of Food Science and
Nutrition, Swiss Federal Institute of Technology, Zurich, Switzerland.

The aim of the present study was to evaluate Mg absorption from a test
meal served with an oxalate-rich vegetable, spinach, as compared with
a test meal served with a vegetable with a low oxalate content, kale.
Mg absorption was measured by a stable-isotope technique based on
extrinsic labelling of the test meals and faecal monitoring of the
excreted isotope labels. Nine healthy adults participated in the
study. The test meals were based on 100 g phytate-free white
bread, served with 300 g spinach (6.6 mmol oxalate; 0.7 mmol (25)Mg
label added, 5.0 mmol total Mg) or 300 g kale (0.1 mmol oxalate; 1.2
mmol (26)Mg label added, 4.8 mmol total Mg). The test meals were
served on days 1 and 3, at breakfast and lunch, using a cross-over
design. The results from the present study demonstrated that apparent
Mg absorption was significantly lower from the meal served with
spinach (26.7 (sd 10.4) %) than the meal served with kale (36.5 (sd
11.8) %)(P=0.01). However, the lower fractional apparent Mg absorption
from the test meal served with spinach can be assumed to be, at least
partly, counterbalanced by the higher native Mg content of spinach as
compared with kale. Although based on indirect evidence, i.e. not
based on an evaluation of added (or removed) oxalic acid, the
difference in Mg absorption observed in the present study is
attributed to the difference in oxalic acid content between the two
vegetables.
PMID: 15035687

J Agric Food Chem. 2005 Apr 20;53(8):3027-30.
Effect of different cooking methods on vegetable oxalate content.
Chai W, Liebman M.
Department of Family and Consumer Sciences (Nutrition), Department
3354, University of Wyoming, 1000 East University Avenue, Laramie,
Wyoming 82071, USA.

Approximately 75% of all kidney stones are composed primarily of
calcium oxalate, and hyperoxaluria is a primary risk factor for this
disorder. Nine types of raw and cooked vegetables were analyzed for
oxalate using an enzymatic method. There was a high proportion of
water-soluble oxalate in most of the tested raw vegetables. Boiling
markedly reduced soluble oxalate content by 30-87% and was more
effective than steaming (5-53%) and baking (used only for potatoes, no
oxalate loss). An assessment of the oxalate content of cooking water
used for boiling and steaming revealed an approximately 100% recovery
of oxalate losses. The losses of insoluble oxalate during cooking
varied greatly, ranging from 0 to 74%. Because soluble sources of
oxalate appear to be better absorbed than insoluble sources, employing
cooking methods that significantly reduce soluble oxalate may be an
effective strategy for decreasing oxaluria in individuals predisposed
to the development of kidney stones.
PMID: 15826055


Asia Pac J Clin Nutr. 2003;12(2):219-24.
Bioavailability of soluble oxalate from spinach eaten with and without
milk products.
Brogren M, Savage GP.
Food Group, AFSD, Lincoln University, Canterbury, New Zealand.
Savage@...

Leafy vegetables such as spinach (Spinacia oleracea) are known to
contain moderate amounts of soluble and insoluble oxalates. Frozen
commercially available spinach in New Zealand contains 736.6+/-20.4
mg/100g wet matter (WM) soluble oxalate and 220.1+/-96.5mg/100g WM
insoluble oxalate. The frozen spinach contained 90mg total
calcium/100g WM, 76.7%of this calcium was unavailable as it
was bound to oxalate as insoluble oxalate. The oxalate/calcium (mEq)
ratio of the frozen spinach was 4.73. When frozen convenience food is
grilled there is no opportunity for the soluble oxalates to be leached
out into the cooking water and discarded. Soluble oxalates, when
consumed, have the ability to bind to calcium in the spinach and any
calcium in foods consumed with the spinach, reducing the absorption of
soluble oxalate. In this experiment 10 volunteers ingested 100g
grilled spinach alone or with 100g additions of cottage cheese, sour
cream and sour cream with Calci-Trim milk (180 g) and finally, with
20g olive oil. The availability of oxalate in the spinach was
determined by measuring the oxalate output in the urine over a 6-hour
and 24-hour period after intake of the test meal. The mean
bioavailability of soluble oxalate in the grilled spinach was
0.75+/-0.48% over a 6-hour period after intake and was 1.93+/-0.85%
measured over a 24-hour period. Addition of sour cream and Calci-Trim
milk reduced the availability of the oxalate in the spinach
significantly (P<0.05) in both the 6-hour and 24-hour collection
periods.
PMID: 12810415

J Nutr Sci Vitaminol (Tokyo). 1995 Dec;41(6):671-85.
The bioavailability of magnesium in spinach and the effect of oxalic
acid on magnesium utilization examined in diets of magnesium-deficient
rats.
Kikunaga S, Ishii H, Takahashi M.
Faculty of Home Economics, Notre Dame Seishin University, Okayama, Japan.

Spinach was evaluated for its bioavailability of magnesium in the
experiment with magnesium-deficient rats. The effect of oxalic acid on
absorption of dietary magnesium was also examined in the same
experiment. After there were significant differences in the body
weight of the rats between the control group and the
magnesium-deficient group, and after the number of dead rats
increased, the magnesium-deficient rats were divided into six groups.
They were pair-fed for 8 days on the magnesium-deficient diet,
magnesium-deficient diet supplemented with raw powdered spinach
(R-sp), boiled powdered spinach (B-sp), or fried powdered spinach
(F-sp), control diet supplemented with oxalic acid (Ox-C), and control
diet (+Mg). On the 10th day, there was no significant difference in
the food intake of the rats between the control group and
magnesium-deficient group. However, the body weight, and body weight
gain of the rats increased more significantly in the control group
than in those of the magnesium-deficient group. Also, the contents of
calcium and phosphorus in the liver and kidneys, and serum calcium
content increased significantly in the magnesium-deficient rats
compared with those of the control rats. However, the serum magnesium
content decreased significantly in the magnesium-deficient rats. An
especially large amount of calcium was accumulated in the kidneys of
the magnesium-deficient rats. At the end of the experimental period,
there were no significant differences in the food intake, body weight
and body weight gain of the rats among the control group and each of
the spinach-added groups. The body weight and body weight gain of the
Ox-C rats decreased significantly in comparison with those of the
control group and each of the spinach-added groups. Although, there
were no significant differences in the concentrations of serum
minerals (Mg, Ca and P) among each of the groups, kidney magnesium,
calcium and phosphorus, and liver magnesium and phosphorus were
significantly higher in each of the spinach added groups than those of
the control, Ox-C and +Mg groups. A large amount of calcium was
accumulated in the kidneys of the rats fed on the R-sp, B-sp, F-sp and
Ox-C diets. However, the kidney calcium of each of the spinach-added
groups markedly decreased in comparison with kidney calcium of the
magnesium-deficient rats on the 10th day, when the magnesium-deficient
rats were separated. There was no significant difference in the
magnesium content of the left tibiae among each of the spinach-added
groups. Also, the magnesium contents of the left tibiae of each
of the additional groups did not reach the level of those of the
control rats. The contents of calcium and phosphorus of the left
tibiae were not significantly different among any of the groups except
for both the R-sp and Ox-C groups, and decreased significantly in the
R-sp and Ox-C groups compared with those of the other groups. A highly
positive correlation between bone calcium and bone strength was not
observed in this study; the breaking force of the left femurs of the
B-sp and F-sp rats increased significantly in comparison with that of
the Ox-C group. The rate of magnesium absorbed by the rats receiving
the control, R-sp, B-sp, F-sp, Ox-C, and +Mg diets was 88.9, 80.2,
88.4, 90.4, 88.1, and 87.7%, respectively. The rate of apparent
absorption of calcium from the control, Mg-deficient, R-sp, B-sp,
F-sp, Ox-C and +Mg diet was 87.0, 84.1, 57.3, 66.4, 66.2, 53.3 and
83.5%, respectively. The data indicate that oxalic acid remained
in spinach after cooking of boil or frizzle was not deleterious to
magnesium availability, and that spinach is one of the most promising
sources of magnesium.
PMID: 8926539

****
Here is an interesting study on mineral balances that found that
spinach eating is not a problem for calicium absorption in general.
****

J Nutr. 1988 Oct;118(10):1197-204.
Mineral balances of men fed a diet containing fiber in fruits and
vegetables and oxalic acid in spinach for six weeks.
Kelsay JL, Prather ES, Clark WM, Canary JJ.
Carbohydrate Nutrition Laboratory, U.S. Department of Agriculture,
Beltsville, MD 20705.

In an investigation of the effects of fiber and oxalic acid on weekly
mineral balances, 12 men consumed two diets consisting of natural
foods for 6 wk each in a crossover design. One diet contained about 25
g neutral detergent fiber (NDF) in fruits and vegetables and included
100 g spinach, which is high in oxalic acid, every other day. The
second diet was a low fiber diet that contained about 5 g NDF and the
same amount of spinach as the first diet. On the basis of mean values
for 6 wk, balances for calcium, magnesium, iron, zinc or manganese
were not significantly different due to diet. Copper balances were
significantly lower when the low fiber diet was consumed than when the
diet containing fiber in fruits and vegetables and consequently a
higher level of copper was consumed. Mineral balances were
significantly different due to week. When results of both diets were
considered together, mean mineral balances decreased at some time
after wk 1 and returned to the wk 1 level during wk 6. Balances for wk
1 and 6 were not significantly different for any of the six minerals.
These results demonstrate the importance of determining weekly
balances for several weeks when conducting a human metabolic study.
PMID: 2846801


--Paul