Hi, everyone. My name is Su A Lee from the Stein Monogastric Nutrition Laboratory at the
University of Illinois. I’ll be discussing effects of dietary calcium on digestibility and
retention of phosphorus and blood biomarkers in late gestating sows. This work was
presented in 2020 ASAS Midwest meeting.

Let’s start with take home messages before we go over to the presentation. First,
increasing dietary calcium decreases the apparent total tract digestibility (ATTD) of
phosphorus. Second, increasing dietary calcium increases the ATTD of calcium. Third,
increasing dietary calcium increases retention of both calcium and phosphorus. Lastly,
blood biomarkers may be useful to predict bone turnover status of pigs.

In growing pigs, it has been known that excessive calcium in diets has negative effects on
growth of pigs, and this has been shown in a number of previous experiments already. For
example, previous data that were published in 2011 indicated that as dietary calcium
increased from 0.33 to 1.04%, the ATTD of phosphorus linearly decreased.

Another study also showed this. This experiment was not conducted to investigate the
negative effects of too much calcium. However, we found that the ATTD of phosphorus in
diets containing the same amounts of corn and monosodium phosphate fed to gestating sows
were much different between the calcium-free diet and calcium carbonate diet. The only
difference was the calcium level in diets: no calcium in calcium-free, but a certain
amount of calcium was provided from calcium carbonate in the second diet.

There are two bone cells that are working on bone turnover. The first one is osteoblast
that is in charge of bone formation, and the second one is osteoclast that is in charge of
breaking down bones. The idea was if byproducts of those cells are analyzed in blood or
urine, we may predict the bone formation and resorption.

Therefore, the objective was to test the hypothesis that increasing calcium in diets fed
to sows affects calcium and phosphorus balance, serum calcium and phosphorus
concentrations, and hormone and biomarker concentrations in blood.

We utilized 36 gestating sows for this experiment. Initial body weight was about 219 kg,
and they were in late gestation period. The sows were fed at 1.5 times maintenance
requirement for energy.

Four diets were formulated. First diet contained corn, soybean meal, and sugar beet pulp.
Calcium carbonate and monosodium phosphate was also added, but the levels of calcium and
phosphorus were only at 25% of requirements. From the second diet, more calcium carbonate
was added to have 50, 75, and 100% of the requirement with a constant level of phosphorus.
The four diets contained 0.18 to 0.72% calcium and 0.56% phosphorus.

Sows were fed a standard gestation diet before they were housed in metabolism crates, and
feces and urine samples were quantitatively collected for 4 days. At the end of the
experiment, sows were bled and blood samples were collected to analyze calcium,
phosphorus, hormones, and biomarkers in serum.

Data were analyzed using Mixed procedure of SAS. In the statistical model, diet was the
fixed effect, and replicate and parity were the random effects. Linear and quadratic
effects of increasing dietary calcium were also tested using contrast coefficients.

Jumping to the results, let me set up the slide. The horizontal axis represents percentage
requirement of calcium that are 25% all the way up to 100%. The vertical axis represents
the ATTD of calcium and phosphorus in percent. As dietary calcium increased, the ATTD of
calcium quadratically increased. This increase can be explained by different contributions
of the basal endogenous loss of calcium when calcium concentration varies from low to
high. What about phosphorus? As dietary calcium increased, the ATTD of phosphorus linearly
decreased and this was also observed in growing pig data. This decrease can be explained
by the calcium and phosphorus precipitation in the intestinal tract of pigs. 

Now, we are looking at the retention of calcium and phosphorus in percent in the vertical
axis. Retained calcium and phosphorus were calculated by subtracting calcium and
phosphorus in feces and urine from intake of calcium and phosphorus, respectively. As
calcium increased in diets, retention of calcium quadratically increased. The quadratic
increase in the ATTD of calcium can be one of the reasons for the quadratic increase in
retention. We found that increasing dietary calcium decreases the ATTD of phosphorus; then
what about retention of phosphorus? As calcium increased in diets, retention of phosphorus
linearly increased, and this was an opposite situation to the digestibility data. It has
been known that the synthesis of bone tissues, which is the primary role of calcium and
phosphorus, requires calcium and phosphorus in the body at the same time. Therefore, as
more calcium was available in the body as calcium increased in the diets, utilization of
absorbed phosphorus increased in the body.

This is clearer when we look at urine data. This graph shows the urine calcium and
phosphorus in g per day. As calcium increased in diets, excretion of calcium in urine was
not changed, and this indicates that the body lacked of calcium, at least in the first
three diets, because the body utilized all digestible calcium by minimizing execration of
calcium in urine. Urine phosphorus was linearly decreased by increasing calcium in diets
because the body needed more phosphorus as more calcium was available.

We also analyzed serum concentrations of calcium and phosphorus. Data indicated that the
serum calcium and phosphorus were not affected by dietary calcium level. This can be
explained by hormonal regulations to keep constant levels of calcium and phosphorus in the
body.

Same story was found for hormone concentrations in blood. Serum levels of calcitonin and
parathyroid hormone were not changed by level of calcium in diets. Estrogen has been known
to be related to calcium regulation, but estrogen level was not affected by increasing
calcium in diets. 

CTX stands for carboxyterminal cross-linked telopeptide of type I collagen, and this
metabolite is released from osteoclasts. This biomarker linearly decreased as calcium
increased in the diets, which means that the body reduced pulling over the calcium and
phosphorus from the bone tissues as more calcium and phosphorus were available in the
body.

Osteocalcin is released from the osteoblasts and may tell us if the body is synthesizing
bone tissues. We saw numbers increased, but they were not significant.

Bone alkaline phosphatase may also tell us if the body is synthesizing bone tissues. We
expected this biomarker to increase as more calcium and phosphorus were available in the
body. But the results indicated the opposite way. Several experiments that were conducted
previously also found this. More research is needed to investigate the correlation between
bone turnover and the bone alkaline phosphatase.

Because the body keeps forming and breaking down bone tissues, we have calculated the
ratio between osteocalcin and CTX-I concentrations. As calcium in diets increased, the
ratio linearly increased, which indicates that there were more bone formation over bone
resorption. This result is in agreement with calcium and phosphorus balance data showing
that more calcium and phosphorus were retained to synthesize bone tissues.

Let’s move into conclusions. First, increasing dietary calcium decreases the ATTD of
phosphorus in gestating sows. Second, increasing dietary calcium increases the ATTD of
calcium and retention of both calcium and phosphorus. Lastly, blood biomarkers,
specifically CTX-I, may be useful to predict bone turnover status of pigs.

This experiment was also published in Journal of Animal Science, so if you would like see
more details of this work, please find the paper. I would like to acknowledge AB Vista for
the financial support and everyone from Dr. Stein’s lab. And if you want to learn about
the research we are conducting in the Stein Monogastric Nutrition Laboratory, please visit
our website. You can also search “Stein” and “pig” on google. Thank you for listening.