The influence of gravity and body orientation on protein digestion and absorption.

From Page 12-13 of the C1 research protocol document:

The regulation of skeletal muscle mass is dependent on the long-term balance of
protein turnover, comprised of the ongoing processes of protein synthesis and
protein breakdown. Improvements in protein balance and/or higher muscle protein
synthesis rates have been reported following the ingestion of various types of
dietary protein: whey protein (16), casein protein (16), soy protein (18),
casein protein hydrolysate (11, 15), egg protein (13), and whole milk and/or
fat-free milk (5, 22). Milk protein and its main isolated constituents, whey
and casein, are the most widely studied dietary proteins. Casein and whey
protein seem to have distinct anabolic properties, which are attributed to
differences in digestion and absorption kinetics (2-4, 11). Whereas whey
protein is a soluble protein that leads to rapid intestinal absorption, intact
casein clots in the stomach delaying its digestion and absorption and the
subsequent release of amino acids in the circulation (7). The fast, but
transient rise in plasma amino acid concentration after whey protein ingestion
can lead to higher protein synthesis and oxidation rates (2, 3, 18). Thus, it
is strongly suggested that the rapidness of protein digestion and absorption is
a primary factor determining the extent to which muscle protein synthesis can
be stimulated.

We have previously demonstrated that dietary protein is digested and absorbed
during an acute period of overnight rest (9). However, the protein digestion
kinetics appeared to be somewhat altered when compared to similar data
collected from our previous work conducted during the daytime. We speculated
that digestion might somehow be modulated if the post-prandial or digestive
phase occurs while lying in a horizontal body position as experienced during
overnight sleep. To gather a further understanding of protein digestion during
the night, we feel it necessary to isolate and study how body position, and
ultimately the acting effect of gravity, affects protein digestion and the
stimulation of muscle protein synthesis.

Alterations in the acting effect of gravity on absorption of orally
administered pharmaceuticals has previously been demonstrated during simulated
microgravity environment (-6° head tilt) (10). Results from the aforementioned
study demonstrate that absorption of the pharmacological agent occurred at a
more rapid rate suggesting that gastric function may actually improve when the
effect of gravity is altered. However, no data pertaining to changes in protein
digestion and stimulation of muscle protein synthesis under modified effects of
gravity (i.e., lying horizontally vs. sitting vertically) currently exist.

The concept of altered protein digestion and absorption and the resultant
stimulatory effect on skeletal muscle protein synthesis caused by altered
effects of gravity is currently understudied. The present study is designed to
detect if there is a clear interaction of post-prandial body position on
gastric emptying and the digestion and absorption of dietary protein.

From Page 14 of the C1 research protocol document:

Primary Objective: To determine if changes in gravity through alterations in
post-prandial body position affects protein digestion and absorption

Secondary objectives: To determine if changes in gravity through alterations in
post-prandial body position affects protein digestion and absorption and if
such effects can be explained by delayed gastric emptying.

Hypotheses: We hypothesize that horizontal body positioning will cause a
decreased and delayed peak plasma amino acid concentration after ingestion of
dietary protein. Lastly, we hypothesize that the inefficient absorption of
amino acids into blood plasma will be explained by impairments in gastric
emptying.

From Page 16-18 of the C1 research protocol document:

The study design of this randomized, placebo-controlled crossover test is
depicted in Figure 1. After the subjects arrive at the University, we will
check if they meet all the prerequisites described in section 3.2 and ask them
to put on comfortable clothing. A catheter will be inserted into one of the
antecubital veins for continuous blood collection. A basal blood sample (10ml,
t= -30) and resting blood pressure and heart rate will be collected. After
these measurements are taken during the inversion test, subjects will then lay
on the tiltable bed and will be slowly reclined and secured at a body position
of 0° (horizontal), measured with an electronic level. After an acclimation
period of 30 minutes, a blood sample will be collected (10ml, t= 0) and blood
pressure and heart rate will be measured at the upper arm and at the lower
leg. The protein drink will then be an ingested through a drinking straw and
the post-prandial period will begin. Further blood sampling will occur at t =
15, 30, 45, 60, 90, 120, 180, and 240 min and blood pressure and heart rate
will be collected from the upper arm and from the lower leg at t = 60, 120, 180
and 240 min. The subjects will be slowly returned to an upright body position
following the collection of the last blood sample. The exact same sampling
protocol will be followed on the upright test-day except that subjects will
remain in a seated, vertical body position for the duration of the experiment.

The main intervention is a manipulation of the effects of gravity on protein
digestion by studying the post-prandial phase in non-upright body positions.

On one test, subjects will lay horizontally at 0deg for 4.5h. On the other
test day, subjects will remain upright (vertical) in a seated position.

From Pages 10-11 of the C1 research protocol document:

The burden and risks associated with participation in the pilot test is low.
Insertion of the catheters is comparable to a blood draw and could result in a
small hematoma. Blood will be sampled 10 times (10 mL) during each test. The
total amount of blood drawn (100mL each test) is minimal when compared to the
amount taken in a single blood donation (500m) and will be completely restored
within 1 month. Each participant will come to the laboratory once for the
orientation visit (1 hour and twice for the tests (~6 hours each). Before
initiating an orientation visit, interested subjects will be asked questions to
ensure they are not lactose intolerant or display any other gastrointestinal
disorders and fill in and return a medical questionnaire. During the
orientation visit, medical questionnaires will be checked and signed and the
informed consent form will also be filled in. Before starting each test day,
subjects will need to be fasted and will be asked to refrain from eating or
drinking (except for water) from 22h00 the evening before. Subjects will also
refrain from physical activity or alcohol consumption. Participants will
consume a milk-based protein beverage (commercially available food product)
once during each test while the researchers will collect blood samples. Blood
pressure and heart rate will be measured each hour. There is no direct benefit
for the participants except for their contribution to the scientific knowledge
of physiology and nutritional strategies that may aid in preventing muscle loss
in clinical and microgravity situations.