What is the meaning of Metabolic Balance Studies in human research?
In vitro conditions or animal experiments allow to examine changes in the organism by observing the cell or the animal - if indicated, even by dissection. Which opportunities do we have to investigate changes in the human body as a reaction to the conducted measures? For this purpose we use the tool of Metabolic Balance Studies. We measure substances (parameters) specific for the process in question that are released into circulation or excreted via skin, saliva, urine or feces. In order to prepare a balance for a specific parameter, we control and document completely everything affecting the subjects participating in our studies. This refers to the entire food and beverages the subjects ingest as well as to all environmental influences (temperature, humidity, daylight). By collecting everything which is released into circulation or eliminated out of the body we calculate balances that allows for estimating what has been metabolised and excreted and what is potentially stored in the human body.
How are we conducting Metabolic Balance Studies?
Our Studies are performed in the simulation facility for occupational medicine research (AMSAN) in our institute. This Metabolic Ward is capable of housing up to 8 subjects at a time. On 300 m² the AMSAN provides test rooms, laboratories, kitchen, living area, bath rooms and 8 single bed rooms. For our balance studies it is of utmost importance that this facility is isolated from the outside world and that humidity and temperature can be kept constant.
Nutrition, in particular energy-, calcium and vitamin D supply as well as the intake of table salt and protein is influencing bone metabolism in different ways and plays a decisive role in our studies. Therefore variations in the supply of these nutrients during the experiments have to be eliminated or according to the respective study design, the intake of a specific nutrient has to be increased or lowered under strict control. Macronutrients as well as Calcium, vitamin D, sodium-chloride and protein intake are individually calculated day by day and kept constant during the entire study. Thus, subjects participating in our Metabolic Balance Studies receive meals that are accurately weighed to the gram and must be consumed without any leftovers. Some substances, like calcium and electrolytes, are eliminated via urine and feces. Therefore, besides blood samples, 24-hour urine and the entire feces are collected and analysed in our biochemical laboratory. Even excretion via the skin can be investigated by the use of whole body cotton suits. Thoroughly rinsing the suits with deionised water allows us to detect specific electrolytes ('Total Body Wash Down Method').
Examples for Metabolic Balance Experiments
Salty Life 6 and Salty Life 7
In our Salty Life 6 (SL6) and 7 (SL7) studies we examined sodium regulation, bone metabolism and acid base balance in ambulatory conditions (SL6) and in head down tilt bed rest (SL7). The acid base balance system is an important regulative system of our body to keep the balance between acids and bases constant (a measurement for this is the blood pH value). Already little variations in blood-pH can cause changes in cell-, organ- and total body-functions up to multiple organ failure.
Not only a high dietary salt intake, but also other elements of today’s eating habits in western countries have an acid excess (high intake of animal protein, less fruit and vegetable intake) which leads to a so called “latent metabolic acidosis”. This is a chronic status characterized by a small shift of blood pH within the normal range (7.35 – 7.45) to a more acidic level with a simultaneous reduction of the buffer capacity of blood. There are no observable clinical symptoms, but other systems of the organism for example the bones react like buffer systems to keep the blood pH constant.
The 28 days Salty Life 6 study was divided into four study periods with three different dietary salt intake levels. During the first 6 days the subjects received 0.7 mmol NaCl/kg body weight (BW)/d, during the next 6 days 2.8 mmol NaCl/kg BW/d, during the following 10 days 7.7 mmol NaCl/kg BW/d and during the last 6 days again 0.7 mmol NaCl/kg BW/d.
Salty Life 7 consisted of two study phases in a cross-over design. Each of the two study phases was divided into a 4-day adaptation-period (non-bed rest; 2.8 mmol NaCl/kg BW/d), 14 days of intervention (head down tilt bed rest; 0.7 mmol NaCl/kg BW/d or 7.7 mmol NaCl/kg BW/d) and 3 days of recovery (non-bed rest; 2.8 mmol NaCl/kg BW/d).
The data show that a high dietary salt intake leads to increased bone resorption in ambulant subjects (SL6) and moreover exacerbates immobilisation induced bone resorption during head down tilt bed rest (SL7). Data from both studies show simultaneously a reduction of the buffer capacity in blood: a so called low-grade metabolic acidosis caused by increased dietary salt intake? Hence, changes in acid base balance might be the reason for exercerbated bone loss during high salt intake. In our current Salty Life 8 study we focus on this hypothesis and potential countermeasures.
Additionally we were able to show, in SL6 as well as in SL7, that a high dietary salt intake leads to sodium storage without compensatory water storage: In SL6 sodium was predominantly stored osmotically inactive. In SL7 the high dietary salt supply was associated with a higher potassium excretion, thus sodium storage could be due to an intracellular potassium exchange.
Salty Life 8
In previous Salty Life Studies we were able to show effects of a high dietary salt intake on bone metabolism as well as on the acid base balance. If dietary salt induced a reduction of the buffer capacity in blood (low-grade metabolic acidosis), as we have seen in SL6 and SL7, this could be the cause of salt induced bone loss. If this holds true an additional intake of alkaline salts might reduce this salt induced increased bone resorption by increasing the buffer capacity in blood.
We examine this hypothesis in our ongoing Salty Life 8 Study. 8 male, healthy test subjects take part in a cross-over experiment with two study phases, both divided into 5 days of adaptation, followed by 10 days of intervention. During the intervention part the subjects receive a high dietary salt diet which is supplemented in one part by an alkaline salt.