This study aims at assessing the effect of standardized dietary fat load and short-term aerobic exercise on systemic lipolysis, flexibility and partitioning of ectopic fat stores (intramyocellular = IMCL, intrahepatocellular = IHCL, intramyocardial lipids = IMCaL) in relation to FFA in endurance trained athletes and hypopituitary patients compared to sedentary healthy control subjects.

Exercise is a powerful stimulation for growth hormone (GH) secretion in health. A standardised exercise test can, therefore, be discriminative for the diagnosis of GH-deficiency in adults. This will be assessed.

Hypothesis (ectopic fat stores)

1. Ectopic fats stores are flexible fuel stores and are influenced by diet and physical activity.FFA availability may play an important regulatory role.
2. There is a tissue specific partitioning of triglycerides and/or FFA among non-adipose organs after fat load and physical exercise
3. The flexibility of ectopic fat stores is related to insulin sensitivity
4. Lipolytic and anti-lipolytic hormones are critical for regulating FFA availability (at rest or during exercise) and therefore also for the regulation of ectopic fat stores.
5. GH is a lipolytica hormone. Lack of GH in adulthood is related to decreased FFA availability thereby influencing ectopic lipid stores Hypothesis diagnosis of GHD
6. A short intensive physical exercise shows a good discriminative power to diagnose GHD.

Background

Obesity, in particular visceral obesity, is associated with impaired insulin action on target tissues (insulin resistance or metabolic syndrome) that, in turn, is related to an increased risk for cardiovascular mortality and morbidity. Pathophysiological mechanism remains unclear. Interventions in patients with impaired glucose tolerance/impaired fasting glucose have consistently demonstrated that by increasing physical activity and reducing calorie intake the risk to convert to frank type 2 diabetes is decreased, even more efficiently than by early medical therapy with metformin.

Physically inactive and overweight subjects do not only store the excess of fat in the intra-abdominal or subcutaneous department but also in non-adipose tissue (="ectopic" tissues), such also skeletal muscle, liver and myocardium, also called intramyocellular lipids (IMCL), intrahepatocellular lipids (IHCL) and intramyocardial lipids (IMCaL).This ectopic lipid accumulation occurs either by increased free fatty acids (FFA) uptake, increase synthesis in the involved tissues or reduced FFA oxidation.

The relative contribution of these factors to ectopic lipid accumulation varies in different physiological conditions (i.e. physical exercise, fasting, postprandial condition) and in different tissues. In addition, it may be influenced by hormones that regulate lipid metabolism. There is increasing evidence that ectopic fat and its intermediate metabolites interfere with insulin signalling, thereby contributing to the impaired insulin action on target tissues such as liver and skeletal muscle. It has been well showed that high fat oxydation capacity allows to keep a lower lipolytic activity and therefore low levels of lipolyse degradation products. Strong evidence is accumulating that these intermediates interact with insulin signalling finally resulting in insulin resistance.

There is still scarce data about the behaviour of ectopic fat stores in the presence of positive(nutritional fat excess) or negative energy balance (physical activity) and the role of hormones in regulating these fat depots.

Additionally,we have previously shown that GH values obtained during a 2h standardized aerobic exercise of moderate intensity revealed a high diagnostic accuracy in predicting severe GHD in adult individuals. Whether a shorter exercise on higher intensity may lead to comparable results has to be assessed.

Objective

The study aims to comprehensively assess the flexibility and partitioning of ectopic fat deposition and to define the role of relevant hormones (especially GH and insulin) in relation to the availability of FFAs as well as their role in the process of ectopic fat deposition and consumption. Another objective is to investigate the role of a standardized fat load and aerobic exercise on IMCaL.

Methods

Using the two-step hyperinsulinaemic-euglycaemic clamp technique hepatic and peripheral insulin sensitivity is assessed.

Lipid depots (skeletal muscle and liver) are repeatedly measured by MR-spectroscopy, subcutaneous and visceral fat mass by whole body MR-imaging.

Exercise capacity ist measured on a bicycle (incl. spiroergometry). Counterregulatory hormones, glucose and free fatty acids are measured during a 2h physical exercise at 50-60 VO2max.

Blood samples to evaluate GH will be taken immediately before and after the Vo2 max-test, as well as 15, 30 and 45minutes after the end of the exercise test.

Inclusion Criteria:

• Male and female patients >18 years old
• Capable to exercise for 120 minutes on a treadmill
• Willingness to participate in the study and to give written consent.
• Normal ECG during ergometry
• Specific for Athletes: VO2max> 50 ml/kg/min
• Specific for GHD patients: no GH substitution in the last 6 months
• Specific for sedentary controls: Matched the GHD patients for age, gender, BMI and waist

Exclusion Criteria

• Abnormal liver or renal function
• Active neoplasia
• Severe cardiovascular disease (unstable coronary artery disease, heart failure NYHA III-IV)
• Haemophilia
• Inability to exercise
• Contraindications to exposure to a 3 T magnetic field
• Abnormal ECG during ergometry
• Women in childbearing age unless on a continuous contraceptive therapy or surgically sterilized
• Depression, psychosis and other severe personality disorders
• Pregnant women
• Excessive alcohol consumption (>60g/d) or drug abuse

Athletes:Endurance trained athletes: minimal >50 mlO2/KG body weight

GH-deficient adult patients: NO GH-replacement therapy during the last 6 months

Sedentary controls: age, BMI, Gender and waist matched (to the growth hormone deficient patients) healthy control subjects