H A Keizer's research while affiliated with Vrije Universiteit Brussel and other places

Publications (174)

Article
Training horses improves athletic capabilities by inducing skeletal muscle-specific and systemic adaptations. However, rest is required to recover from exercise or else overtraining may occur and affect performance and welfare. Biomarkers would be useful to identify early chronic overtraining in animals. The objective of the current study was to in...
Article
Introduction: Resistance training has been well established as an effective treatment strategy to increase skeletal muscle mass and strength in the elderly. We assessed whether dietary protein supplementation can further augment the adaptive response to prolonged resistance-type exercise training in healthy elderly men and women. Methods: Health...
Article
To determine the influence of intensified training and subsequent reduced training on glucose metabolism rate and peripheral insulin sensitivity in horses and identify potential markers indicative of early overtraining. 12 Standardbred geldings. Horses underwent 4 phases of treadmill-based training. In phase 1, horses were habituated to the treadmi...
Article
The objective of this study is to assess the influence of acute exercise, training and intensified training on the plasma amino acid profile. In a 32-week longitudinal study using 10 Standardbred horses, training was divided into four phases, including a phase of intensified training for five horses. At the end of each phase, a standardized exercis...
Article
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Defining normal Growth Hormone (GH) secretory dynamics in the horse is necessary to understand altered GH dynamics related to issues like welfare and disease. Twelve healthy yearlings and two mature Standardbreds were used to quantify GH secretion. Endogenous GH half-life was determined after administration of 1.0 µg/kg BW GH releasing hormone (GHR...
Article
Gastric ulceration can be caused by different pathophysiological mechanisms including dietary factors, psychological stress and exercise. Overtraining is a medical syndrome in performance horses associated with altered hormone levels, altered feed intake, altered behaviour and decreased performance. These components might lead to a higher incidence...
Article
Serial blood samples were collected from three dwarf Friesian foals to examine their endogenous growth hormone (GH) profiles, and the integrity of the GH-insulin-like growth factor-1 (IGF-1) axis was tested in one of them by examining its responses to the administration of GH-releasing hormone (GHRH) and to 10 days of treatment with recombinant equ...
Article
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The influence of intensified and reduced training on nocturnal growth hormone (GH) secretion and elimination dynamics was studied in young (1.5 yr) Standardbred geldings to detect potential markers indicative for early overtraining. Ten horses trained on a treadmill for 32 wk in age-, breed-, and gender-matched fixed pairs. Training was divided int...
Article
To evaluate alterations in skeletal muscle carnitine metabolism during exercise and training by measuring changes in plasma acylcarnitine concentrations in Standardbreds. 10 Standardbred geldings with a mean +/- SD age of 20 +/- 2 months and weight of 384 +/- 42 kg. In a 32-week longitudinal study, training on a treadmill was divided into 4 phases...
Article
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Too intensive training may lead to overreaching or overtraining. To study whether quantitative needle electromyography (QEMG) is more sensitive to detect training (mal)adaptation than muscle enzyme activities, 12 standardbred geldings trained for 32 wk in age-, breed-, and sex-matched fixed pairs. After a habituation and normal training (NT) phase...
Article
The aim of this study was to investigate the association between insulin resistance and physical fitness, leptin concentration, body composition and family history for diabetes in non-selected young children. Physical fitness, fasting plasma glucose, insulin and leptin concentrations, anthropometric characteristics and medical history were availabl...
Article
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To determine the feasibility and the benefits of combined resistance and interval exercise training on phenotype characteristics and skeletal muscle function in deconditioned, type 2 diabetes (T2D) patients with polyneuropathy. Short-term, single-arm intervention trial. Eleven male T2D patients (age: 59.1+/-7.5 years; body mass index: 32.2+/-4.0 kg...
Chapter
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From an operational standpoint, overtraining can be defined as stress > recovery (regeneration) imbalance, that is, too much stress combined with too little time for regeneration –In this context, stress summarizes all individual training, non-training, and competition-dependent stress factors,–Particularly, additional exogenous non-training stress...
Article
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Overtraining is an imbalance between training and recovery leading to symptoms associated with a neuroendocrine dysbalance called the overtraining syndrome, a disease characterized by behavioral, emotional and physical symptoms similar with depression. Although the prevalence of overtraining is high in human and equine athletes, at present no sensi...
Article
Skeletal muscle uncoupling protein-3 (UCP3) is reduced in type 2 diabetes, and in the pre-diabetic condition of impaired glucose tolerance (IGT). Here we examined whether intervention programs known to improve insulin sensitivity are paralleled by an increase in skeletal muscle UCP3 protein levels. Skeletal muscle UCP3 protein content was measured...
Article
It was hypothesized that both vibration frequency and muscle length modulate the strengthening of muscles that is assumed to result from whole-body vibration (WBV). Length of knee extensor muscles during vibration is affected by the knee joint angle; the lengths of the knee extensors increase with more flexed knee joint angles. In an intervention s...
Article
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Disturbances in substrate source metabolism and, more particularly, in fatty acid metabolism, play an important role in the aetiology and progression of type 2 diabetes. However, data on substrate source utilisation in type 2 diabetes are inconclusive. [U-(13)C]palmitate and [6,6-(2)H(2)]glucose tracers were used to assess plasma NEFA and glucose o...
Article
The impact of exercise on blood glucose homeostasis has not been assessed in long-standing type 2 diabetes patients receiving exogenous insulin treatment. To study the effects of an acute bout of exercise on the subsequent 24-h blood glucose excursions under free-living conditions in insulin-treated type 2 diabetes patients. Eleven male type 2 diab...
Article
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The aim of the present study was to assess the level of glycaemic control by the measurement of 24 h blood glucose profiles and standard blood analyses under identical nutritional and physical activity conditions in patients with Type II diabetes and healthy normoglycaemic controls. A total of 11 male patients with Type II diabetes and 11 healthy m...
Article
To study the possible long-term effect of improved glucose tolerance in horses after long-term training, as the impact of exercise training on glucose metabolism is still unclear in the equine species. It is not known whether there is a direct long-term effect of training or if the measurable effect on glucose metabolism is the residual effect of t...
Article
To investigate the effects of exercise on activation of mitogen-activated protein kinase (MAPK) signaling proteins in horses. 6 young trained Standardbred geldings. Horses performed a 20-minute bout of exercise on a treadmill at 80% of maximal heart rate. Muscle biopsy specimens were obtained from the vastus lateralis and pectoralis descendens musc...
Article
Coingestion of protein and/or free amino acids with carbohydrate has been reported to accelerate postexercise muscle glycogen synthesis due to an increase in the insulin response. To determine the extent to which the combined ingestion of carbohydrate and a casein protein hydrolysate with or without additional free leucine can increase insulin leve...
Article
This study investigates the molecular mechanisms underlying the blood glucose-lowering effect of a 2-day very low-energy diet (VLED, 1883 kJ/d) in 12 obese (body mass index, 36.3 +/- 1.0 kg/m2 [mean +/- SEM]) type 2 diabetic (HbA(1C) 7.3% +/- 0.4%) patients simultaneously taken off all glucose-lowering therapy, including insulin. Endogenous glucose...
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To determine the effects of short-term IV administration of hydrocortisone or equine growth hormone (eGH) or long-term IM administration of eGH to horses on tissue sensitivity to exogenous insulin. 5 Standardbreds and 4 Dutch Warmblood horses. The euglycemic-hyperinsulinemic clamp technique was used to examine sensitivity of peripheral tissues to e...
Article
Objective-To determine the effects of short-term IV administration of hydrocortisone or equine growth hormone (eGH) or long-term IM administration of eGH to horses on tissue sensitivity to exogenous insulin. Animals-5, Standardbreds and 4 Dutch Warmblood horses. Procedure-The euglycemic-hyperinsulinemic clamp technique was used to examine sensitivi...
Article
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This study investigates the consequences of inhibition of adipose tissue lipolysis on skeletal muscle substrate use. Ten subjects were studied at rest and during exercise and subsequent recovery under normal, fasting conditions (control trial, CON) and following administration of a nicotinic acid analog (low plasma free fatty acid trial, LFA). Cont...
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In the present study, we investigated the consequences of adipose tissue lipolytic inhibition on skeletal muscle substrate use in type 2 diabetic patients. We studied ten type 2 diabetic patients under the following conditions: (1) at rest; (2) during 60 min of cycling exercise at 50% of maximal workload capacity and subsequent recovery. Studies we...
Article
The aim of the present study was to determine whether a single session of resistance exercise improves whole-body insulin sensitivity in healthy men for up to 24 h. Twelve male subjects (23 +/- 1 years) were studied over a period of 4 days during which they consumed a standardized diet, providing 0.16 +/- 0.01 MJ.kg(-1).day(-1) containing 15 +/- 0....
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It has been suggested that adiponectin regulates plasma free fatty acid (FFA) clearance by stimulating FFA uptake and/or oxidation in muscle. We aimed to determine changes in plasma adiponectin concentration and adiponectin receptor 1 and 2 mRNA expression in skeletal muscle during and after prolonged exercise under normal, fasting conditions (high...
Article
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The present study was designed to determine postexercise muscle protein synthesis and whole body protein balance following the combined ingestion of carbohydrate with or without protein and/or free leucine. Eight male subjects were randomly assigned to three trials in which they consumed drinks containing either carbohydrate (CHO), carbohydrate and...
Article
The purpose of the study was to investigate whether severe fatigue, possibly leading to overreaching, could be diagnosed at an early stage by a combination of parameters. Seven well-trained male subjects (age [mean +/- SD]: 25.3 +/- 4.7 yr; body mass: 76 +/- 6.6 kg; VO2max: 61.1 +/- 7 ml.kg(-1).min(-1)) increased their training load by doubling the...
Article
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Recent evidence suggests that intramyocellular lipid (IMCL) accretion is associated with obesity and the development of insulin resistance and/or type 2 diabetes. However, trained endurance athletes are markedly insulin sensitive, despite an elevated mixed muscle lipid content. In an effort to explain this metabolic paradox, we compared muscle fibe...
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Simultaneous analyses of glycogen in sections with other subcellular constituents within the same section will provide detailed information on glycogen deposition and the processes involved. To date, staining protocols for quantitative glycogen analyses together with immunofluorescence in the same section are lacking. We aimed to: (1) optimise PAS...
Conference Paper
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y mother is a walking miracle. I'm not the kind of person who tries to be cool or trendy, I'm definitely an individual. If you can do what you do best and be happy, you're further along in life than most people.
Article
To investigate the expression and localization of glucose transporter 4 (GLUT4) and fatty acid translocase (FAT/CD36) in equine skeletal muscle. Muscle biopsy specimens obtained from 5 healthy Dutch Warmblood horses. Percutaneous biopsy specimens were obtained from the vastus lateralis, pectoralis descendens, and triceps brachii muscles. Cryosectio...
Article
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To investigate the effects of two different regimens of androgenic-anabolic steroid (AAS) administration on serum lipid and lipoproteins, and recovery of these variables after drug cessation, as indicators of the risk for cardiovascular disease in healthy male strength athletes. In a non-blinded study (study 1) serum lipoproteins and lipids were as...
Article
Limited information exists about the putative role and expression in human skeletal muscle cells of the 88-kDa integral membrane protein fatty acid translocase (FAT), highly homologous to the human leucocyte differentiation factor CD36. Therefore, we investigated in healthy male individuals the muscle (m. vastus lateralis) fibre type specific expre...
Article
To investigate whether protein kinase C (PKC) isoforms are expressed in equine skeletal muscle and determine their distribution in various types of fibers by use of immunofluorescence microscopy. 5 healthy adult Dutch Warmblood horses. In each horse, 2 biopsy specimens were obtained from the vastus lateralis muscle. Cryosections of equine muscle we...
Article
Both stable isotope methodology and fluorescence microscopy were applied to define the use of intramuscular triglyceride (IMTG) stores as a substrate source during exercise on a whole-body as well as on a fibre type-specific intramyocellular level in trained male cyclists. Following an overnight fast, eight subjects were studied at rest, during 120...
Article
In prior exercise tests in two independent laboratories we observed a VO2 overshoot at the onset of constant-load exercise in well-trained cyclists. The purpose of this study was to investigate this previously unreported phenomenon. Fifteen male elite cyclists (age 26.3±3.3 years, height 180.7±5.3 cm, body mass 72.5±3.6 kg) underwent a constant-loa...
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To investigate in glycolytic and oxidative muscles of trained (nine weeks) and untrained hyperglycaemic female rats the effect of hyperandrogenicity and/or endurance training on energy metabolic properties. Glycogen content and activity of muscle enzymes with regulatory functions in glycogen synthesis were examined. Testosterone treatment increased...
Article
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The data of the present case demonstrate that the abuse of androgenic anabolic steroids (AAS) may lead to serious health effects. Although most clinical attention is usually directed towards peripheral side effects, the most serious central side effect, hypothalamic-pituitary-dysfunction, is often overlooked in severe cases. Although this latter ce...
Article
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Insulin secretion in response to carbohydrate intake is blunted in type 2 diabetic patients. However, it is not clear whether the insulin response to other stimuli, such as amino acids, is also diminished. Recently, we defined an optimal insulinoptropic mixture containing free leucine, phenylalanine, and a protein hydrolysate that substantially enh...
Article
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Comparisons of visual perception, response-selection, and response-execution performance were made between Type 2 diabetes mellitus patients and a matched nondiabetic control group. 10 well-controlled male patients with Type 2 diabetes without diabetic complications (M age 58 yr.) and an age and IQ-matched non-diabetic control group consisting of 1...
Article
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Recently, we observed that impairments exist in skeletal muscle free fatty acid (FFA) utilization during exercise in obese subjects with Type II diabetes. The main objective of the present study was to investigate whether plasma FFA oxidation is impaired during exercise in non-obese Type II diabetic patients. Stable isotope tracers of palmitate and...
Article
In patients with familial combined hyperlipidemia (FCHL) and type 2 diabetes (DM2) organ-specific differences in insulin resistance may exist. In FCHL and DM2 in vivo insulin mediated muscle glucose uptake and inhibition of lipolysis were studied by euglycemic hyperinsulinemic clamp. Insulin mediated glucose uptake was impaired to the same extent i...
Article
The purpose of this study was to monitor general and individual changes in hematological variables during long-term endurance training, detraining and altitude training in elite Olympic distance triathletes. Over a period of three years, a total of 102 blood samples were collected in eleven (7-male and 4 female) elite Olympic distance triathletes (...
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The objective of this investigation was to study the effects of androgenic-anabolic steroid (AAS) misuse on deltoid muscle fiber characteristics in experienced, male strength-trained athletes. In a double-blind study, 15 volunteers were administered nandrolone decanoate (ND) for 8 weeks (200 mg/week, intramuscularly). In an additional study, 12 sub...
Article
(a) To investigate the effects of reduced training on physical condition and performance in well trained cyclists; (b) to study whether an intermittent exercise programme would maintain physiological training adaptations more effectively than a continuous exercise programme during a period of reduced training. Twelve male cyclists participated in a...
Article
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Recently, a role for uncoupling protein-3 (UCP3) in carbohydrate metabolism and in type 2 diabetes has been suggested. Mice overexpressing UCP3 in skeletal muscle showed reduced fasting plasma glucose levels, improved glucose tolerance after an oral glucose load, and reduced fasting plasma insulin levels. However, data regarding the expression of U...
Article
In general, physical activity benefits health. However, long-term intensive physical training may have detrimental effects on the health of some individuals. In cyclists, changes in the femoral arteries may occur leading to stenoses that are manifested in claudication type symptoms. Some endurance athletes may experience atrial fibrillations that a...
Article
The aim of the study was to validate a new, fast metabolic measurement system (the Oxycon-Pro) during low and high exercise intensities against the Douglas bag method. For this purpose twelve highly trained subjects performed an incremental cycle ergometer test. In the third minute of each exercise step simultaneous measurements of the Douglas bag...
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Muscle fiber-type specific expression of UCP3-protein is reported here for the firts time, using immunofluorescence microscopy
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In whole muscle homogenates, the glucose transporter-4 (GLUT-4) content is reported to be higher in muscles consisting predominantly of oxidative (type-1) muscle fibres than in muscles consisting predominantly of glycolytic (type-2) fibres. From these findings, it has been deduced that in rat muscle, oxidative fibres have an intrinsically higher le...
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Physical activity has a beneficial effect on insulin sensitivity in normal as well as insulin resistant populations. A distinction should be made between the acute effects of exercise and genuine training effects. Up to two hours after exercise, glucose uptake is in part elevated due to insulin independent mechanisms, probably involving a contracti...
Article
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The purpose of this study was to evaluate the role of exercise intensity in the effect of physical training on insulin sensitivity. The insulin tolerance test (ITT) was applied to quantify insulin sensitivity. Eighteen healthy, young, untrained men and women participated in a 4-week, five times per week, 1-h per session bicycle-ergometer training p...
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Plasma total homocysteine (tHcy) has been identified as an independent risk factor for cardiovascular diseases (CVD). The difference in tHcy between the sexes has most often been related to the sex hormones, but also to a higher muscle mass in men. The purpose of this study was to assess the effects of acute exercise, brief exhaustive training, and...
Article
In several studies, artery wall properties have been shown to differ between men and women. It has been hypothesized that these differences may result from hormonal influences but, in a previous study, we were unable to detect any influence of the menstrual cycle on artery wall properties. Therefore, we investigated the differences in artery wall p...
Article
To study the occurrence and contributing factors of transient hypoglycemia after pre-exercise ingestion of glucose after a 4-hour fast, 19 well-trained cyclists ingested 50 grams of glucose dissolved in water around noon after having a normal breakfast. The ingestion of the glucose solution was followed by 30 minutes rest after which the subjects c...
Article
The effect was investigated of treadmill training of moderate intensity on the fatty acid-binding protein (FABP) content in relation to parameters of oxidative and glycolytic metabolism. To this end, the cytoplasmic FABP content and the activity of beta-hydroxyacyl-coenzyme A dehydrogenase (HAD), citrate synthase (CS), and 6-phosphofructokinase (PF...
Book
Most of the research on endurance exercise has focussed on the physiological and metabolic demands during an event and on factors causing fatigueFatigue during and after prolonged submaximal running may have many origins. The conventional explanation for fatigue after running a marathon or ultra-marathon is that glycogen is depleted in the skeletal...
Article
The purpose of this study was to determine if there were significant differences in lumbar bone mineral density (L2-L4, g/cm2) or several hormones among 3 groups of adolescent females: 10 amenorrheic runners, 10 eumenorrheic runners, and 10 eumenorrheic controls. Experimental design: comparative. Cooper Clinic, Aerobics Center, Dallas, Texas. The s...