== Metabolic Syndrome: What is it? ==
Metabolic Syndrome is not a specific diseased state, but rather a cluster of conditions including central or visceral adiposity and dyslipidaemia, type II diabetes and insulin resistance, and cardiovascular disease and hypertension. Metabolic syndrome is defined by the International Diabetes Federation as central obesity as well as two of the following states: triglycerides greater than 150 mg/dL, HDL cholesterol levels lower than 40 mg/dL for males, 50 mg/dL for females, blood pressure greater than 130/85 or undertaking treatment for hypertension, and fasting plasma glucose levels greater than 100 mg/dL or type 2 diabetes. The consequences of metabolic syndrome are extensive, including the patient being five times more likely to develop diabetes, an increased risk of cancers partially due to elevated insulin (a growth factor) levels, the development of non-alcoholic fatty liver, and chronic inflammation. Here, exercise as a means to improve the factors comprising Metabolic Syndrome are discussed with regard to the somewhat controversial High Intensity Interval Training (HIIT) exercise method.
== HIIT evidence for Obesity, Dyslipidaemia and Chronic Inflammation ==
With increased adiposity comes an increase in inflammation due to pro-inflammatory cytokines stimulated by increased M1 macrophage levels, including those released from adipose tissue itself, such as interleukin 6 and c-reactive protein, a key marker of inflammation. To reduce the chronic inflammation associated with obesity, adiposity must be reduced. A number of studies have shown a significant increase in the body’s ability to oxidise fat resulting from HIIT interventions, even for as short a time as 2.5 hours over a 2 week period. These results all show similar or superior effects to continuous type training with regard to oxidative enzyme activity and mitochondrial capacity. Of interest, peroxisome-proliferator activated receptor γ coactivator PGC-1α levels significantly increase following low volume HIIT, which is associated with increased oxidative capacity and mitochondrial enzyme expression, and importantly, anti-inflammatory pathways, including the increase in circulating adiponectin.Addressing dyslipidaemia, visceral fat stores are comparatively less ‘stable’ than others, tending to release excess free fatty acids, which is a contributing factor in the instance of dyslipidaemia. A reduction in visceral fat storage would therefore be desired to contribute to the reduction in severity of dyslipidaemia. In addition to the increased oxidative capacity observed with HIIT, a study conducted on metabolic syndrome patients revealed HIIT the superior training choice when compared to continuous training, with results including greater decreases in lipogenesis, fatty acid transport into adipose tissue, waist circumference and body weight, indicating a reduction in visceral fat storage and an increase in the body’s ability to metabolise fat with a HIIT exercise intervention.
== HIIT evidence for Type 2 Diabetes and Insulin Resistance ==
The increased mitochondrial enzyme activity discussed above, while increasing oxidative capacity and ability to oxidise fats, also benefits the metabolic syndrome patient with a reduction in the risk and severity of other metabolic disorders, including insulin resistance. HIIT in metabolic syndrome patients has been found most effective in the improvement of insulin sensitivity, fasting blood glucose and cell function when compared to continuous training. The earlier mentioned PGC-1α not only benefits muscle oxidative capacity and anti-inflammatory mechanisms, but also has a positive effect on glucose uptake.HIIT has been found to increase skeletal muscle glucose transport capacity in patients with type 2 diabetes, through increased insulin sensitivity, and by the increased skeletal muscle content of the chemical mediator GLUT4. Following a HIIT program, subjects were also found to have a higher muscle glycogen content, a lower rate of glycogen utilisation, and an increased capacity for lipid oxidation after just 2 short weeks comprising six HIIT sessions.
== HIIT evidence for Cardiovascular Disease and Hypertension ==
Though often perceived as an inappropriate and unsafe form of training, interval based training techniques have provided numerous benefits in applications to a range of populations, including cardiovascular conditions such as coronary artery disease, hypertension and post-infarction heart failure. It is thought that short intense bursts of activity, while causing increases in peripheral vascular stress, may in fact ‘insulate’ the heart from such stresses due to the brief exercise duration. Compared to continuous training, HIIT has been found to have a greater flow on effect with the reduction of blood pressure, and increases in endothelial function, and of the two types of training, HIIT was the only one found to decrease arterial stiffness.Aerobic capacity has been well documented to be superiorly developed using HIIT training when compared to continuous training, particularly on a time to results basis. An increase in cardiovascular fitness has been noted when applying HIIT techniques to coronary artery disease patients. Aerobic capacity determined by VO2 peak was found to increase significantly with HIIT when compared to continuous training (46% and 14% respectively) in a study of patients with stable post-infarction heart failure. Additional findings revealed declines in end-diastolic and end-systolic volumes, and increases in left ventricular ejection fraction brachial artery flow with HIIT, and quality of life.
== Recommendations ==
Previously regarded as an unsafe and inappropriate exercise tool for many populations, HIIT has gained much credit in the literature as a potentially beneficial mode of exercise for the improvement of a number of conditions, including those comprising Metabolic Syndrome. The exact volume, frequency and intensity is yet to be determined as HIIT focus is relatively new. Studies as short as 2 weeks with 3 sessions per week have proven HIIT benefits significantly superior to continuous training. However, as a general consensus, a HIIT exercise intervention of 10–16 weeks, involving 3 HIIT sessions per week, with achieved intensities of 80-95% peak HR during 2-4 minute high intensity periods and 40-70% peak HR during 2-3 minute active recovery periods, repeated 4-6 times provides sufficient training stimulus for positive adaptations in those presenting with Metabolic Syndrome factors. However, before undertaking any physical activity program, a medical exam must first be conducted to determine the appropriateness of a HIIT intervention, and to provide any individual limitations, for example, for diabetics with conditions such as retinopathies. Any program will be of most benefit when tailored to each individual, and additionally overseen by a physician. Where appropriate, a combination of exercise equipment involved in the provision of HIIT is ideal, as this provides total body training, stimulating the greatest amount of skeletal muscle tissue prospective of adaptation, as well as benefiting the patient with whole body exercises improving personal strength and range of motion, and follows cardiac rehabilitation procedures.
== Further reading ==
International Diabetes Federation's 'The IDF Consensus Worldwide Definition of the Metabolic Syndrome' (2006) provides valuable and clear information regarding the classification, diagnosis and management of Metabolic Syndrome.
== References ==