INCREASE AEROBIC THRESHOLD (AET)
Despite various naming conventions, the Aerobic Threshold (LT1, VT1, closely related to FATmax ... etc) is essentially the intensity at which 'easy' training becomes 'moderate' training. It is the approximate location where your reliance on fat to provide energy, is exceeded by your reliance on carbohydrates. Changes in AeT will always track very closely with changes in AnT, as it will also benefit from increasing VO2max and decreasing VLamax. However, there are also additional strategies that can provide modest (but potentially significant) increases in AeT.
If increasing AeT is the objective of an easy endurance session, that is, increasing the maximum intensity that your body prioritises fat as a fuel source - it is important to avoid high intensity efforts that will accumulate lactate. Whenever lactate has accumulated, your aerobic metabolism will prioritise lactate as fuel, rather than utilising fat. When lactate is present, fat will be pushed out of the aerobic metabolism, and will not return until lactate levels have decreased. It is therefore extremely important, during endurance sessions targeting improvements in AeT - to avoid high intensity efforts.
Another (potentially hazardous) tactic that can be employed to elevate AeT, is to perform endurance training with relatively low glycogen (stored carbohydrate) availability. Low carbohydrate diets and training in a low carbohydrate state, have become increasingly popular in recent years, due to the findings that they can significantly increase mitochondrial biogenesis - that is, they can increase the number of 'powerhouses' in your cells.
Reducing carbohydrate availability, will inevitably result in a reduced capacity for carbohydrate metabolism. In doing so, fat combustion and the AeT will be elevated. However, it is critical to note, that during periods of low carbohydrate intake, high intensity sessions must be minimised - as these rely on carbohydrates to fuel the demand for high power. Frequent training in an under-fuelled state, can be extremely hazardous to both your general health and athletic performance. For this reason, at THP, we will always consider training in a low carbohydrate availability to be very much a potential cherry on top of a cake that must be baked and iced to perfection first. It is also important to note, that when carbohydrate availability is back to normal, the positive adaptations received through the low carb availability, will be quickly reversed - so effectively timing such strategies is extremely difficult to get right.
Increasing your VO2max will always have the greatest impact on improving your fat metabolism and aerobic threshold. A high aerobic capacity means more mitochondria, improved oxygenated blood flow to working muscles, and a shift from using 'fast-twitch' to 'slow-twitch' fibres. All of this results in a reduced involvement of your anaerobic metabolism - and an increased ability to utilise fat. Whether it is achieved through high volumes of low intensity training, or through lower volumes of high intensity training - increasing your VO2max will always have the greatest impact on your aerobic threshold.