Long Slow Distance: is FatMax training Too good to be true?

Running slower for longer distances might provide a counterintuitive improvement in speed. A slower pace can stress your fat burning to the max, resulting in fitness improvements that are beneficial at a wide range of intensities. Most people do their Long Runs at a slower pace, often called "Long Slow Distance" (LSD). This page focuses on the slower pace, with the Long Run and The Science of the Long Run focused on the endurance aspects.

1 Introduction

It's counter intuitive that slower running would make you faster. However, there's good evidence that slower paced running improves fitness and speed. The mechanism behind this improvement is most likely that lower intensity stresses the fat burning processes. As exercise intensity increases from rest, fat burning rates increase to provide the energy. The percentage of calories from fat decreases with intensity, as the body uses more carbohydrate for fuel. This results in a peak fat burning occurring at a relatively low intensity. This peak is often called "FatMax" and is the intensity where the greatest benefit occurs. At FatMax, the metabolic pathways that burn fat working as hard as possible. This stress then improves the ability to burn fat. Even in relatively short races, there is some fat metabolism that contributes energy, and fat metabolism is critical for longer races such as the marathon. It seems likely that training at FatMax also improved oxygen delivery, which in turn improves overall performance at all intensities. So, is FatMax training the silver bullet, or is it too good to be true? Let's look at some of the issues.

2 FatMax varies between runners (a lot)

Research into FatMax has shown that the intensity that corresponds to maximum fat burning varies between different runners quite a lot. Depending on how you measure intensity, FatMax occurs in the range 36-62% of V̇O₂max, 50-74% of Max HR, 20-50% of Watts at V̇O₂max. That's a huge variation between runners, and so far, there is no way of predicting why different individuals have particular values. The only option at the moment would be a laboratory test (maybe).

3 FatMax varies day to day

FatMax measured on one day is generally within 90% on other days. While that's not a huge variation, it makes the value of a laboratory test rather lower than you might expect. That variation is more than enough to move you from one heart rate zone to another. Some of the day-to-day variation will depend on Glycogen stores and carbohydrate intake. The more carbohydrate you eat, and the more glycogen stores you have available, the more your body is willing to burn.

4 FatMax varies during a run

FatMax shifts during just an hour's exercise. This has been shown in several experiments, where metabolism shifts to burning more fat as carbohydrate stores are depleted. This means that FatMax will occur at lower and lower intensities.

5 What Fat is being burned at FatMax?

The fat being burnt at FatMax isn't all body fat (adipose). A rough estimate is that about half of the fat is from intramuscular triglycerides, and the other half from body fat. If you look at the graphs below, you can see that plasma free fatty acids (FFA) have a maximum burn rate at quite a low intensity. Most of the FFA is probably from body fat, unless you're still digesting food which is providing fat into your blood stream. You can also see that the burn rate of FFA increases as exercise progresses, so the longer you go, the more body fat you burn as a percentage.

The contribution of different energy sources changes with exercise intensity.

Another study also looked at the utilization of different energy sources during exercise and produced similar results.

Changes in substrate usage over 120 min period at 65% V̇O2max.

6 FatMax and weight Loss

Is FatMax good for weight loss? There's evidence that exercise overall might help modestly with weight loss, with some analysis finding High Intensity Interval Training more effective or similar to FatMax style lower intensity. In the long term, exercise might help with weight loss, but it's no silver bullet. To a large extent, it seems that appetite increases to match the energy expenditure of exercise. Personally, I suspect that the benefits of exercise on weight loss are motivational, with people motivated to keep their weight down to benefit their exercise. Alternatively, it could be that people who like to exercise are more likely to lose weight. Either way, I'd suggest using FatMax to improve fitness and health, rather than primarily for weight loss.

7 FatMax is similar over a broad range of intensities

FatMax tested at five different intensities, between 55-95% of anaerobic threshold and showed similar levels of fat metabolised. This suggests that it's not worth being too focused on a specific intensity beyond going slow enough that you can keep going for a long time.

8 The intensity gap

The most obvious problem with FatMax training is that you have to train at a much lower intensity the most people are comfortable with. Some runners will struggle to even run with their heart rate low enough to be in FatMax. Runners may find that FatMax intensity falls into the gap between the lower intensity of walking and the higher intensity of running. It's often said that the most common problem in recreational runners is their easy runs aren't easy enough, and their hard runs aren't hard enough. The two problems are related in many runners, where their easy runs or too hard to allow them to fully recover and run hard on their hard days.

9 Practical Advice

My practical advice is that FaxMax training is worthwhile, but don't get hung up on the exact intensity. Instead of looking at heart rate zones, simply ensure you go slow enough that you can keep it up longer without feeling exhausted. The pace should be slow enough you can hold a conversation, which seems to be one of the best intensity metrics, even now with so much available technology. How long will depend on your fitness, but longer is better. Decades ago, coaches recommended "2-3 weekly interval sessions, a weekly long run, and as much low intensity as you can handle", which still seems to be true.

10 Tangent – Defining Intensity Levels

When reading any research into FatMax, it's worth noting that studies typically define intensity as a percentage of V̇O₂max. That's fine in a lab but doesn't help much in the real world. Often a percent of V̇O₂max is assumed to be the same percentage of Heart Rate, but that's not quite true. To convert from %V̇O₂max to %HR isn't a simple one and doesn't work the same for all people. Really, we need to use a percentage of the reserve capacity. The well-known Heart Rate Reserve has a parallel with V̇O₂max Reserve. If you can reasonably convert from a percentage of one reserve to the other. But too few research papers use the reserve numbers. Hopefully, this will change over time.