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Fasted cardio: is it an empty promise?

Written By

Colin Gentry


General Fitness, Nutrition, Personal Training

Posted On

5 September 2016

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Slow down the pursuit of fat loss and give your personal training clients their fill of the facts first

As personal trainers, many of us have asked clients at the start of a session what they have eaten that day and received a list of terrible food choices. And then there are those who simply say: nothing.

One of the major hurdles to achieving our fitness goals is eating the right food at the right times. Some people over-eat, some under-eat and some swing between fasting and binging.

​While the thought of not having downed a protein shake before a workout might be unfathomable to regular gym-goers, what to eat before exercise becomes an interesting question when clients raise the concept of fasted cardio with their personal trainer.

For clients wanting to lose body fat, the idea of not eating and then exercising to burn even more calories can sound like an attractive proposition, and one their personal trainer should be championing. Responsible personal trainers, however, need to question whether clients have more to gain from a regular healthy diet than an approach to weight loss that has yet to receive unanimous support from science.

Running on empty

Fasted cardio goes beyond training on an empty stomach. Fasted cardio means cardiovascular exercise performed when the body is no longer "fed" – as in insulin levels are elevated and food is still being processed – but when the body is in a fasted state. Insulin is at its baseline level and nutrients are no longer being absorbed. As there is no more food in the tank to act as fuel, in the fasted state the body turns to its fat stores to power the workout.

So, what exactly can fasted cardio achieve?

In a study for the Journal of the International Society of Sports Medicine, twenty women who regularly performed aerobic exercise performed the same cardio routine. Half did so in a fed state and half in a fasted state. All twenty women lost body fat. The conclusion? Fed or fasted, the real key to fat loss is a controlled diet and exercise.

Some studies have found fasted cardio to have a more significant impact. Research in 2010, for example, found fasted cardio resulted in a 28% increase in GLUT4. Fed cardio saw an increase of just 2%. GLUT4 is a glucose transporter that allows glucose to enter muscle cells for energy rather than being stored as fat.

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Adding weight to the theory

Fasted training is not exclusive to cardio. Fasted weightlifting is also an option, which may sound appealing to those who want to burn calories in the squat rack rather than on the elliptical.

A study published in the International Society of Sports Nutrition examined the anabolic response to fasting by looking at Muslim bodybuilders during Ramadan. It was found that fasted weightlifting has no detrimental effect on the anabolic response at all. This conflicts with another study, however, which argues that fasted training can dramatically increase muscle breakdown.

Those who choose to lift in a fasted state may find an added benefit is a reduction in DOMs intensity. Recent research in the scientific journal Experiment Gerontology has found that fasting can aid in inflammation resistance in muscle fibres. This means recovery after a workout is not only less painful but more rapid, which is good news to those who find it hard to walk after legs day.

Fat chance of success

Jim Stoppani – the U.S personal trainer armed with a ph.D from Yale - believes fasted cardo has a place in training programs, but only for men and women who already have low body fat and wish to lose the last stubborn areas. He has suggested that fasted cardio could benefit men at around 5% body fat and women at around 13% body fat, triggering the release of resistant fat stores for fuel. Such a group of people, however, are likely to be found in bodybuilding and fitness professionals rather than the general public at the start of their journey.

"If you're a male with roughly 8% body fat or more, or a female with 16% or more, fasted cardio probably won't make a massive dent in your fat-loss efforts," Jim Stoppani concludes. "Instead, go high intensity with some form of HIIT and watch the fat melt."

All in all, if your client inquires about fasted cardio, consider their goals and the point they are at in reaching them. For most clients, eating healthily and at regular intervals, combined with an active lifestyle, will serve them far better than an empty plate. Educating your personal training clients on the benefits of a pre-workout shake or meal instead could be far more valuable food for thought.

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