After 17 days of incorporating fasted aerobic workouts into my regimen I've lost six pounds, which is about 5 percent weight loss.
To put this into context, I finished my first half marathon a year ago and have finished a total of five since then. In December 2015 I finished my first marathon.
Through it all I hadn't lost a single pound.
If you'd asked me 18 days ago if I felt like I needed to lose weight, I would have told you I was comfortable with my weight. For me, exercise -- and, in particular, running -- is so much more than about weight loss. There's the rush that stays with me throughout the workday and the mindfulness benefits of cardiovascular exercise but above all, exercise gives me a powerful sense of wellbeing.
And maybe even a sense of, well, power.
Still if you ask me how I feel about my performance as a runner, I would say I might be a more efficient runner, and therefore a better runner, if I could adjust my running posture.
But that would also mean chipping away at the layer of fat clinging to my abdomen like I'm its BFF.
As in lose weight.
I had the fat look before I started running so looking skinny fat sounds like an improvement.
"Skinny fat?" I asked.
It's being thin but having a layer of fat around the waistline, Greenfield said. While taking into account an individual's genetic ability to burn fat, endurance exercise alone can decrease a person's metabolism and increase the deposition of fat on the waistline, he added.
This is one reason why Greenfield and most fitness experts recommend that runners incorporate strength training, especially if one of their goals is to lose fat. Adding resistance as you work your body's big muscles such as the gluteus and pectoral muscles for example has the potential to burn more calories and more fat than cardio workouts on their own.
We know this but getting in extra mileage can often be at the expense of strength training sessions at the gym or at home.
Another way -- or better, in addition -- is to train your body to burn fat more efficiently by incorporating fasted aerobic workouts into your regimen.
According to Greenfield, when you do any kind of aerobic exercise after you have not eaten for a minimum of four hours but up to 12 to 16 hours you are forcing your body to tap into its own fat stores for energy.
What's behind fasted aerobic workouts?
What Greenfield is talking about is adapting the body to access stored fat for energy instead of glucose.
Glucose and carbohydrates are your body's main sources of fuel, and when glucose and carbohydrates aren't being used for energy they are stored as glycogen in your liver and skeletal muscles.
Typically runners load up on carbohydrates because the body can store a limited amount of glucose as glycogen in the liver and muscles. Runners can burn through those glycogen stores pretty quickly, which is where energy gels or powders or bars come in to quickly replace glycogen so runners don't experience that "bonking" or that feeling of hitting the infamous wall.
But if you want to lose weight as a runner and burn fat stores you can begin to train your body to burn fatty acids as fuel by exercising in a depleted state of glycogen.
In order to get into fat adaptation without feeling drained, for most of his clients, Greenfield recommends a morning aerobic session after a 12- to 16-hour overnight fast that is 20-30 minutes long that isn't stressful -- an easy swim or easy jog or walk or even a yoga session.
Another reason to do this gradually and gently, he said, is that once you begin to exhaust your liver and muscle glycogen stores and your blood levels of amino acids fall low enough during exercise you run the risk of breaking down lean muscle and causing other types of potentially harmful hormonal imbalances.
"It's a fine line," he said. "And in this case, more is not better."
You also don't want to strength train on a fast, added Greenfield, who recommends doing your fasted aerobic exercise in the morning and your resistance work later in the day once you've had an appropriate amount of nutrition.
And a word of caution: If you have diabetes or certain other health conditions, consult your physician beforehand.
Weight loss and other benefits of fasted workouts
If you can do a fasted aerobic workout and then limit your frequency of meals to three as opposed to the typical five or six and restrict your carbohydrates to no more than 30 percent of your total daily intake, Greenfield said, you can get yourself to the point where you can do aerobic exercise while fasting for longer periods of time without deleterious effects.
And lose weight.
Greenfield also recommends eating most if not all of your carbohydrates later in the day. An ideal breakfast after a fasted aerobic workout would include healthy fats and plants, such as a smoothie. After an overnight fast, your blood sugar and insulin levels are low, and this is important, Greenfield said, because when insulin levels are high the body's fat burning processes are shut down.
For people who prefer a piece of toast with breakfast, Greenfield recommends naturally fermented sourdough, which is low glycemic and doesn't cause a spike in blood sugar.
Greenfield also said that fasting in general -- sometimes called intermittent fasting (IF) -- may be associated with antiaging effects by accelerating autophagy, the process of cleaning up cellular debris, and by decreasing the rate at which telomeres shorten.
Another possible benefit of IF, which is significant for athletes, Greenfield added, is that fasting may decrease the production of inflammatory cytokines, which are the molecules that cause joint pain when you exercise, so you are able to recover more quickly from an intense workout.
In terms of performance, Greenfield noted that one of the newer concepts of fat adaptation, or ketosis, refers to the point when your body becomes more efficient at burning fatty acids as a fuel. When that occurs, he said, you can go for longer periods of time without exhausting your body's relatively finite liver glycogen and muscle glycogen levels.
"You really only have 1,500-2,000 calories of storage carbohydrates in the form of glycogen in your tissues," he said. "If you're able to tap into fatty acids efficiently as a fuel during exercise it'll take you longer to hit that wall. And you also have more matches to burn for brief spurts of time when you go hard without exhausting your glycogen stores as quickly because during the periods of the race when you're going easy, you're burning more of your own fat as a fuel."
While the performance implications for athletes are significant, Greenfield pointed out that it can take six to 12 months to reach a state of fat adaptation where you can go for long periods of time exercising while feeling good on a depleted or low glycogen state, Greenfield said.
You can read more about Ben Greenfield on The Huffington Post.