Fasting and Exercise

A common question and the underlying uncertainty that gave birth to Fastingsport, my blog is—What happens if you are fasting and chose to work out? Can you work out and fast? Will, you go into some catabolic state and lose all your muscle gains or endurance? Are you going to end up with brittle nails, hair loss and sickly? You here these myths often in conversations involving nutrition and exercise. Most athletes wouldn’t even consider fasting. They operate on a caloric surplus, eating once every couple of hours, up to seven meals a day. Some eat huge amounts of food to “fuel their workout." Is this necessary?

The answer is an absolute no. The most common assumption is that our bodies do two things when we fast. Those assumptions are that we go into what is called “starvation mode” or become “catabolic” and that our basal metabolic rate drops, meaning our metabolism slows down to a crawl. These assumptions have been echoed over time. But do they hold any weight?

“Starvation mode” is a real thing. But, it only affects people who have a very low body fat percentage. If you are walking around at 4 percent body fat and fast for a long period of time, you would most likely experience “starvation mode” and the body would begin to break down muscle tissue. (Stote KS et al). A controlled trial of reduced meal frequency without caloric restriction in Healthy normal weight, middle-aged adults (AM J Clin Nutr 2007). This study found that adults who were restricted to one meal a day, without reducing caloric intake, decreased body weight by 6 percent over the duration of the trial. But, fat mass decreased 11.4 percent, and skeletal muscle did not change.

The myth of “starvation mode” has roots in the Minnesota Starvation Experiment. The study was aimed to better understand starvation amongst soldiers in post-WWII America. It was set up so that healthy subjects who were at or near their “ideal” weight were first given a diet of just over 3000 calories. Then the subjects had the number of calories cut in have after 12 weeks. The reduced calorie period lasted 24 weeks, and the subjects were placed on a strict “lower” calorie diet to simulate starvation experienced by WWII soldiers. They were, however, given vitamin supplements.

The results of the study were staggering. The subjects exhibited severe depression, a preoccupation with food, hysteria, hypochondriasis, social withdrawal, a decline in concentration, comprehension and basic cognition, decreased sex drive and isolation. Physically the subjects exhibited a low Basal Metabolic Rate, reduced energy expenditure including temperature. They had poor respiration and heart rates and edema. There were reports of hair loss, pale skin, and brittle nails. They were sick.

The misconceptions of the Minnesota study have reverberated throughout the nutrition world and have taken up root in the fitness industry. With little to no supporting evidence suggesting that fasting has even the slightest negative side effect, the practice of fasting and exercise is frowned upon and usually comes with a heavy warning that you will lose all of your muscle and become sick.

The subjects of the Minnesota Starvation Experiment were not fasting though. They were on a severe calorie restricted diet. There is a huge difference between fasting and caloric restriction. To understand that difference we have to take a look at the body’s response to a fast. While doing so, we will also see why State found that his subjects in 2007 were healthy and exhibited no muscle loss, and why Keys and his Minnesota subjects were ill.

Between six and twenty-four hours marks the beginning of the “fast” period. Your liver will break down glycogen and begin to produce glucose for energy. Under normal activity, your glycogen stores will last about twenty-four hours.

Over the next day or two, your body will convert non-sugar molecules such as glycerol and amino acids into glucose through the liver’s process of gluconeogenesis. Insulin and blood glucose levels will be near normal levels.

Ketosis begins anywhere from the first to the third day. This is where your energy is going to come from for the duration of the fast. You are now converting fat into energy.

After four or five days your body produces high levels of growth hormone. This hormone protects your muscle fibers and lean tissues. But, it also boosts your metabolic rate, which at this point is completely fueled by fatty acids and ketones from breaking down body fat. At this stage and beyond the body produces stem cells to heal the body. You easily rid your body of toxins stored in your body fat through your body’s natural cleansing processes. The stress of toxins in your bloodstream is vanquished

What happens while fasting is, your body begins to detox and heal itself from the damages you’ve done to it by constantly eating.

Interestingly enough, and this comes as a huge blow to the calories in versus calories out fans, it is a caloric restriction that slows down the metabolism, not fasting.

Training in the Fasted State

Our bodies have an amazing ability to adapt to situations we give it. When we train in a fasted state we dramatically lower our insulin levels. But we also increase our adrenalin. This process stimulates our ability to break down and oxidize fat for fuel. You are not going to “hit a wall." You won’t go into “starvation mode." Many elite levels athletes train in a fasted state, as well as eat a low carbohydrate, moderate protein, and high-fat diets. This not only helps keep their performance levels at a peak. But, it helps with inflammation of joints and the wear and tear of their long competitive seasons. You can absolutely train fasted. You may take a couple of weeks to become used to using fat as an energy source. But once you have exercise and fasting only combine to use more body fat. And who wouldn’t mind that?