Fasting has become an ever more popular way of losing weight and staying healthy in recent years. There are a million different workouts and diets for a weightlifter to choose from, but fasting in and of itself is pretty simple: it’s going without food for a significant length of time. But is it worthwhile to fast while trying to build muscle, and can it impair muscle growth?
Fasting while trying to build muscle may help if your goal is to alter body composition. Fasting can increase insulin sensitivity, increase the rate of autophagy and the presence of fat-burning ketones, increase mental clarity, and promote fat adaptation.
How should you fast while building muscle, and how do these effects manifest in training? Explore these questions below as I seek to explain how and why fasting may be beneficial as part of your weightlifting lifestyle.
How Eating & Fasting Works
Fasting is an ancient practice that began with the very first humans, although at the time it was simply the result of having sporadic access to food. All of us fast to some degree between the time we eat our last meal (or midnight snack) and breakfast. The question then is whether or not being in a “fasted” state for extra time is beneficial.
The body relies on a few different fuel sources to keep your energy up. The first of these is blood sugar, followed by glycogen stores in your muscles and liver. These stores will last the body roughly 24-48 hours, depending on what you’ve been eating lately and how much exercise you get. This means that fasting will only put you into ketosis after a corresponding amount of time, namely one to two days. This then begs the question, why bother to fast for any shorter period?
When you fast for shorter but significant periods, you avoid rapidly increasing your blood sugar constantly. While normal, blood sugar spikes incurred from eating release insulin, which can increase fat storage in the body (it prevents the breakdown of fat). Fasting for some amount of time every day can help make you more sensitive to insulin, which is beneficial for avoiding prediabetes and diabetes.
However, it is important to note that insulin is needed by our bodies. It promotes the storage of glycogen in the muscles, which in turn makes those muscles look bigger. Just as importantly, it also helps the body to utilize the nutrients in our food, including the all-important amino acids that combine to form the building blocks of muscle.
Eating leads to a release of insulin, serotonin, and then melatonin, all of which combine to put the body in a postprandial or post-meal state. This state is referred to by many as the ‘anabolic’ state.
In the anabolic state, the body undergoes protein synthesis and your muscles grow, largely due to the protein mTOR, or Mammalian Target of Rapamycin, which gets going when the body detects an increase of nutrients like protein and energy. In turn, mTOR increases hormones such as IGF-1, which in turn allows you to gain muscle and fat faster and easier. In other words, it helps with ‘bulking’, hence why so many bodybuilders eat constantly to remain in this anabolic state.
Low blood sugar, which occurs when you go long periods without food, increases cortisol levels in the body. In conjunction with a rise in noradrenaline, this raises your metabolic rate and increases fat loss. At the same time, cortisol leads to myostatin release, a substance responsible for breaking down muscle tissue. Testosterone is also lowered by the presence of cortisol.
Alpha cells then release glucagon as a rejoinder to the lowered blood sugar levels in the body. Glucagon in turn leads to the utilization of energy from stored glycogen from the liver and from the muscles. Once this is gone, any activity you do still requires the body to burn ATP or adenosine triphosphate, which in turn raises AMP or adenosine monophosphate levels.
This then revs up AMP or activated protein kinase, which fights against mTOR to reduce protein synthesis. Insulin levels are at their lowest after 18 to 24 hours, which encourages fat burning. When this happens, you are in what bodybuilders refer to as a ‘catabolic’ state.
Why then, would anyone want to fast if they are trying to build muscle? There are several unique and important benefits that may warrant the moderate use of fasting.
Benefits of Fasting for Bodybuilders/Weightlifters
Fasted State Calorie Burn
As mentioned above, fasting increases the body’s metabolic rate as a result of the release of cortisol, which means that even the calories you burn sitting still will increase when they are in a fasted instead of a fed state.
Fat adaptation is one side effect of fasting that can be very beneficial for those looking to lose weight. Fat adaptation increases the body’s consumption of lipids and typically takes 2-3 weeks of frequent fasting to achieve.
While remaining in an anabolic state is great for muscle building, constantly eating to keep blood sugar levels high decreases insulin sensitivity, which can put someone at risk for diabetes, obesity, cardiovascular disease, fatty liver disease, PCOS, and metabolic syndrome. This also means that the body stops utilizing the energy from food as readily.
Fasting works to increase insulin sensitivity, essentially reversing this process. Thus, fasting can be an excellent strategy for increasing your ability to bulk in the long term and remain healthy while doing so.
Autophagy is the body’s way of recycling waste material in the body. When somebody fasts, their body still needs amino acids and other nutrients, so damaged organelles and toxic waste are broken down and used up by the body. Mitophagy is the breakdown of mitochondria that are malfunctioning, and it decreases oxidative stress on the body in the long term. It also reduces inflammation in the long term by consuming antigens in the body. Autophagy can occur from 18-24 hours after somebody starts fasting and is explained in more detail here.
Alternatives: Do they Work?
Modified fasting refers to eating an extremely limited amount of calories while fasting, and usually gets brought up when discussing alternate-day fasting. The fact is that modified fasting will only provide you the same benefits of calorie restriction, and might decrease your metabolic rate, slowing down the fat-burning process.
The Keto Diet
The keto diet often involves fasting, as it naturally makes you more full as a result of all the fat consumed. However, Keto (or a high-fat, very low-carb diet) can be a great option for those looking to diet while building muscle even without fasting, as most studies find that it either increases or does not affect testosterone levels. Calorie restriction and fasting both reduce testosterone, as mentioned when discussing cortisol’s effect on the body.
While it might be difficult for some due to the dietary restrictions inherent, Keto can be a great alternative to fasting due to its fat-burning benefits and the lack of detrimental androgenic hormonal changes.
Protein-Rich Diets/Diet Modifications
Some may find that they benefit more from a few subtle tweaks to their diet than they would from frequent intermittent fasting. Some people simply can’t fast for health reasons, and different people respond differently to various diets and health interventions. If you are looking to lose weight while building muscle, it is worthwhile taking a look at how much protein you consume daily.
It is recommended that strength athletes eat about 1.2 to 2 grams of protein per kilogram of their body weight, every day. Doing so will aid in building and retaining muscle, especially if the protein you eat contains all nine of the essential amino acids that must be gained from food, especially leucine.