Muscle vs. FatWe compare and contrast muscle and fat more than any other tissues in the body because it seems like these tissues are in some sort of competition, as if there is a war in your body between muscle and fat and on the line are your strength, health, and fitness goals. The truth is that your body needs and uses fat just as it needs and uses muscle. Both are necessary, and both are the product of what you do and what you eat.
Muscle vs. Fat
Most people understand a few basic facts and thoughts about the differences between muscle and fat:
- One pound of muscle is equivalent to one pound of fat on the scale.
- Muscle is more dense than fat, meaning that muscle takes up less space on the body and looks harder and less “flabby.”
- People with more muscle are stronger than those with more fat.
We compare and contrast muscle and fat more than any other tissues in the body because it seems like these tissues are in some sort of competition, as if there is a war in your body between muscle and fat and on the line are your strength, health, and fitness goals. The truth is that your body needs and uses fat just as it needs and uses muscle. Both are necessary, and both are the product of what you do and what you eat. In this article, I hope to bring a clearer understanding of the two types of tissue, diving into some commonly held beliefs, looking at how and why your body creates or stores these tissues, answering some frequently asked questions, and overall painting a clearer picture of the relationship between muscle, fat, and your body as a whole.
Does “muscle turn to fat” when you age or if you don’t use it?
Many people still think that muscle turns to fat as you age. Despite this belief, older people tend to exhibit resistance to advice like eating more protein, lifting weights, and gaining muscle mass. Yes, we do tend to lose muscle mass, and we do tend to gain fat. However, it’s not like there is a fairy that is stealing your muscle and turning it into fat. So why does it happen frequently? The answer is a combination of alterations in lifestyle habits and physiological changes.
As we age, muscle loss often occurs with observable decreases in both strength and physical performance. According to most estimates, we lose about 3-8% of our muscle mass per decade after the age of 30, although Donald D. Hensrud, associate professor of preventive medicine and nutrition at the Mayo Clinic College of Medicine, reports that we may lose closer to 1% of our lean body mass per year as we age.(“Website” n.d.); (English and Paddon-Jones 2010)
This tends to happen for a couple of reasons.
- At any age, if you are not using your muscle and stop training, it will tend to decrease in size, a phenomenon called muscle atrophy. As most people age, they also tend to become more sedentary. The good news is that muscle size can be regained with the combination of proper lifestyle habits, resistance training, and diet.
- As we age, there are a few metabolic changes that occur, including a decrease in muscle protein synthesis, an increase in insulin resistance, and a decrease in metabolic rate. These changes concurrently occur with decreased physical activity levels, lower hormone excretion, nutritional deficits, and possibly chronic inflammation.(English and Paddon-Jones 2010; Evans 2010)
Regardless, the result is a loss of muscle mass, strength, and function….and metabolic rate.(Siparsky, Kirkendall, and Garrett 2014) As we age, it is estimated that there is a progressive decline in whole-body resting metabolic rate of roughly 1–2% per decade after 20 years of age.(Elia, Ritz, and Stubbs 2000)
At the same time as this muscle loss is occurring, many people end up gaining weight and fat. Physiologically, there may be some legitimate reasons why this happens:
- The biggest reason is a decrease in the metabolic rate that occurs as we age.
- A 2019 study suggests that as we age, there is a natural decrease in lipid turnover in adipose tissue (where the body stores fat), slowing down the removal of fat, which could result in greater levels of fat mass.(Arner et al. 2019)
- It is well accepted that changes in hormones occur with aging—testosterone in men and estrogen and progesterone in women. While it tends to accumulate in the gut, it can occur all over the body.
However, it would be ignorant not to consider the tendency for people to have reduced physical activity levels and, at the same time, gradually have higher calorie intakes (typically due to a shift to a higher calorie, more palatable diet). We are not mere victims of aging. Age changes us in ways that make it more difficult to build muscle and more prone to storing fat, and the notion that muscle turns into fat isn’t necessarily the full truth.
With proper lifestyle habits (think resistance training and nutrition, but also stress management, sleep quality, etc.), these changes can be minimized to maintain a more favorable body composition.
Are there any benefits to having body fat? In other words, what is the evolutionary “value” of fat?
With the overwhelming prevalence of overweight- and obesity-related health issues in our society, we are afraid of having excess body fat, especially as health-conscious strength athletes. But, there is actually a wide assortment of benefits that having a normal amount of body fat provides for the body. I will leave it to a fellow RD, Rachel Hartley, to share some facts:(“Website” n.d.)
- “Body fat helps to protect internal organs against trauma by acting as a physical cushion. Fat also helps keep internal organs stable inside your body.
- Body fat is the primary storage for energy. While your body is pretty much always using energy from multiple sources (cellular glucose, muscle, glycogen stores), the body taps more significantly into energy stores from fat during endurance exercise, so you’re able to move your body for long periods of time without stopping to eat every five seconds. Energy from adipose tissue is especially helpful if you’re ever sick, and it’s difficult to obtain adequate energy from food. Having fat reserves is protective, and having less fat meant someone had fewer nutrition stores to weather an illness.
- Adipose tissue is important for hormone regulation. It may represent the largest endocrine tissue in humans. Hormones secreted by adipose tissue include leptin (“fullness” hormone that inhibits hunger), adiponectin (plays a protective role in the development of diabetes and cardiovascular disease), tumor necrosis factor (pro-inflammatory and may contribute to insulin resistance), angiotensinogen (precursor to the hormone that regulates blood pressure and electrolytes), interleukin 6 (pro-inflammatory and anti-inflammatory and plays a role in the immune system), just to name a few. It also plays a role in the regulation of hormones that affect fertility, which is one of the reasons why eating disorders can lead to fertility problems.
- Fat, especially around the midsection (yes, that so-called “toxic” fat), can help protect against the side effects associated with menopause. When the ovaries stop producing estrogen, adipose tissue takes over, which helps mitigate the symptoms caused by estrogen depletion.
- Body fat helps with temperature regulation. Think of it like insulation.
- Fat tissue plays important roles in immune function. It communicates the metabolic state of the body to the immune system via hormones called adipokines, which control immune cell activity.
- There’s nothing wrong with having wrinkles – it’s a natural part of aging. However, body fat can be protective against skin’s aging by providing more structure to the skin.”
As people interested in strength development, we also know the anecdotal value of having a certain amount of body fat on improving our leverages for our resistance training exercises. For example, some extra mass packed into the gut simulates a tight belt that can function as a sturdy column for holding up seemingly absurd weights
So, what is an ideal body fat percentage? We know that being too lean can be detrimental to overall health and performance. But, having excess levels of body fat can also lead to poorer health outcomes and work against you (depending on your sport or athletic goals). Measuring body fat also is not a simple matter: BMI is an imprecise mathematical measure that has nothing to do with body fat, and direct measures of adipose do not always correlate with obesity-related diseases. So, it’s totally individual and something that can only be determined by YOU, your functionality and health markers, with the help of a competent medical professional.
How is muscle tissue created? What about adipose tissue (fat)?
The process of growing muscle is typically referred to as hypertrophy.
We all know that it is very hard, if not impossible, to build muscle with our daily activity and movement. Therefore, the first step in the process is to provide enough muscular tension to create damage. Additionally, metabolic stress and extreme stretching can create this damage. (Read more about how stress leads to strength gains, “Homeostasis: A Framework for Strength Gains.”)
After damaging the muscle fibers, the body responds with inflammation and sends cells called cytokines to repair the tissue. It does this by fusing the muscle fibers together to form new muscle protein strands or myofibrils. Over time, the buildup of these myofibrils results in bigger (and usually stronger) muscles.
However, this can’t be done without proper nutrition. Protein, specifically the amino acids, are the building blocks for muscle tissue. However important the amino acids are, you need energy to fuel the process as well. This is where the energy macronutrients (carbohydrates and fats) come into play. It’s an old analogy, but you can think of the carbohydrates and fats as the construction workers that use the protein you consume as the building blocks.
Additionally, you don’t grow muscles in the gym. You are creating the stimulus for muscle growth in the gym, but you need to let the adaptive process take place. This takes place outside of the gym, especially as you sleep.
Here is a short, simplified video that details the process.
In essence: Tear your muscles up. Eat right. Rest and repeat.
Muscle building seems like a simple process, eh? What about how our bodies create fat?
Adipose tissue is composed of adipose stem cells (the precursor cells that give rise to new adipocytes), adipocytes (the fat-storing cells), and various other cell types, which include mural, endothelial and neuronal cells.(Berry et al. 2013) The best way you can think of a fat cell as a tiny plastic bag that holds a drop of fat.
Just as with muscle, body fat is first developed as you grow in your mother’s womb. It is also able to be added throughout our entire lifespan. Interestingly though, the number of fat cells an individual has is usually stable after late adolescence and rarely changes during adulthood. Instead, it’s primarily the size of the fat cells that changes as we add body fat.
Trying to keep things as simple as possible, whenever you eat, your body breaks down the carbohydrates, fats, and proteins into their building blocks. Insulin, made in the pancreas, is the primary dictator of what your body tissues are utilizing or storing from those nutrients.
Typically, when insulin levels are high, carbohydrates are the primary fuel source and low levels of fatty acids are being burned. However, when insulin levels are low, fatty acids become the primary fuel source.
(Other hormones and biological inputs play supporting roles in these processes too)
As you see, the body is in the constant flux of burning incoming and stored nutrients, both carbohydrates and fats. So, how does the process of accumulating body fat happen?
Over time, when energy intake exceeds energy expenditure…in the presence of a calorie surplus, body fat is added to the body. In technical terms, this is called lipogenesis.
Does fat make us fat? Technically, yes. The fat you ingest in the form of food is broken down into fatty acids and combined along with cholesterol molecules to form vesicles called chylomicrons. They are then released into the circulation where they can either go to the liver or be stored in fat cells throughout the body.(“Website” n.d.)
However, carbohydrates can also be stored as fat. When you eat more glucose or carbohydrates than your body needs, your system creates and uses molecules called acetyl CoA to turn the excess into fat. While many people think protein is free calories, this same process can occur with excess dietary amino acids, as the building blocks are broken down into glucose, shuttled through the same process, and turned into body fat. The conversion of carbohydrates and amino acids to adipose tissue is referred to as de novo lipogenesis.
Excess dietary fat can be stored extensively in adipose tissue and is the easiest for the body to convert to body fat. However, the storage of excess dietary carbohydrates is more limited, and the metabolism of dietary carbohydrates is energetically expensive.(Leitch and Jones 1991) Despite what many low-carbohydrate zealots will tell you, the base of evidence to date suggests that carbs do not contribute significantly to increased fat balance, although experts are proposing that de novo lipogenesis may play a more significant role in the general increase of fat stores at a population level with excess calorie intake as a result of the modern hyper-palatable diet.(Hellerstein 1996); (Read more about the problems with a Traditional Western Diet here.)
Like I mentioned above, amino acids from excess protein can be stored as body fat. Recent studies seemed to disprove this theory, as protein intakes double to triple what is recommended by even physique enthusiasts resulted in lean mass gain and losses in body fat.(Jäger et al. 2017) However, a 2019 study showed that select amino acids (glutamate, glutamine, and leucine) induced de novo lipogenesis. But, as the authors of the paper noted, “At present, the precise amino acid components that are responsible for these effects, and the extent to which amino acids are able to induce DNL are not known.”(Charidemou et al. 2019)
In essence, body fat can be created from all of the food that we eat. Dietary fat is the easiest to be stored, with carbohydrates and proteins being more difficult for the body to add layers of body fat. However, the most important thing to remember is that this all occurs in the context of a CALORIE SURPLUS.
Is it possible to gain muscle without gaining body fat?
The holy grail of body composition…gaining muscle without adding fat. Many people seek to gain weight and muscle without gaining body fat due to competing in a certain weight class, aiming to limit the amount of body fat needed to be removed at a later time, and/or just because of our society’s fear of body fat in general.
From a physiological basis, there is technically no way to gain muscle without adding fat. The rationale? Aside from water (75%), skeletal muscle is made up of protein (20%), with the remainder from other materials including fat, glycogen, inorganic salts, and minerals.(Frontera and Ochala 2015)
As expected, In a review from Antonio and Leaf, the researchers reported that gaining weight in the context of overfeeding results predominantly in more fat mass gained (~60-70% of mass gained) in the absence of resistance training.(Frontera and Ochala 2015; Leaf and Antonio 2017) Well duh, right?
There are some caveats to these findings, however. In addition to the fact that these studies were not done in the context of exercise, the studies that reached this conclusion used sedentary populations, inadequate protein intakes and very high caloric surpluses (baseline energy intake plus 1000 kcal or 40–70% of baseline), making it hard to relate their findings to most of the BLOC readers.
What about studies that look at those who fit the BLOC mold? To date, there is a sparse but growing amount of research looking at body composition changes when correcting for most of these flaws (resistance-trained individuals, reasonable energy surplus, and adequate protein intake). Interestingly, there are some studies that suggest that it is possible to gain weight and lean body mass without adding fat mass. The main commonalities between these studies? Adequate protein (>2.2g/kg), a moderate energy surplus (370-800 kcal), and resistance training.(Leaf and Antonio 2017)
One key confounder in these resistance training studies is that these populations tend to be in relatively untrained subjects. In untrained subjects, a substantial energy surplus of up to 2000 calories combined with resistance training has been shown to provide robust weight gain where the contribution from lean body mass was up to 100%.(Rozenek et al. 2002) Due to their relatively new exposure to a muscle-building stimulus like resistance training, individuals who are novices tend to put on more muscle mass and less fat mass.
It should be noted that there are still resistance training studies that show that fat mass is part of the muscle gaining process. These studies tended to include lower protein intakes and higher caloric surpluses than those that found less fat mass gained.(Spillane and Willoughby 2016)
Additionally, as an individual becomes more experienced, they may need to be more concerned with variables such as the size of the energy balance and distribution of macronutrients on keeping gains as lean as possible.
For example, one 2019 review detailing recommendations for bodybuilders notes that, “in trained subjects, substantial energy surpluses might not be necessary or beneficial.” The authors attribute this to the fact that more advanced lifters have a more limited capability to gain muscle mass, factoring into an aggressive surplus resulting in higher rates of body fat gain. As one of the referenced studies showed, the smaller the surplus, the proportion of fat mass gained was reduced.(Garthe et al. 2013)
Thus, regardless of training age, it is still generally well-accepted in the lifting and scientific communities that gaining fat mass is part of the muscle gaining process.(Leaf and Antonio 2017; Slater et al. 2019) However, as one recent study suggests, “combining a positive energy balance with resistance training provides the most effective method to ensure the anabolic effects are directed toward increasing skeletal muscle mass.”(Iraki et al. 2019)
As an individual becomes more experienced, they may need to be more concerned with variables such as the size of the energy balance and distribution of macronutrients on keeping gains as lean as possible.
Thus, there needs to be a balance. Too high of a surplus may result in too much fat gain, where too little surplus may result in either reduced amounts, slowed rates, or no muscle gain at all. Based on the current evidence, the “sweet spot” for an energy surplus that facilitates optimal rates of muscle gain relative to fat mass is recommended to be ~1,500–2,000 kJ·day− (350-475 calories), or ~10–20% above maintenance calories per day.(Slater et al. 2019); (Iraki et al. 2019)
To be on the safe side, I would still recommend a higher protein intake. The studies by Antonio and colleagues suggest that consuming higher protein intakes results in greater proportions of lean mass gained. The line of rationale is that higher protein intakes cause increases in non-exercise activity thermogenesis and/or diet-induced energy expenditure, thus aiding in reducing fat mass accrual.
However, it should be noted that this is still inconclusive. As the 2019 paper on muscle hypertrophy reports, “so long as minimum guidelines for macronutrients advocated for resistance training individuals are achieved, there does not appear to be any metabolic or functional benefit to the source of the energy surplus.”(Slater et al. 2019) This is based on earlier findings in a very well controlled study where protein affected energy expenditure and storage of lean body mass, but not body fat storage.(Bray et al. 2012)
For people aiming to gain the most amount of muscle mass while limiting fat mass accrual, dietary protein intake is recommended to be 1.6–2.2 g/kg/day with a focus on sufficient protein at each meal (0.40–0.55 g/kg/meal) and an even distribution throughout the day (3–6 meals).(Iraki et al. 2019)
Thus, while many people are scared of gaining body fat, it is a required part of the process for many lifters. Aiming to minimize fat accrual typically results in suboptimal rates of muscle gain and extends the process of moving forward.
Gaining muscle should be thought of as an investment. When you gain weight, both muscle and fat, you are able to increase strength and add size to your muscles. As I have heard someone say before, …you are gaining “pounds for a purpose.”
Anecdotally, many people will find that it is a lot easier to lose fat than gain muscle. Once you get to a good level of muscle mass, you can shed the body fat that you gain in the process.
However, this does not mean being reckless, either. As mentioned above, there is a rationale in aiming for a limited amount of body weight gain over time. Gain too fast and you put on too much fat, but you can also shoot yourself in the foot and not make progress by not eating enough. Therefore, the recommendations above apply with the generally accepted goal of gaining ~0.25–0.5% of body weight per week to maximize the amount of lean mass to fat mass gained over the process.(Iraki et al. 2019)
In summary, gaining fat should be an accepted part of the process of gaining muscle and strength. However, with appropriate considerations and lifestyle choices, the amount of fat mass accrued can be limited and not impact long-term body composition outcomes.
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