Fat loss is very simple and very complex at the same time. In this article, we take an evidence-based look at fat, what it means to “burn fat”, and how to maximize fat loss through diet and exercise.
By Dr. Adam M. Gonzalez
SHIFTED’s Chief Scientific Officer
Key Points:
- Dietary fat is an important and essential macronutrient that serves several functional roles in the body.
- We primarily eat and store fat in the form of triglycerides - a glycerol molecule attached with three fatty acids.
- The fat that we store on the body is primarily made up of cells known as adipocytes that specialize in storing energy as fat.
- Fatty acids are constantly undergoing the process of storage and release into and out of adipocytes depending on factors such as your dietary intake and exercise habits.
- This article gives you the understanding of how fat cells grow and shrink and how to use diet and exercise to optimize “fat burning”.
What is Dietary Fat?
Fat is an umbrella term when it comes to diet, exercise, and body composition. In the diet, fat is an important and essential macronutrient that belongs to a category known as lipids. Although dietary fat is often demonized, it is required for smooth functioning in our body.
While lipids are comprised of other important categories such as cholesterol and phospholipids, dietary fat is primarily made up of triglycerides – a glycerol molecule attached with three fatty acids.
Fat serves several functional roles in the body including:
- Major fuel source
- Provides insulation and a layer of protection
- Aids the absorption of fat-soluble vitamins
- Incorporated into cell membranes
- Play a role in maintenance of sex hormones
- Important for optimal functioning of the nervous system
- Provides substrates for several other essential compounds in the body
The downside of dietary fat, however, is that it is the most calorie-dense macronutrient – offering 9 calories per gram – and when consumed in excess can lead to a caloric surplus and an increased storage of body fat.
What is Body Fat?
The fat that we store on the body is called adipose tissue and is primarily made up of cells known as adipocytes that specialize in storing energy as fat. While most people associate body fat with that stored around the waistline, there are different locations and types of fat storage. Based upon its location, body fat can be defined as:
- Subcutaneous fat – The layer of fat directly beneath the skin
- Visceral fat – Located around the organs in the abdominal cavity
- Intramuscular fat – Fat located inside the muscle
There are also different types of fat cells:
- White fat cells – Primarily functions as a storage site for extra energy in the form of triglycerides. This is the most common type of fat in the body.
- Brown fat cells – Contain more mitochondria and primarily functions to generate heat. In adults, brown fat cells tend to cluster around cardiac arteries and in the upper back and lower abdomen.
- Beige fat cells – Fat cells that are thought to be in transition between a white fat cell and a brown fat cell.
Almost all subcutaneous fat is made up of white fat cells that grow and shrink as a result of increasing or decreasing stores of triglyceride (14). When the body requires more energy, it often relies on this “storage tank” of triglycerides to provide fatty acids to the bloodstream to be metabolized to energy in the form of ATP. However, during times of prolonged overfeeding, these white fat cells fill up and increase in size (known as adipocyte hypertrophy).
There are also mechanisms by which we can increase the number of fat cells (known as adipocyte hyperplasia). It is believed that fat cells have a limit to how much they can expand to store triglycerides. Once full capacity is reached, the body responds by increasing the number of fat cells available to store energy. Ultimately, high levels of subcutaneous fat storage elevate disease risk and negatively impacts body composition.
Visceral fat is more metabolically active compared to subcutaneous fat, yet high levels of visceral fat are associated with additional health risks. Excess fat in the abdomen, particularly when out of proportion to total body fat, acts as an independent predictor of disease (5). In other words, high levels of fat stored in the abdomen and waistline are even more associated with obesity-related chronic diseases than just overall body fat percentage. Therefore, the World Health Organization (WHO) recommends classifying disease risk on both body mass index (BMI: kg/m2) and waist circumference.
Disease risk based upon BMI and Waist Circumference
Intramuscular fat is the lipid droplets found inside the muscle. We all have some stores of triglycerides in the muscle, and this can be particularly important for an endurance athlete to provide energy to the muscle more rapidly than would be possible from subcutaneous fat. Although, excessive storage of fat inside the muscle can lead to problems such as muscular dysfunction, insulin resistance, and oxidative stress.
There is no such thing as a perfect or even optimal body fat percentage, but it is well-appreciated that there are health consequences associated with high levels of body fat including an increased risk of chronic diseases and conditions such as type 2 diabetes, cardiovascular diseases, high blood pressure, and even some types of cancer. On the other hand, maintaining a severely low body fat level for too long can lead to negative consequences such as poor reproductive health and low bone density. Generally, for individuals who are overweight, following a sustainable and healthy weight loss plan to shed some unwanted body fat is widely considered beneficial and improves overall health.
What Causes You to Gain Fat? How do fat cells grow?
Fatty acids are constantly undergoing the process of storage and release into and out of adipocytes depending on factors such as your dietary intake and exercise habits. The net difference of whether more is being taken up and stored or broken down and released comes down to your total net energy balance. Overconsumption of dietary fat and, more importantly, total calories, promotes the uptake of triglycerides into adipocytes making them grow. You can learn more about calorie balance and how many calories you need to lose weight here.
After a meal, you break down the macronutrients into their smallest components to make key compounds required for bioenergetics (i.e., energy supply). Fat storage and utilization is regulated by several metabolic feedback loops. “Signals” such as fluctuations in intracellular concentrations of metabolic byproducts, actions of the sympathetic nervous system, and hormones bring about changes in the activity of specific enzymes involved in fuel utilization and storage. For example, protein, carbohydrates, and fat converge on the creation of acetyl-coenzyme A (acetyl-CoA), which is the starting material for energy production in the mitochondria. During times when we have a lot of acetyl-CoA available, it signals to the body that we can store excess energy away to be saved for a later time when we might need it. When we are chronically in a caloric surplus (usually due to a combination of eating too much and being inactive), it sets the stage for excess storage and fat mass accumulation.
What does it mean to “Burn Fat”? How do fat cells shrink?
To lose weight, you must consume less calories than you are expending. Move more and eat less, right? This is probably not a new concept for you. Beyond a calorie deficit, “burning fat” requires two key steps:
- Liberation of fatty acids from the stored triglycerides in subcutaneous fat and
- Oxidation of the fatty acid (12).
In other words, the fatty acid must be released into circulation and then be put through the metabolic furnace to harness its energy to be “burned off”.
First, there must be a signal to the body that we need to release fat from fat cells. The process of liberating fatty acids from the stored triglycerides is call lipolysis. During this step, triglycerides are broken down into their basic parts: 1 glycerol and 3 fatty acids. Once this occurs, the fatty acids are released into the bloodstream and transported to various parts of the body for use as a fuel substrate (i.e., oxidation). The rate of lipolysis is regulated by an enzyme called hormone sensitive lipase (HSL), which can be activated by low energy availability and hormones such as epinephrine, glucagon, cortisol, and growth hormone. HSL activity is blunted by caloric surpluses and hormones such as insulin.
Just because you have broken down triglycerides from stored fat, does not mean that you have “burned fat” yet. You are still only halfway there. If there is not an energy demand, the rate of oxidation will not meaningfully impact fat loss and the liberated fatty acids can be tucked back away as body fat. While lipolysis and liberation are critical aspects of fat metabolism, it does not equate with fat loss. The actual “burning” occurs when the fatty acids are taken up by the mitochondria and converted to water, carbon dioxide, and energy in the form of ATP via oxidative metabolism. Simply put, this will occur when you are active and in a calorie deficit.
How Fast Can You Burn Fat?
A general recommendation for rate of weight loss is 0.4-0.8% of body weight per week (1% of body weight per week maximum). For most people, this equates to a weight loss of 1-2 pounds per week. This rate of weight loss is viewed as being sustainable and helps limit some of the negative metabolic adaptations that occur with rapid weight loss. So, in a 6 month period, that’s 24-48 pounds. Even if you lost at half that rate, a 12-24 pound weight loss in 6 months would still make a very meaningful impact on your body composition and overall health. Remember, most people spend decades tacking on the fat mass. We cannot expect to lose it all in the blink of an eye.
More aggressive dieting and exercise strategies can sometimes be warranted, but you must consider the negative implications associated with low energy availability. And let’s not forget about the part that you will be very hungry if you drop calories too low. I like approaching a diet with a mindset of “if you can’t see yourself sticking to the diet long-term, it probably is not worth starting in the first place”. This is because it is far too common for people to crash diet and lose a bunch of fat mass only to hit a wall, return to old habits, and put the weight back on. This often occurs because the diet was not sustainable.
What Matters More? Diet or Exercise?
Creating a caloric deficit requires a reduction of calorie intake via dieting and/or an increase in caloric expenditure via exercise. Exercise is great for creating a calorie buffer and has several health benefits irrespective of calorie burn and weight loss (e.g., improved heart health, bone health, insulin sensitivity, fitness, and general well-being), however it is simply not feasible for most people to expend the daily amount of energy required to lose a significant amount of weight (>500 calories per day). Therefore, dieting is often a requirement, and it seems to be a more powerful tool for reducing fat mass.
When studies compare dieting alone vs. exercise alone, caloric restriction through dieting shows to be a superior tool for fat loss (10). One way to grasp this concept is to think about how it is easier to skip a 400 calorie slice of pizza than it is to hit the gym and burn 400 calories (but, yes, I appreciate that this is easier said than done).
What is the Best Type of Exercise to Burn Fat?
While dietary restriction shows to be superior for fat loss, exercise is definitely beneficial for a successful weight loss journey. Before getting too overwhelmed with hitting the treadmill at a certain heart rate zone, the priority should be to simply get more active. Schedule at least 150-300 minutes per week for at least moderate-intensity physical activity. Any form of movement and cardiovascular activity will help improve overall health and calorie burn.
When the goal is maximal fat oxidation (MFO) during exercise, there are some considerations worth noting. Fuel utilization during exercise follows a crossover concept depicted below. As exercise intensity increases, we rely less on fat and more on carbohydrate to meet the energy demand (12). MFO occurs primarily at exercise intensities between 45 and 65% VO2max (this equates to a moderate intensity for most people). When exercising at an intensity that exceeds the crossover point (~65% VO2max), carbohydrate becomes the predominant fuel source for energy supply. However, fat oxidation will also be influenced by training status, exercise duration, sex differences, and nutritional intake.
This does not mean that exercising outside of this zone will be ineffective for burning fat. All intensities of exercise promote fat oxidation during and in the post-exercise period (7). For example, while it is true that you rely less on fat (and more on carbohydrates) when exercising at a higher intensity, you are burning more total calories per unit of time and still burning a high absolute amount of fat. In fact, the more carbohydrate that you burn during exercise, the more fat you will burn in the post-workout period. This is why high-intensity interval training (HIIT) has gained popularity. Although you burn more fat during moderate intensity steady-state cardio, HIIT training has shown to spark more fat burning in the post-workout period as compared to steady-state cardio (15).
So, which is best for fat loss – moderate steady-state cardio or HIIT? The most recent review of the literature indicates virtually identical changes in fat loss when comparing the two training styles (13). The upside for HIIT however is that the benefits were accomplished in less than half the time as in the steady-state cardio group. Nevertheless, the best type of cardio must be individually tailored considering fatigue accumulation, time availability, and the incorporation of other training styles such as resistance training.
Resistance training is a critical component of any fat loss protocol and arguably more important than cardio. This is because resistance training preserves muscle mass while losing weight. It is not weight loss that we are striving for – it is fat loss. We do not want to lose muscle mass when dieting, and resistance exercise is the most effective tool for maintaining muscle mass while losing weight.
Several studies have shown that people who engage in resistance exercise while restricting calorie intake lose more total body weight and a higher percentage from fat compared to muscle.
For example, one study (11) reported similar weight loss among a diet group, a diet + cardio group, and a diet + resistance training group. However, the diet only group lost 69% of the weight from fat, the diet + cardio group lost 78% of the weight from fat, and the diet + resistance training group lost 98% of the weight from fat. Resistance training attenuates the loss of muscle mass during caloric restriction and sets the stage for a better long-term weight management.
Can You Spot Target Fat Burn from Specific Body Parts?
How do I burn belly fat? How do I lose the fat from my arms? These are questions that will be asked until the end of time. Unfortunately, there is no specific exercise, cream, supplement, or diet that you can use to preferentially shed fat from a certain part of your body.
The liberation of fat from adipose tissue is not site specific. Therefore, you are better off focusing on total calorie burn through multi-joint movement patterns (e.g., squats, hinges, pushes, and pulls) than isolated single-joint exercise that focus on the target area (e.g., sit-ups, bicep curls).
Foods To Help You Burn Fat Faster
There is no magic list of foods. Sure, I could recommend that you eat celery all day, but the truth is that all foods can fit in a weight loss diet as long as you are able to maintain a caloric deficit. However, when thinking about food selection, there are a few factors that can influence how many calories you consume in a day:
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Energy density – the number of calories per gram of food. Regularly consuming foods with higher energy density will likely lead to a higher caloric intake that drives a caloric surplus. Educating yourself about the energy density of foods can be an effective strategy to promote weight loss. There are several practical ways to decrease the energy density of meals such as increasing your intake of high-water foods, high-fiber foods, vegetables, fruits, whole grains, and legumes, while decreasing high-fat foods, fried foods, processed snacks, pastries, and foods/beverages with added sugars.
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Nutrient density – the amount of nutrients per gram of food. Foods such as green leafy vegetables, berries, and salmon have a high nutrient density, whereas foods such as potato chips, crackers, candy, pastries, and soda have a low nutrient density. Focusing on nutrient density will help assure that you are getting all the vitamins and minerals you need while staying in your caloric budget. The energy density of a food will more directly relate to caloric intake, however diets higher in nutrients have been associated with lower calorie intakes and lower hunger levels (6).
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Palatability – a property related to how much enjoyment or reward is derived from consuming a given food. This can be affected by energy density, mouthfeel, sweetness, saltiness, and texture. You do not have to give up every food that you enjoy, but highly palatable foods can impact the number of calories you consume. In a meal, high palatable foods can increase energy intake and result in less satiety (4, 9). In other words, it drives you to eat more and does not make you feel full for long.
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Processed vs. whole foods – Simply, try to eat more whole foods and less ultra-processed foods (i.e., junk food). Ultra-processed foods will be high in energy density and low in nutrient density. Even when equating for calories between a whole food meal and processed food meal, the processed foods can lower energy expenditure following a meal by up to 50% (2). Plus, several studies show that regular consumption of ultra-processed foods are associated with excess body weight (1).
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High protein – The best tools we have for maintaining muscle mass while losing weight is to resistance train and eat adequate protein. Protein also has a stronger effect on satiety, a lower likelihood of being converted to fat, and higher thermogenic properties than dietary fat and carbohydrates. Higher protein intakes can attenuate reductions in muscle mass and promote greater reductions of fat and total body mass. Shoot for at least 1.3 to 1.8 g of protein per kilogram of body mass per day (8).
- High fiber – Foods higher in fiber can increase food volume without increasing calories. This is a big win for increasing satiety throughout the day. High fiber intake is also associated with better weight management and reduced risk of heart disease, type 2 diabetes, and some cancers (3).
7 Key Tips for Burning Fat
- Prioritize protein at each meal.
- Walk more. For some, this might mean taking breaks from your desk at work. For others, it might mean taking a walk after dinner with your spouse. Find ways to move more and enjoy doing so.
- Train to build muscle – not just burn calories.
- Drink more water. Staying hydrated can help reduce hunger.
- Eat more fiber-rich, whole foods to increase satiety.
- Don’t let 2 days go by without some form of exercise.
- Find a diet that works for YOU for maintaining a calorie deficit.
Can a Supplement Help Burn Fat?
There are no magic ingredients that you can supplement with to rapidly strip away fat mass. However, there are supplemental ingredients that can mechanistically influence fat loss by elevating caloric expenditure and influencing the liberation and oxidation of fat. Another powerful target for weight loss supplements is appetite regulation. Hunger obviously plays a role in caloric intake throughout the day, and reducing appetite can aid in creating a caloric deficit. When considering the use of a weight loss supplement, I recommend understanding what ingredients you are taking, their mechanisms of action, and the research supporting their use.
About the Author
Adam M. Gonzalez is an associate professor in the Department of Allied Health and Kinesiology at Hofstra University. He earned a Ph.D. in Exercise Physiology from the University of Central Florida in 2015 and holds certifications as a Certified Strength and Conditioning Specialist (CSCS), along with a Certified Sports Nutritionist Certification (CISSN).
His primary research interests include exercise and nutritional strategies to optimize body composition, maximize health, and enhance adaptations to exercise. He was also awarded the 2022 Nutritional Research Achievement Award by the National Strength and Conditioning Association.
References
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- Bulsiewicz, WJ. The importance of dietary fiber for metabolic health. American Journal of Lifestyle Medicine 15598276231167778, 2023. https://journals.sagepub.com/doi/abs/10.1177/15598276231167778
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