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The Best Diet for Fat Loss

Thomas Lackie, M.S. Nutrition Science, @lackie_nutrition

So, you want to lose some weight, huh? It can seem daunting. The number of fad diets and the frequency of how often new ones become popular can be incredibly overwhelming. It’s hard to know what to do. It’s not like our school systems place a ton of effort on nutrition education during middle and high school. Most people have likely never attended a nutrition course. It also seems like the newer fad diets tend to be getting more extreme. Ones that consist of plants with no meat, while others consist of meat with no plants. Diets that consist primarily of fat and diets that just consist of fasting. Really, anything you can think of is out there, and if it’s an extreme iteration, it’s likely viral. I’m unsure why it seems to work that way. So, what kind of diet should you follow? What type of exercise should you do? What’s the BEST plan? If you’re going to start dedicating time towards things like tracking foods you eat, waking up early to exercise, and turning down donuts in the break room, you want to follow the most efficient diet and workout program possible. You want to get the most out of your efforts. This article will explain the best strategy to burn fat and improve body composition in the shortest time.

For starters, we need to know the rules of the game to play it well. Rule number one of weight loss is that you must be in a calorie deficit to lose stored body fat. A calorie is a unit of energy, so we can think of a calorie deficit as an energy deficit or a negative energy balance.

The body demands a certain amount of energy each day to maintain life. Therefore, to lose body weight, you must consume fewer calories (or energy) than you burn. When this happens, your body will have to use stored energy from body fat, carbohydrates (glycogen), and protein (muscle) to provide fuel for bodily functions. That is how all the diets referenced above “work.” They manipulate a variable that affects your energy consumption, resulting in a negative energy balance. This can include restricting a macronutrient (carb, protein, and fat), food sources, or feeding window (fasting).

In Figure 11 above, you can see energy balance represented as a scale. If we want to be in an energy deficit, we can decrease the calories we eat to achieve this. However, notice that we can also create an energy deficit by increasing the calories we burn through exercise. We could also do a combination of the two. Your body will use stored body fat, carbohydrates (glycogen), and protein (muscle) to fulfill energy demands and keep you alive. That is how all the diets referenced above “work.” Each manipulates a variable that affects the energy you consume, resulting in an energy deficit. Diet strategies can include restricting macronutrients (carb, protein, and fat), food sources, or feeding window (fasting).

If you burn 500 calories more than you ate in a day, you will have lost 500 calories of stored body weight, regardless of the diet strategy. Similarly, for exercise, if you burned 500 calories on a slow jog or 500 calories lifting weights, you lost the same amount in both scenarios. In other words, ALL diets and exercise programs can work and be effective strategies to help you lose weight, so long as it results in an overall negative energy balance. A deficit of energy.

Here is the crucial part. Where you lose stored energy from matters. If you go on a diet to lose weight, you are probably referring to fat weight or fat mass (FM). You’re also made up of lean body mass (LBM), such as muscle, organs, bones, fluid, etc., contributing to your body weight. But you’re probably not interested in losing weight there; you want to lose body fat. If you are in a 500-calorie deficit, you want all 500 calories to be burned from stored body fat. If you could do that, it would be maximum efficiency. Therefore, the best diet and exercise plan to follow for fat loss would be the one closest to 100% efficient. Would you believe a diet and exercise strategy of ~100% efficiency already exists? Let’s talk about it by first introducing a scenario. In this example, we will have the same person (29 y/o female) try two different diet and exercise strategies for fat loss and discuss the difference in outcomes.

Scenario A

You just got off the phone with some old college friends and planned a beach trip in the upcoming summer three months away. College was a few years ago, and you have gained 20 pounds. You decide you want to lose those 20 pounds and buy that new bathing suit you have been eyeing at Victoria’s Secret. You’ve wanted to lose this weight for quite some time now, but you now have the motivation to do it, and you’re going to start today. You can dedicate four days out of the week to exercise for 30 minutes. You follow a few fitness accounts on Instagram. A girl you like follows a vegan diet and posts low-calorie recipes on her stories that look delicious. You’ve even saved some recipes on your phone over the past few weeks. She looks great and has a body similar to what you want. Each morning she uploads videos of ab workouts and park runs with her dog, Max. You buy a pair of running shoes and stop by the grocery store to buy an assortment of fruits and vegetables on the way home. You pull up the recipes on your phone and prepare meals to take to work for the week. After prepping your meals, you lace up your new shoes and go for a run. Over the next 12 weeks, you really stuck to it. You didn’t deviate from the diet or workout plan once. You ran four times a week and adhered to the new vegan diet. You ate four low-calorie meals daily consisting of fruits and vegetables, and you even started incorporating intermittent fasting protocols halfway through the diet. You lost 20 pounds.

Scenario B

Let's think of the same scenario above, but this time replace running with resistance training and the vegetarian diet with a diet high in animal protein. Everything starting out is the same. You spent the same amount of time exercising and meal prepping each week. You ate the same number of calories and burned the same in each scenario. You had the same amount of willpower and discipline each day. In both scenarios, you will have lost 20 pounds, but there will be a difference in weight loss from stored body fat. Your body composition will look much different if you compare at the end of both diet scenarios. Why is this the case? Well, let’s look at some numbers.

During a typical weight-loss diet, it is estimated that roughly 30-50% of the total weight lost is from lean body mass (LBM) and 70% from stored body fat2,3,4. LBM consists of all non-fat tissues such as bone, muscle, and organ - so it’s not exclusively muscle. However, muscle is a big part of it, and the food you eat throughout your weight loss diet will significantly impact your body composition. It should also be noted that fruits and vegetables contain small amounts of low-quality protein, and the intermittent fasting protocols leave long periods without protein. This is important for a few reasons. First, high protein diets help preserve LBM in a calorie deficit5,6,7,3. But not all protein is created equal. Second, animal protein is superior for stimulating muscle growth compared to plant-protein8. So, protein quantity, quality, and lastly, frequency matter. Third, intermittent fasting leaves long periods without protein ingestion, resulting in increased rates of muscle protein breakdown9. Therefore, a diet high in animal protein, distributed evenly across multiple meals throughout the day, appears to be the optimal diet strategy for preserving/increasing LBM10. Now that we know scenario A was not the most efficient diet and exercise strategy, we can estimate that out of the total 20 pounds lost over 12 weeks, 30% (6 pounds) came from LBM and 70% (14 pounds) from fat mass. Now let’s compare to this to scenario B.

In scenario B, weekly resistance training and a high protein diet were the only two things that changed. We discussed how high protein diets help preserve LBM during a weight loss diet. Once resistance training is included, body composition changes become even more dramatic. Humans adapt to repeated bouts of resistance training by building bigger and stronger bones and muscle tissue. After a resistance training session, affected muscles will want to repair and grow at an accelerated rate for the next few days. The tissue can repair itself and build stronger if a constant supply of high-quality protein is available. So, a high protein diet alone can preserve LBM when calorie-restricted, and when combined with resistance training, it is now possible to increase LBM. This is especially true if you are overweight/obese or untrained, like in Scenario B5. But the first law of thermodynamics remains. The weight you lost must come from somewhere, and the only two options initially were FM or LBM, but I just told you LBM will stay the same or increase. So what option is now left? Your brain should be churning now. We can estimate that out of the 20 pounds lost in Scenario B; all 20 pounds were from body fat and 0 pounds from LBM. This is what 100% efficiency looks like. Of the lost weight, ~100% comes from stored body fat, not LBM.

So, imagine that. For the same amount of time spent dieting and exercising, you are burning potentially 30-50% more body fat by switching to a high protein diet and switching the exercise from cardio to resistance training. In both Scenario A and B, 20 pounds were lost over 12 weeks. 4 sessions of exercise at 30 minutes a session comes out to be 1,440 minutes of total activity. The only two variables that changed were the diet and the type of exercise, yet these adjustments are the difference of six extra pounds of fat loss and eight extra pounds of LBM. Start imagining how different scenarios A and B would look compared to each other. Let’s see how this example looks on a graph.

*Pre diet and exercise intervention

Physical Characteristics of Example Female in Scenario A & B

Fat Mass (lbs.)


Fat-Free Mass (lbs.)


Body Mass (lbs.)






*Post diet and exercise intervention

Scenario A (Post-12-week diet)

Scenario B (Post-12-week diet)

Fat Mass (lbs.)


Fat Mass (lbs.)


LBM (lbs.)


LBM (lbs.)


Body Mass (lbs.)


Body Mass (lbs.)


Here is one last thing to think about… Throughout any weight loss diet, your metabolism will adapt and attempt to conserve energy. You would eventually die if you were in a calorie deficit every day. As time goes on in a calorie deficit, your body will adjust its metabolism, resulting in less energy being burned each day. As a result, you become more energy efficient. LBM largely contributes to the total energy you burn each day, and If you lost 6 pounds of LBM, your metabolism would be different than the version that didn’t lose any LBM. In the following article, we will go into this more in-depth. We will also talk about the components that make up your metabolism, how it adapts to a calorie deficit, and what we can do from a diet and exercise standpoint to influence it to our advantage. Let’s end this article by going over the key points.

1. To lose body weight, you must have a negative energy balance. You MUST burn more energy than you take in.

2. Any diet and exercise program will work with the caveat of if they induce a negative energy balance. But from where you lose that weight matters. We want to lose weight, but we want it to be fat weight.

3. Be efficient. A high protein diet combined with weekly resistance training will be ~100% efficient. There is no better or faster way to lose body fat and improve your body composition.


1. mdsamo. DIET AND NUTRITION. Hipatía... Biología y Geología. Published October 8, 2019. Accessed June 12, 2022.

2. Hernandez-Reyes A. Changes in body composition with a hypocaloric diet combined with sedentary, moderate and high-intense physical activity: a randomized controlled trial - PMC. Accessed June 1, 2022.

3. Willoughby D, Hewlings S, Kalman D. Body Composition Changes in Weight Loss: Strategies and Supplementation for Maintaining Lean Body Mass, a Brief Review. MDPI. Published online December 3, 2018.

4. Wycherley TP, Brinkworth GD, Clifton PM, Noakes M. Comparison of the effects of 52 weeks weight loss with either a high-protein or high-carbohydrate diet on body composition and cardiometabolic risk factors in overweight and obese males. Nutr & Diabetes. 2012;2(8):e40-e40. doi:10.1038/nutd.2012.11

5. Longland TM, Oikawa SY, Mitchell CJ, Devries MC, Phillips SM. Higher compared with lower dietary protein during an energy deficit combined with intense exercise promotes greater lean mass gain and fat mass loss: a randomized trial. The American Journal of Clinical Nutrition. 2016;103(3):738-746. doi:10.3945/ajcn.115.119339

6. Miller T, Mull S, Aragon AA, Krieger J, Schoenfeld BJ. Resistance Training Combined With Diet Decreases Body Fat While Preserving Lean Mass Independent of Resting Metabolic Rate: A Randomized Trial. International Journal of Sport Nutrition and Exercise Metabolism. 2018;28(1):46-54. doi:10.1123/ijsnem.2017-0221

7. Wycherley TP, Moran LJ, Clifton PM, Noakes M, Brinkworth GD. Effects of energy-restricted high-protein, low-fat compared with standard-protein, low-fat diets: a meta-analysis of randomized controlled trials. Am J Clin Nutr. 2012;96(6):1281-1298. doi:10.3945/ajcn.112.044321

8. Phillips SM. The impact of protein quality on the promotion of resistance exercise-induced changes in muscle mass. Nutr Metab (Lond). 2016;13:64. doi:10.1186/s12986-016-0124-8

9. Williamson E, Moore DR. A Muscle-Centric Perspective on Intermittent Fasting: A Suboptimal Dietary Strategy for Supporting Muscle Protein Remodeling and Muscle Mass? Front Nutr. 2021;8:640621. doi:10.3389/fnut.2021.640621

10. Norton LE, Wilson GJ, Moulton CJ, Layman DK. Meal Distribution of Dietary Protein and Leucine Influences Long-Term Muscle Mass and Body Composition in Adult Rats. J Nutr. 2017;147(2):195-201. doi:10.3945/jn.116.231779

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