What it takes to burn fat
This blog post will discuss weight loss.
When we want to talk about weight loss, what we really need to focus on is the question: how do I get my body to use fat for fuel? There are a few basic biological functions we should first understand in order to wrap our heads around the answer to this question.
Basic thermodynamics: matter cannot be created nor destroyed. Losing fat will require a calorie deficit.
The body’s preferred source of energy is glucose (sugar). Our bodies will always prefer to burn glucose for energy even if there is plenty of fat available. Glucose is easily released into the bloodstream, where cells can take it up and use it as energy. Fat (triglycerides) have to undergo lipolysis (splitting of triglycerides into fatty acids and glycerol) then those components can enter into the Krebs cycle and produce ATP (energy).
The body cannot effectively burn fat for energy in the presence of insulin.
With all these factors considered, there are some strategies for encouraging the body to use fat for fuel:
1. Exercise: 50-65% of your VO2 Max (or max heart rate) tends to be an optimal zone for burning fat vs. higher intensity workouts which use anaerobic respiration, which requires glucose. Of course, any additional physical movement helps increase energy expenditure. Many studies have shown that HIIT (High-intensity interval training) increases metabolism and energy expenditure at rest, and is highly correlated to weight loss as well. You can calculate your VO2 max with exertion tests, or estimate your level using online calculators.
2. Adapt your body to burn fat: training your body to use fat as energy and rely less on glucose takes time. A diet higher in fat and lower in carbohydrates can “re-train” the body to burn fat for energy more readily.
“One such strategy is “fat adaptation”, an intervention in which well-trained endurance athletes consume a high-fat, low-CHO diet for up to 2 weeks while undertaking their normal training and then immediately follow this by CHO restoration (consuming a high-CHO diet and tapering for 1–3 days before a major endurance event). Compared with an isoenergetic CHO diet for the same intervention period, this “dietary periodization” protocol increases the rate of whole-body and muscle fat oxidation while attenuating the rate of muscle glycogenolysis during submaximal exercise. Of note is that these metabolic perturbations favouring the oxidation of fat persist even in the face of restored endogenous CHO stores and increased exogenous CHO availability.” (4)
In layman’s terms: endurance athletes consumed a high-fat, low-carbohydrate diet for up to 2 weeks. Compared to a high-carbohydrate diet of the same caloric intake, the high-fat diet athletes saw an increase in fat burned for energy and slower use of muscle glycogen during exercise. Even when they returned to a higher carbohydrate diet, the rate of fat oxidation (aka burning fat for energy) remained increased. For endurance athletes, this is a huge benefit - depletion of muscle glycogen stores is what causes muscle fatigue. Tapping into two sources of fuel (body fat and muscle glycogen) simultaneously allows longer endurance before running out of glycogen.
What does this mean for the average person? A diet higher in fat and protein and lower in carbohydrates could make it easier for your body to burn fat stores for energy, even if you only eat this way periodically.
3. Make your body fat easier to burn: there are different types of body fat. “White” adipose tissue, and “brown” adipose tissue. Brown adipose (Fat) cells have mitochondria, which means they are more readily available to use as energy. Research is showing that it is possible to make white adipose tissue turn in to beige or brown adipose tissue, making fat stores more readily used for energy. Compounds that encourage this phenomenon include: capsaicin, green tea, berberine, resveratrol, curcumin, omega-3 fatty acids, irisin, retinoic acid, menthol, and melatonin. (6)
4. Sauna and cold exposure (extreme temperature): heat from saunas stimulate the body to burn fat the same way exercise does, increasing core body temperature. Conversely, cold plunges and other cold temperature exposure require the body to engage fat stores for thermogenesis, rapidly generating heat to prevent hypothermia.
5. Building muscle: even at rest, muscle mass uses more energy than the same weight in fat mass. Every kilogram of muscle mass burns 13/kcal per day. Gaining 10 lbs of muscle would cause the body to burn almost 60 calories per day just at rest, and even more with increased physical activity. Building and retaining lean muscle mass is being recognized as a key factor in successful weight maintenance after significant weight loss. (7)
As a person loses weight, their basal metabolic rate (BMR) decreases. BMR is the amount of energy your body uses at rest just to keep you alive. Body weight is a major factor in this equation. Muscle mass helps increase BMR, which can help combat common issues with weight maintenance. People losing significant weight often report struggles with “plateauing” - BMR decreases with weight loss to a point where it is difficult to remain in a calorie deficit without feeling tired and hungry.
Increased muscle mass also promotes stable blood sugar. Skeletal muscle is one of the few places that the body can store glucose (stored as glycogen in muscle). The more muscle mass a person has, the more glycogen can be stored. Once glycogen stores are depleted via exercise (70% VO2 Max exertion favors use of glycogen for anaerobic energy), intake of carbohydrates will favor storage as glycogen rather than storage as body fat (de novo lipogenesis). Thus, increased muscle mass allows the body to store more energy before beginning to store as body fat.
Sources:
(1) Jensen J, Rustad PI, Kolnes AJ, Lai YC. The role of skeletal muscle glycogen breakdown for regulation of insulin sensitivity by exercise. Front Physiol. 2011 Dec 30;2:112. doi: 10.3389/fphys.2011.00112. PMID: 22232606; PMCID: PMC3248697.
(2) https://pmc.ncbi.nlm.nih.gov/articles/PMC10054577/
(4) Wee KianYeoW.K. Yeo, Andrew L.CareyA.L. Carey, LouiseBurkeL. Burke, Lawrence L.SprietL.L. Spriet, and John A.HawleyJ.A. Hawley. 2011. Fat adaptation in well-trained athletes: effects on cell metabolism. Applied Physiology, Nutrition, and Metabolism. 36(1): 12-22. https://doi.org/10.1139/H10-089
(5)https://nutritionandmetabolism.biomedcentral.com/articles/10.1186/s12986-022-00694-0
(6) https://www.degruyterbrill.com/document/doi/10.1515/hmbci-2017-0016/html