Most people just want to achieve good healthy fat loss with lasting results. They search on the internet how to lose fat quickly, and inevitably comes across fasted cardio. Most dive right in because the “science” makes sense.
The science being:
When in the fasted state, fat is used for fuel instead of carbs, thus burning more fat leads to faster fat loss. The fallacy of such thinking is that, burning fat doesn’t mean losing fat. If someone eats 2000 calories per day to maintain their weight (which is already too little for most people for weight maintenance purposes) and does some cardio that burns about 300 calories, they’ll lose fat because they are in a negative energy balance. Instead of eating, they decide to do their cardio fasted. How many calories do you think they’re going to burn? That’s right…300. Does this mean better fat loss? Nope.
Some people even take it one step further and deplete their glycogen the previous day, or eat low carb diets, to increase the fat oxidation even more. As a result, the body burns more fat. The body has to increase or decrease enzymes that utilize different fuel sources in order to produce energy to survive. If you eat a high-fat diet your body has to increase enzymes and pathways that burn fat. If you eat a high carb diet, your body reduces enzymes that burn fat and increases glucose oxidation. Both diets lead to the same fat loss even though the high-fat low carb diet people will burn more fat.
Burning fat ≠ fat loss.
“We conclude that negative energy balance is a pre-requisite for weight reduction and increased fatty acid oxidation per se has little, if any, effect to reduce adiposity (R).”
Your resting metabolic rate determines your energy expenditure per day. Other factors also influence your total daily energy expenditure, such as the thermic effect of food, exercise thermogenesis and also non-exercise activity thermogenesis (NEAT). NEAT can account for about a 2000 calorie difference between individuals.
What influences NEAT?
Mostly how much energy you have. If you’re tired you’re most likely not going be very active. But it’s not just being active that burns energy, but also your metabolic rate. There are many metabolic brakes, such as cortisol, serotonin, low androgens, low thyroid, heavy metals, polyunsaturated fats, etc., and they will all reduce the metabolic rate.
But back to fasted cardio.
Fasting is stressful to the body and it downregulates thyroid hormones, increases the sympathetic nervous system, increases cortisol and lowers your androgens and uncoupling proteins (which burns calories and produces heat) just to name a few.
Adding cardio to that will just potentiate the negative effect of fasting and result in a net negative effect on your metabolic rate. Remember, fat burning doesn’t equal fat loss, but a high metabolic rate does.
We don’t want to do things that negatively influence our metabolic rate. We want our metabolic rate high, and as a result, we’ll burn more calories, which includes both fat and carbs.
Just for an example, AMPK, which is an enzyme that boosts fat oxidation, but also increases mitochondrial biogenesis and uncoupling proteins (UCPs), are of much interest as of late because of its potential for treating obesity, diabetes, metabolic disease, etc.
But…AMPK promotors, which increases fat oxidation, don’t promote fat loss. Burning fat instead of carbs just shifts the balance to another fuel source. It doesn’t mean anything, because fatty acid oxidation doesn’t increase energy expenditure.
“A major caveat is that thermodynamically the only way to achieve net weight loss, assuming energy intake remains constant, is to increase energy usage or decrease energy efficiency by diverting metabolic energy to heat (R).
FAO doesn’t increase energy expenditure. Increasing fat oxidation decreases glucose oxidation, which produces lactate instead.”
In the short term AMPK promotors doesn’t result in fat loss. In the long term, it does.
By increasing mitochondrial biogenesis and UCPs that lead to a higher metabolic rate. As a result, more mitochondria will be burning more fuel and creating more heat thus leading to fat loss.
Promoting the oxidation of fat alone does nothing.
To prove my point, let’s look at the uncoupling drug, DNP. DNP promotes heat production in the mitochondria by increasing UCP. This significantly increases the metabolic rate and the demand for both fats and carbs.
AMPK, which doesn’t boost the metabolic rate directly, increases the oxidation of fat at the expense of carbs, whereas DNP, which increases the metabolic rate, increases both the oxidation of carbs and fat, leading to significant fat loss.
Back to fasted cardio again :).
Cardio, or just being active in general, increases the oxidation of fuel, both carbs and fats. This can increase the resting metabolic rate by about 7-10% compared to sedentary people, although only if done in a fed state (R). Doing the cardio in a fasted state will only lead to the oxidation of more fat, but not more fat loss. Fasted cardio will also elevate the cortisol:testosterone ratio, lower thyroid hormones and negatively affect the metabolic rate in the long run.
I need not remind you that if the body is “taught” to burn fat, it becomes insulin resistant.
Many other studies have found that prolonged cardio and the excessive oxidation of fats can lead to kidney damage (R), liver damage and disease (caused by the oxidation of polyunsaturated fat) (R) and insulin resistance to name a few.
Another downside to fasted cardio is that it lowers insulin (which people think is actually good). Insulin is very anti-catabolic and people with lower insulin lose more muscle, but also more fat. Keeping insulin a little elevated will help to preserve muscle loss but might prevent you from getting super lean. The reason I say might is because insulin inhibits lipolysis, but it doesn’t inhibit UCP. UCP is expressed in the adipose tissue and can continue to get rid of excess fat even if there is still insulin floating around. The thing is that UCP gets downregulated after a prolonged caloric deficit and stress (such as fasted cardio). Thyroid and androgens increase UCP and keep the metabolic rate high.
Just to cement my claims with some actual scientific proof, Schoenfeld et al. showed that fasted cardio doesn’t lead to better fat loss than doing cardio in the fed state (R).
For fat loss, a high metabolic rate is essential.
Doing exhaustive exercise, long term fasting and fasted cardio will negatively affect your metabolic rate, thyroid, androgens, muscle mass, insulin sensitivity, etc.
Fasting cardio increases the cortisol:testosterone ratio and in the long run will destroy muscle mass and lower the resting metabolic rate. As a result, a significant portion of weight loss can be from muscle mass and not fat mass.
A caloric deficit is key to fat loss, regardless if more fat is burned or not.
A high metabolic rate is essential for proper fat loss. Fasted cardio increases fat oxidation, but also slows the metabolic rate. Let’s look at how to preserve the metabolic rate during fat loss:
Pre-workout protein: “…dietary protein exerts an anti-catabolic stimulus when ingested before or during exercise Hackney and colleagues  noted that this effect of pre-exercise whey protein feeding on resting metabolism appears to last for at least 24 h after resistance exercise, though rates of fat oxidation were not different between carbohydrate or protein treatments.” (R)
“Likewise, Paoli et al. reported that consuming a protein-rich meal prior to moderate-intensity cardiovascular exercise resulted in significant increases in resting metabolism for 24 h after exercise.”
Both casein and whey are effective but I’d rather use casein as it releases amino acids slowly into the bloodstream over time, instead of spiking insulin and possibly causing hypoglycemia a bit later during exercise.
Pre-sleep protein: High dose protein supplementation (48g vs 20g) consumption increases resting metabolic rate the following day with casein being more effective than whey (R).
Caffeine: Caffeine increases the resting metabolic rate by 13% (dose-dependent effect) (R). Caffeine increases uncoupling as well as the oxidation of both fats and carbs. Caffeine can also help improve glycogen stores before and after training (A).
Sleep: Sleep loss reduces the metabolic rate (R). Get good quality sleep every night. Consistency is key.
Fructose: Fructose speeds up the metabolic rate, replenishes glycogen stores, keeps stress hormones low and potentiates the thermic effect of protein. Fructose is the least insulinogenic, so high fructose fruits such as apples and pears can be a great addition of your morning meal or smoothie (A).
MCT oil, capsaicin & green tea extract: These three ingredients can help to boost fat loss, but together, they work synergistically to boost thermogenesis and energy expenditure. They also potentiate the thermogenic effect of protein. Remember, a pre-workout meal will increase the resting metabolic rate for the day which is further potentiated by exercise. No pre-workout meal, no increase in metabolic rate for the rest of the day.
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