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How Low Carb High Fat (LCHF) Diet Affects Your Metabolism and Weight Loss

Low Carb High Fat or LCHF diets are gaining popularity. From fat loss, energy boost, neuroprotection, and alertness – pick your weapon. But is this weight loss sustainable? How does Keto Diet affect your metabolism? What about Thyroid Hormones on a High-Fat diet?

There are certainly some pros of LCHF or Keto diets, in terms of fat loss, energy efficiency, mitochondrial function, and even cognition. Low-carb diets can bring many metabolic changes, and sugar starvation can be a great catabolic strategy. This is especially true for the short-term (in a sedentary, sugar-pumping world), hence why we’ll cover the importance of cyclic ketosis and metabolic flexibility. Let’s dive in.

LCHF vs High-Carb Diet on Energy

There’s this sentiment that one diet must be superior, it’s either Keto or high-carb. Today we’ll try to debunk that myth. There are pros and cons to each, and it’s on us to find the best instances to eat a certain way.

Carbohydrates are the primary source of fuel for energy production in the body. They are superior to fats in terms of fast energy creation, in a process of glycolysis. For anaerobic exercise, performance will be better if we fuel on carbs. These are your jumping, fast running, and lifting weight type of activities. (1) (2) (3)

Carbs are an important source of fuel, by which our body can produce energy (for skeletal muscles) very fast. The body stores glycogen in the muscles and liver, which we break down and use for energy.

Fun Fact

The simpler the carbohydrate is, the higher the blood glucose spike will be, the more intense energy we’ll feel, and the larger the crash that will follow. This is why eating complex, fiber-rich, low-glycemic carbs can have better metabolic effects, as it’ll reduce the sugar roller-coaster in your blood.

LCHF diet

On the other hand, fueling on fats can be superior in other ways. Keto or LCHF diet, which essentially limits sugar intake, will cause metabolic adaptation in which the body can burn fat for fuel. Since fats provide more energy per 1 g (9 kcal) compared to carbs (1g = 4kcal), it takes fewer fats to produce more energy. Also, we use less oxygen to produce the same units of energy when metabolizing fat, meaning higher efficiency.

The Keto diet (or LCHF) can:

  • improve mental alertness, cognition, and aid in neuroprotection. (4) (5)
  • It can be used as fuel for ultra marathons, or low-intensity aerobic exercise. (6) (7) (8)
  • It can accelerate fat loss and weight maintenance, (9) (10)
  • Reduce hunger or sugar cravings by reducing appetite, etc. (11) (12)

LCHF or Keto diets can be great fuel for low-intensity aerobic performance. But in most cases, fats won’t be sufficient to fuel pro athletes, even in many endurance sports. The superiority is the ability to reduce inflammation & fatigue, be efficient at energy creation, maintain longer low-intensity aerobic activity, and stimulate fat loss.

Metabolism and Metabolic Rate

Metabolism is the process of turning food into energy. From the mechanical breakdown of food to digestion and further absorption, metabolism is the endpoint. How can you turn that avocado-bacon sandwich, into chemically usable energy, ATP?

You’ve heard fast metabolism makes you a fat-loss machine, haven’t you? Metabolic rate (or metabolism speed) is the number of calories, your body burns to sustain physiological functions at rest. This is known as BMR or Basal Metabolic Rate.

If you eat hot and chilly foods, more protein, perform high-intensity exercise, etc., you can speed up your metabolism. Why would you want that? Because you’ve heard it burns more calories, which is partially true.

I say partially because Keto or LCHF appears to slow down your metabolism (in some instances) but makes you a better fat-burning machine. How? They improve energy efficiency, or the ability to produce more energy from less food (more fats, fewer carbs). This happens because fats produce more energy (ATP) per unit of oxygen than carbs.

Metabolism of Fats vs. Carbohydrates

Your body can run dominantly on carbs or fats.

If it runs on carbs, you’ll mainly get energy from glucose, by breaking down glycogen stored in the muscle, or liver. When you eat sugars, insulin will increase to transport that glucose into tissues (lowering your blood sugar). Excess glucose will be transformed into fats, and triglycerides, the ones no one likes.

To run on fat for fuel, your body needs to be in sugar starvation mode. When sugar is low enough, the body starts mobilizing fats to use them for fuel. In a process called the Krebs cycle, it turns ketones into energy. Boom.

Fun Fact

The point of LCHF or Keto is to shift (fully or partially) into ketosis. So instead of burning glucose (glycolysis) for energy, you’ll burn fat (lipolysis). This means mobilizing excess triglycerides and fats from adipose tissue, to use them for energy. Sounds good to burn your own fat for your own energy, doesn’t it?

  • The cons of too many simple carbohydrates are fast spikes of insulin, which can be detrimental to cardiovascular and brain health, as well as lead to diabetes and obesity.
  • The cons of too many fats for extended periods are potential ketoacidosis or having excess ketones which are more acidic. This may lead to brain fog, headaches, keto flu, etc.

This is why I see the keto diet as a reset diet done for 1-6 months, to restore insulin sensitivity along with the other potential benefits (alertness, fat loss, neuroprotection, energy).

That being said, the Keto diet is definitely not for everyone. This is where LCHF might come in handy. Some of the beneficial effects of reducing sugars will also be present with LCHF diets.

Point is, If we didn’t live in a hyper-sugary world with sedentary lifestyles, a low-carb diet wouldn’t even become popular. But nowadays, many people could have various metabolic and physiological benefits (related to sugar problems) from going low-carb and high-fat.

lchf on metabolism

Why Fast Metabolism Helps You Lose Weight

A faster metabolism means your energy expenditure is higher. Simply put, you’ll burn more calories. Your body burns calories to stay alive and perform basic physiological functions, that’s BMR. RMR or REE is similar, it’s the calories your body burns at rest.

On top of that we have NEAT, non-exercise activity thermogenesis which is you usually biking to work, or cleaning at home – not considered exercise, but still burns calories.

Plus, we burn calories through food thermogenesis (DIT) which is diet-induced. These are the calories you burn to digest (break down) the food you ate. The more proteins you eat, the more calories you’ll roughly burn through DIT.

Last but not least, we have exercise. This is the activity that burns more calories than you, just existing in the world. This is when you jump rope, swim, run, cycle, or perform similar activities. Some exercises, like HIIT, even come with an EPOC effect, meaning you burn calories after your workout has finished. Cool.

The speed of your metabolism will be pre-determined partially by your genetics, height, weight, lean muscle mass, and other factors. Your diet and exercise, which are lifestyle factors over which you have influence, can highly affect your metabolism too.

A faster metabolism means higher energy expenditure. It means burning more calories and producing more energy. Think about that skinny dude in high school who ate all the burgers.

Let’s see what Keto or LCHF does in this situation, shall we?

Keto and Thyroid Hormones: T3, T4

Thyroid hormones regulate your metabolism. The hypothalamus and pituitary gland dictate the release of thyroid-stimulating hormone, TSH which activates the thyroid gland to produce T3 and T4. These are important hormones for energy, metabolism, temperature regulation, metabolic function, and bone health. (13)

Lower thyroid hormone means a slower metabolism and less energy. Caloric deficit reduces thyroid hormones, slowing down your metabolism. Anytime you’re on a calorie-restrictive diet, your metabolism will slow down.

Increased thyroid hormone levels are associated with fat loss, due to increased fat mobilization. But also, anytime we increase our carbohydrate intake, T3 levels will increase because T3 is needed for the metabolism of glucose. This isn’t the case for ketones.

Because of this, we tend to see a drop in T3 hormone during an LCHF or Keto Diet. But is that detrimental in terms of slowing down your metabolism? Not necessarily.

While on LCHF diets T3 decreases, T4 levels in some instances, stay the same. In one study, T4 was elevated on a carbohydrate-restrictive diet (8% carbs). People lost 3.4 kg of fat mass and gained 1.1 kg of lead body mass, plus there was a significant drop in insulin by 34% and an increase in T4, by 11%. This is practically impossible for a slow metabolism (or hypothyroid function). (14)

Because we use fats for fuel and don’t eat glucose, we don’t need such elevation in the T3 hormone.

As posted in the Marks Daily Apple, most of the negative sentiment on keto and thyroid function comes from research on rats that ate corn oil, soybean, and safflower oil. On the other hand, those who fed on omega-3-rich fish, improved thyroid signaling in the liver. (15) (16)

Another study has shown that 55% fat, 10% protein, and 35% carb diet reduced T3 levels, reduced TSH levels, without changing T4 levels. (17)

Not to build a case for Keto or LCHF diets, but the argument is: Some Keto or LCHF diets have shown a reduction in T3 levels, partially because we don’t need it for glucose metabolism. High-fat, low-carb diets elevated T4, or it stayed the same.

So far, if on the LCHF diet TSH and T4 levels stay the same, while T3 drops, our ability (as shown below) to lose weight, isn’t compromised.

Key Point:

  • Point is, keto or LCHF diets might make us more efficient at using food (fats) for fuel, to a degree where the drop in T3 hormone won’t be associated with slower metabolism in terms of inability to lose weight (or fat).
  • Also, lower T3 levels might not be such a bad thing after all, if T4 and TSH stay the same during ketosis. Research shows that in a weight-reduction diet, 400 kcal of proteins (as supplements) showed significant weight loss.
  • The most significant changes occurred in those who had the highest T4 level increases, and the greatest decrease in T3. Point is, at lower T3, people still lost weight.

This doesn’t mean that keto or LCHF diets will boost your metabolism. The results show that even if there’s a reduction in T3 levels if TSH and T4 stay the same, our ability to lose weight is unlikely to be compromised. During Keto, the metabolism will probably slow down, especially with caloric restriction. But this may improve our efficiency in energy creation, meaning per lower T3 we can still produce the same amount of energy. Point is, that slowing the metabolism in Keto isn’t necessarily going to compromise the ability to lose fat.

LCHF or Keto on Metabolism

The research surrounding LCHF and Keto for metabolism is two-sided. We can find studies that show a high-carb diet will increase energy expenditure AKA burning calories, meaning faster metabolism. Also, we can see the same for keto diets in some instances.

Since caloric deficit causes energy drop, it slows down the metabolic rate. There is a connection between our energy-production source (fuel) and metabolic rate.

Comparing LCHF to LHCF, the High-carb group in this study had higher energy expenditure, while in the high-fat low-carb diet T3 was reduced, and T3 uptake was higher. (18)

On the other hand, we have research that supports Keto or LCHF’s potential to increase energy expenditure AKA burning more calories.


  • In overweight and obese men, the isocaloric keto diet was more effective than the high-carb diet for increasing energy expenditure. It also reduced respiratory quotient, which means their bodies ran more on fats (less on sugar) during the isocaloric keto diet. Keto wasn’t superior to high-carb for fat loss, but increased energy expenditure. (19)
  • Excess carbs will be turned into fat in the adipose tissue. For those with hyperinsulinemia, energy expenditure can be suppressed. Switching the fuel source (from carbs to fat), may theoretically improve fat mobilization and oxidation, but also reduce the physiological stress that comes with insulin spikes and glucose metabolism.
  • Another interesting study, compared low-carb 20%, moderate-carb 40%, and high-carb 60% diets, during weight loss maintenance in people with 25+ BMI. The low-carb diet was superior in terms of total energy expenditure, higher by 209 kcal/d more than the high-carb diet. (20) However, there’s a critique of the methods and data representation of the study above, here. (21)
  • Short-term, low-carb diets can have positive effects on fat loss, and also improve some markers associated with type 2 diabetes. In the long-run, low-carb didn’t seem to have additional benefits for metabolic health or weight loss. Also, LCHF has lower adherence, and without sufficient fiber can compromise the quality of the gut microbiota. (22)

Keto or LCHF isn’t superior in increasing metabolic rate or energy expenditure. The narrative that seems to prevail, that a low-carb diet will lead to lower energy expenditure, holds true for the most part and is supported by science. But the association between lower metabolic rate impairing fat loss wasn’t strong either. Even with a potentially slower metabolism, LCHF or keto-eaters experienced significant fat loss, due to their fat-burning efficiency o(fat mobilization & oxidation). However, in some instances in obese or diabetic patients, there’s a potential for keto, as an alternative fuel source that may increase EE.

Although a high-carb diet or reverse dieting (where you eat more to boost your metabolism) can work, (especially when putting on muscle) this is a more anabolic diet. Keto or LCHF on the other hand, are efficient at burning more fat specifically. Also, they get you in a sugar-starvation mode, which can improve insulin and sugar control. Keto can reduce T3 levels, which may translate into better metabolism-efficiency, in terms of energy creation.

LCHF or Keto on Mitochondria Function

One of the main reasons why keto is promoted as an anti-aging or longevity diet has to do with its catabolic nature. Matter of fact, intermittent fasting or a ketogenic diet can stimulate autophagy in the body, which allows for the removal of junk cells, toxins, and dysfunctional cells. This lowers inflammation and oxidative stress.

But, mitochondria play a massive role in health. Mitochondria are the energy-producing power cells in the body. The more you have, and the better their function, the more energy you can produce, and fight oxidative stress theoretically.

The Keto diet seems to incorporate or engage the mitochondria more in energy creation. It can stimulate or induce mithormesis, which is the mitochondria generating low ROS levels for an adaptive response, which can then improve mitochondria function. (23)


  • Research even shows the potential of improving neurotransmitter function and enhancing mitochondrial respiration with the Keto diet. (24)
  • The Keto diet tested in 29 participants for 12 weeks in combination with supervised training reduced HOMA-IR, a model assessing insulin resistance. It also decreased RQ, which shows the use of ketones and fat oxidation for energy. Plus, keto causes an increase in mitochondria protein, meaning better energy production and efficiency. (25)
  • In a very specific scenario, in the hippocampal neurons of rats or mice (with specific genes and induced stress), those fed on a keto diet seem to have improved markers of mitochondrial biogenesis, and a calorie-restricted keto diet leads to better brain metabolism. (26) (27)

Keto on Mitohormesis & Mitochondrial biogenesis

While we can’t easily replicate all these studies and all examples on the population, keto’s potential impact on mitochondrial biogenesis and mitohormesis seems like an interesting direction to research. Also, fat-oxidation and energy efficiency per unit of oxygen is a very well-known pros for keto and the mitochondria collaborative work.

LCHF for Weight-Loss & Fat-Loss

The key idea behind LCHF or Keto weight loss benefits has already been discovered. It shifts our metabolism into using fat for fuel, instead of glucose.

Few things to mention:

  • When shifting to a keto diet, the immediate weight loss isn’t fat-burning but water loss, because water binds to carbs (glycogen)
  • Keto is not for everyone, and it may have different hormonal effects.
  • LCHF in general seems to be a versatile, easy enough diet for higher adherence while still providing many weight-loss (28) and insulin/sugar control benefits.
  • Keto (or LCHF) diet will in many cases prove superior to high-carb, in terms of fat loss and catabolism but it isn’t the best long-term approach. (29)
  • Keto or LCHF is better when used intermittently, to stimulate metabolic flexibility (shifting from carbs to fats for fuel).

Weight loss happens when we are in a caloric deficit. Metabolism is a bit more complex.

  • In a massive meta-analysis including 6499 adults, it was shown that a low-carb diet was effective for improving HDL, weight loss, and triglycerides. The low-fat diet was better for LDL and total cholesterol. (30)
  • For maintaining body weight, a better long-term approach might be the LCHF diet, especially in weight-dependent sports, where maintaining lean muscle mass is important. (31)

One more thing about muscles and metabolic rate. Muscles are more metabolically active, meaning they burn more calories. The more muscle you have, the more place for the carbs you eat to get stored. If you are lean, and eat LCHF, maintaining a good level of protein is crucial to preserve that muscle mass. Why? To stay metabolically more active and burn more calories.

LCHF has shown superior in some instances, mainly in people who are obese or overweight. Compared to high-carb, LCHF produced 1.5% more body mass reduction and 8.6% fat reduction. (32)

In terms of reducing triglycerides, insulin resistance, increasing HDL, and weight loss, in many instances LCHF diets seem superior, at least for the short term. Point is, it’s not just about fat loss, but also about improving metabolic health which can lead to better glucose utilization later. (33)

Last thing, Cholesterol. Many studies show Keto or LCHF increases both LDL and HDL. One is bad, the other one good. While it’s logical to increase cholesterol and fat, when you eat fat, the foods you eat matter a lot. Going for omega 3 rich foods like salmon, avocado, walnuts, and beef liver will provide much more benefit than fueling on bacon, sausage, and mayo. (the latter is detrimental for total, VLDL, LDL, and HDL cholesterol). We just aren’t that clear on the kind of foods people ate in many studies, it’s just controlled for macronutrients.


In terms of weight loss, keto or LCHF seems to be superior in some instances for obese and overweight people. While the first weight drop is due to water loss, at least in the short-term keto and LCHF may provide many metabolic health benefits related to glucose control (insulin). LCHF is more versatile and less restrictive, often a better choice for the general population. Keto and LCHF diets can target fat-loss better, as they accelerate fat mobilization and oxidation. For the long-term, there is conflicting evidence regarding Keto & fat loss, but as an intermittent diet, shows a lot of potential.

Metabolic Benefits of LCHF / Keto

The superpower of both the Keto and LCHF diets lies in excluding sugar. In today’s world, we are bloated with carbs and sugar everywhere. While carbs are essential and not bad at all, fueling on refined sugars has become a tremendous problem. It is one of the triggers for diabetes, and obesity and even increases inflammation, which may increase the risk of cancer, tumors, dementia, leaky gut, and many more.

By removing sugars from our diet, we have seen tremendous benefits in the way we can respond to sugar later on. LCHF diets may improve insulin response and sensitivity, reducing glucose levels in the blood, and potentially reducing the risk of insulin resistance. (34)

High-fat low-carb diets, with healthy fats dominating the scene, can improve cholesterol profile. This means reducing LDL, increasing HDL, and lowering triglycerides. (35)

Keto diet may improve brain function. Not just enhancing your cognition, making you more alert, improving wakefulness, energy efficiency, etc. but also it is neuroprotective, it may reduce plaque build-up in the brain and enhance mitochondrial function. (36) (37) (38) (39) 

Another big pro of Keto or LCHF is reducing inflammation. Eating healthy fats, like omega-3-rich foods are anti-inflammatory. But these diets reduce inflammation mainly because we exclude refined sugars, which are causing inflammation in the first place. (40) Partially, this effect comes due to improved insulin sensitivity and weight loss too.


Keto or LCHF diet may come with a myriad of metabolic benefits. Benefits include reducing inflammation, improving cognition, enhanced glucose metabolism, improved insulin sensitivity, improving cholesterol profile, enhanced mitochondrial function, etc.

The Long-Term Keto Problem (6 + months)

Depending on genetics, types of foods, metabolic flexibility, and other factors, the Keto diet will work differently for everyone. In the long-term, around 6 months+ keto diet may have some limitations.

Read the full article here: The Low-Carbohydrate Diet: Short-Term Metabolic Efficacy Versus Longer-Term Limitations (41)

  • First, it’s the fiber problem, where it isn’t so easy to get all the needed fiber to feed your gut microbiome on keto.
  • Over the long term, the keto diet can cause ketoacidosis, which is an increase in acidity due to too many ketone bodies in the blood.
  • In terms of adherence, it’s relatively hard to only eat fats and protein. Carbs are everywhere in our diet, and they acutely boost mood and make us feel good.
  • Nutrition imbalances are the next one. Although keto can be great intermittently or short-term, in the long-term in some people it may cause dyslipidemia and increased risk of insulin resistance.

The good news is, that many of the keto benefits can be experienced with the shift towards an LCHF diet. Far easier to adhere to, far more sustainable, and comes with similar effects. Also, Keto is still a powerful weapon for the short term. Mitochondria efficiency, energy production, fat loss, alertness and cognition, HDL increase, metabolic benefits… I can go on and on.

Key Points

  • Metabolism
  • T3 Levels
  • Weightloss
  • Fuel
  • Sugars
  • Benefits
  • Autophagy

Keto or LCHF diets aren’t superior to high-carb in terms of increasing metabolic rate. High-carb diets will increase T3 levels (for glucose utilization), support anabolism, and speed up metabolism.

On a keto diet, levels of T3 hormones will most likely decrease, which doesn’t negatively impact fat loss. In fact, keto-eaters are better at burning fats and are more efficient at creating energy per unit of oxygen.

Keto and LCHF have shown positive weight-loss benefits, in some instances superior to high-carb (especially in obesity or diabetes) for the short-term, but not for the long-term.

Low-Carb diets can shift our body to use and burn fat for fuel, by increasing fat mobilization and oxidation.

Excluding refined sugar (as in Keto, LCHF) leads to reduced inflammation and oxidative stress. Some of the “keto-benefits” aren’t specifically keto-related but come from excluding refined sugars.

Keto and LCHF diets have shown metabolic benefits like improving cholesterol profile, weight loss, improving insulin sensitivity, neuroprotection, enhanced mitochondria function, energy efficiency, and cognitive boos

Keto or LCHF diets are catabolic and many of their mechanisms are similar to how Intermittent fasting works. By stimulating autophagy (done in a calorie restriction, partially sugar-starvation too), we remove dysfunctional cells, plaque, junk liquids, toxins, etc.

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