3 Rules for Successful Low-Carb Diets

Low-carb diets work for fat loss, if you don’t wreck your Testosterone, cortisol, and thyroid. Here are 3 rules for success.

No Carbs = No Gains?

People have used low carbohydrate and very low carb ketogenic diets (VLCK) for decades to improve body composition and increase aerobic performance.

The evidence, both anecdotal and scientific, has shown low carb and VLCK diets to be powerful tools when used properly. But using low carb or VLCK diets incorrectly can wreak serious havoc on your hormonal profile.

They can negatively impact the levels and function of testosterone, cortisol, and thyroid hormones. The end result is decreases in muscle gains and declines in body composition.

Yes, you can actually get fat and flabby on a low carb diet.

But with a proper understanding of how low carb and VLCK affect your hormones, you can use these diets to improve body composition, which means both leanness and muscle growth.

Cortisol & Testosterone

Testosterone is a game changer when it comes to body composition. It’s known for its anabolic effects on muscle and ability to increase basal metabolic rate (BMR). It’s also important for overall health and well-being.

Specific to muscle growth, it’s helpful to think of excess cortisol production as having the opposite effect of testosterone.

Cortisol acts to increase glucose availability by mobilizing amino acids from muscle (gluconeogenesis), essentially “stealing” muscle to fuel high-intensity exercise.

It also has the ability to increase circulating sex-hormone binding proteins, essentially “locking up” testosterone and reducing its ability to signal muscle growth.

Thus, the anabolic effects of testosterone can be offset by the catabolic effects of excess cortisol. Your testosterone-to-cortisol ratio may well determine whether you’re in an anabolic or catabolic state.

This ratio, specifically the cortisol piece, is perhaps the biggest part of the low carb diet and muscle growth puzzle.

Do Low-Carb Diets Tank Your Testosterone?

Low carb diets may not directly reduce your post-workout anabolic window or response, but it can reduce your baseline anabolic environment.

Long term low carb dieting has been shown to result in lower basal levels of testosterone.

Reduced basal levels of testosterone, as a result of low carb dieting, can impact your ability to make long-term muscle gains, especially when coupled with elevated cortisol.

Cortisol: Enemy Number One for Low Carb Diets

Cortisol is released from the adrenal glands to mobilize amino acids from tissues (primarily muscle) to increase the availability of glucose through gluconeogenesis, making cortisol catabolic.

In the context of training, cortisol is released during high-intensity, anaerobic exercise to maintain normal glucose levels. The amount of glycogen you’ve stored directly impacts the release of exercise-induced cortisol.

The more glycogen you have stored, the less cortisol is released, and the less glycogen you have, the more cortisol is released.

Intuitively, this gives us reason to suspect that long-term glycogen depletion resulting from long-term low carb diets may lead to chronically elevated cortisol levels.

Low carb diets also result in an increased exercise-induced cortisol response.

There’s evidence that proper nutrient timing could definitely reduce increased exercise-induced cortisol release. Anecdotally, we observe this all the time with successful protocols that manipulate nutrient timing while still being considered low carb.

Thyroid Hormones & Muscle Growth

There are two hormones produced by the thyroid: thyroxine (T4) and triiodothyronine (T3). Together, these are the primary regulators of your metabolism. Much like testosterone, the thyroid regulates our BMR and increases fat metabolism.

Research has shown that T3 also plays a role in the creation of fast-twitch fibers in muscle tissue. In other words, it impacts muscle growth.

The thyroid primarily excretes T4,which is then converted to T3. While both T4 and T3 are important, T3 is the most metabolically active thyroid hormone. Thus, the conversion of T4 to T3 is critical in maintaining optimal thyroid function and promoting a fat burning, anabolic environment.

If any of these hormone systems are knocked off kilter by a bad low carb diet, you could jeopardize not only your overall health but also your body composition goals.

The Thyroid Drives the Metabolic Engine

T3 is the most metabolically active thyroid hormone. It increases fat oxidation, improves our mood, and most importantly, it increases the production of fast-twitch muscle fiber.

The thyroid primarily produces T4, which is then converted to the more “active” T3. When there’s excess cortisol, the conversion of T4 to T3 is blocked.

As we discussed earlier, low carb diets increase cortisol, so it stands to reason that low carb diets may result in low T3 and a reduced ability to burn fat and gain muscle.

Research has shown that seven days of a low carb dieting didn’t influence T4 but significantly decreased T3 levels.

Studies also showed that decline in T3 wasn’t as significant when replacing those calories with more protein (rather than more fat).

More research showed the same thing when combined with exercise. It’s likely the presence of additional amino acids from high protein intake reduced cortisol production by converting amino acids to glucose via gluconeogenesis.

Long term low carb diets can result in elevated cortisol, resulting in low T3 impairing our ability to burn fat and build muscle.

Fortunately, smart nutrient timing can prevent such issues.

Three Rules for Losing Fat Without Losing Muscle


In high-intensity, anaerobic exercise, your body relies mainly on glucose from blood glucose, muscle glycogen, hepatic glucose output, and gluconeogenesis for fuel. Your body will also utilize glucose to replenish muscle and liver glycogen post exercise.

Fortunately, your body is cheap and thrifty and will utilize the easiest, most available source first and save your muscle tissue for last.

To reduce the need for cortisol production and to keep your body from using amino acids from your muscles as fuel, increase your carb intake (on Amazon) at workout time.

Eating carbs prior to training will increase blood glucose levels, reducing the amount of cortisol needed to meet energy demands.

You can maintain a low carb diet (~25-30% of your total calories from carbs) by reserving carbs for only workouts (before, during, after) and eating high protein and fat meals the rest of the day.

Not only will this timing of carbohydrates suppress cortisol and its catabolic effects, it’ll also result in secretion of the anabolic hormone insulin.


Increase protein to reduce cortisol. Providing the body with additional amino acids for gluconeogenesis from dietary sources may reduce the need to derive them from our muscle tissue.

Generally, when I use a low carb approach I try to replace most of the carbohydrate calories with both protein and fat. But keep in mind there are 9 calories per gram of fat, and only 4 calories per gram of protein, so I consume far more grams of protein than fat, but the calories are about equal.

For example, if I reduce my carb intake from 250 grams a day to 50-75 grams a day, I’ll replace about 400 of those calories of carbohydrate (100 grams) with 400 calories of protein (100 grams of protein) and the remaining calories will come from fat.


There’s strong evidence to support the use of medium chain triglycerides (MCTs) when adopting a low carb or VLCK diet.

MCTs perform a great trick by bypassing certain metabolic processes and end up being used for fuel during exercise (if necessary), thereby reducing the overall need for glucose and potentially reducing the need for cortisol.

While this is speculative, there’s ample anecdotal evidence to support the use of MCT from this perspective alone. Additionally, MCTs have been shown to be effective in increasing fat oxidation and improving body composition by reducing fat storage.

Dietary sources of MCT include coconut oil, palm oil, and butter. You can also find MCT in various supplements.

Low-Carb Warning

Low carb diets are powerful when used combined with nutrient timing and partitioning. But going on one without knowing about your hormones may result in a slowed metabolism and sluggish muscle growth.

But with the right tools, you can change your body comp for the better.