Are Low-Carb and Keto Diets Good for Fat Loss? 

Introduction

MacroFactor is diet-neutral and recognizes that different approaches work for people depending on their goals and preferences. However, it’s always important to understand how each diet works. This article explores the use of low-carb and ketogenic diets for fat loss. Historically, these diets can generate both hype and skepticism, making it challenging to decide if they’re right for you. We’ll break down the differences between low-carb and ketogenic diets, how they might affect your body composition, and what you should consider if you’re trying to use them.

Let’s dig in!

A quick look at how fat loss works 

This article focuses on fat loss, which is important to distinguish because diet advice might differ if your goal is muscle growth or endurance training. For fat loss, all diets (regardless of macronutrient configuration) must manage the balance between Calories in and Calories out (CICO) to create an energy deficit. While the methods to achieve that balance vary based on personal preferences, a net negative energy balance is essential for fat loss.

When you’re in a Calorie deficit, your body taps into stored fat for energy. To create a purposeful deficit for fat loss, you must consume fewer Calories than your total daily energy expenditure (TDEE).

While any discussion of fat loss requires understanding how a deficit works, this is especially important when discussing low-carb or ketogenic diets. In some low-carb and ketogenic circles, the concept of energy balance is contested, with some low-carb advocates arguing that suppressing insulin levels will cause weight loss independent of energy balance. However, even the researchers who promote low-carb and ketogenic diets now acknowledge that an energy deficit is necessary to achieve significant weight loss. They now simply argue that restricting carbohydrate intake impacts hunger cues in a way that makes it easier to achieve an energy deficit (though that’s also an oversimplification). So, it’s important to recognize that the Calorie value and the energy equation are still at play, even if you believe one diet helps you more easily achieve an energy deficit.

What does it mean to be a “good” diet for fat loss? 

Several key factors must be considered when evaluating whether a diet is effective for fat loss. 

• Does the diet help you maintain a Calorie deficit? 
• Does it provide enough energy to support your current activity level, contributing to your overall TDEE? 
• Is the diet too restrictive, potentially leading to periods of overeating as a reactive response?

Ideally, a good diet preserves muscle mass, doesn’t exhaust you to the point of hindering activity, and is sustainable enough to follow consistently until you achieve your fat loss goals. Moderate-to-high protein intakes tend to support all of those goals, and virtually any ratio of fat-to-carbohydrate intake can be effective or ineffective in these areas.

So, when deciding if a diet is right for you or aligns with your fat loss goals, remember these considerations. This is why MacroFactor takes a neutral stance on diet and adherence — what works best often depends on the person and their unique circumstances.

With that said, let’s dive into low-carb and ketogenic diets.

Defining low-carb & ketogenic diets

The definitions of low-carb and ketogenic diets are ongoing debates with different considerations and macronutrient makeups. I like this scoping review of clinical studies that define low- and very-low-carbohydrate diets as:

Low-carbohydrate diet (LCD)*

• Generally defined as ≤30% of daily energy intake from carbohydrates. 
• Typically, it ranges from 60 to 130 grams of carbohydrates per day. This aligns with the threshold below which several nutrition authorities might consider a low-carb diet.

Very-low-carbohydrate diet (VLCD)*

• Defined as <10% of daily energy intake from carbohydrates. 
• Typically ranges from 20 to 50 grams of carbohydrates per day and is often aligned with ketogenic diet parameters.

*In this article, the “C” in LCD (low-carb diet) and VLCD (very low-carb diet) refers to carbohydrates. In some studies, however, “C” can represent Calories, which can be confusing. I’ll clarify this as it comes up, but consider this distinction when reviewing individual studies.

Ketogenic diets 

If you examine ketogenic diets from a purist standpoint, you’ll find that they were originally developed to help manage epileptic seizures. This creates a distinction between the therapeutic use of the diet and its standard use for weight loss or less severe medical conditions.

Therapeutic ketogenic diet (KD): The therapeutic KD was designed for medical conditions like epilepsy. It focuses on a stricter macronutrient ratio that aims to maintain a constant state of ketosis for managing seizures.

Standard ketogenic diet and variants: The standard KD are more for weight loss and general health. These diets reduce carbohydrate intake but allow for more flexibility. Some versions gradually reintroduce carbohydrates over time. Therefore, not all of these diets aim to maintain ketosis.

Types of ketogenic diets

Type	Main characteristics
Therapeutic KD	Approximately 80–90% energy from fat, with most of the remaining energy from protein.
Standard KD	Carbohydrate <50g per day Approximately 70% of energy from fat, 10–15% from carbohydrates, and 15-20% from protein
Atkins diet	Often divided into 4 phases: Phase 1: <20g carbohydrates per day Phase 2: 25–50g carbohydrates per day Phase 3: 80g of carbohydrates per day; remain in the phase until the desired body weight is reached Phase 4: consume up to 100g of carbohydrates per day Ketosis is often observed in phase 1 only
Modified Atkins diet (Dietary version)	Maintain phase 1 of the Atkins diet indefinitely to sustain the ketosis state Restrict carbohydrate and energy intake only, with no limitation on fluid and protein consumption
MCT KD	40–65% of energy from medium-chain triglycerides Based on the rationale that medium-chain triglyceride is more effective for production of ketone bodies
Low glycemic index treatment	40–60g carbohydrates per day Restricted to carbohydrates with a low glycemic index (<50)

A quick point on water loss and low-carbohydrate diets

When evaluating successful weight loss trials involving aggressive carbohydrate restriction, it’s important to consider how much of the measured weight loss is due to temporary glycogen and water displacement. This effect can occur in any weight loss study, even those that don’t restrict carbohydrates or induce a ketogenic state, as most long-term diets result in some loss of food volume and glycogen. The duration of a caloric deficit can also influence total water loss. However, this effect is often amplified, sometimes significantly, in low-carb (LCD) and ketogenic diets.

Why?

When you cut carbs, your body sheds water weight due to decreased glycogen stores. Glycogen binds with 3-4 grams of water per gram, so when glycogen levels drop, water weight follows. This explains why rapid weight loss on low-carb diets is often just water, not fat. For instance, muscle mass stores about 500g of glycogen, and the liver holds around 80g, fluctuating throughout the day. So, a severe carb restriction could drop your weight by 1-3kg from water loss.

This effect can be further exacerbated with the more restrictive styles of ketogenic diets due to dehydrating effects. 

This effect is evident in a study by Wilson et al, where 25 resistance-trained males were studied over 11 weeks to compare the effects of a ketogenic diet (KD) versus a Western diet (WD). The study assessed strength, muscle mass, fat mass, and triglycerides. After a two-week diet adaptation phase, participants then followed an eight-week periodized resistance training program. In the final week (weeks 10-11), carbohydrates were reintroduced to the KD group to observe the impact. During these two weeks, lean mass increased by nearly 5% due to increased glycogen and associated water weight. Or, stated conversely, following a ketogenic diet decreased lean body mass by about 5% (and total body mass by about 4%) by decreasing glycogen levels and associated water weight.

Looking at these studies, you can imagine how this could feel if you’re a lifter or dieter unaware of the effects of water regain. If you’ve been on a low-carb diet and suddenly reintroduced carbohydrates, the resulting jump on the scale can be unnerving — especially if you don’t understand why it’s happening. This lack of understanding of this effect can lead to an unnecessary fear of carbohydrates and a loss of confidence in your dietary approach.

So to quickly recap, low-carb and ketogenic diets produce fat loss comparable to diets with more moderate carbohydrate levels in the long term. However, people often think they’re more effective when dieting because they lose more water weight early (which people mistake as faster fat loss). Furthermore, people often think they’re regaining fat when they reintroduce more carbs into their diet while regaining some of the previously lost water weight. 

Does this mean ketogenic or low-carb diets aren’t good for fat loss?

Not necessarily. It’s important to recognize that low-carb and ketogenic diets can lead to greater weight loss in the short term, primarily due to reduced water weight. This water loss can create the perception of more rapid fat loss, but it’s not always reflective of long-term outcomes.

For instance, in a meta-analysis by Silverii et al, low-carb diets were associated with a larger reduction in body weight than moderate-carb diets at 3-4 and 6-8 months. However, the differences in weight loss between these diets were not discernible in studies lasting longer than 10 months, suggesting that while low-carb diets might give a head start in weight loss, their long-term effectiveness is similar to moderate-carb diets.

A systematic review of systematic reviews on low-carbohydrate diets for obesity also found that higher-quality studies showed little to no difference in long-term results between low-fat and low-carb diets.

This reinforces the importance of considering long-term data and understanding that early weight loss may be influenced by temporary factors like water loss rather than sustained fat reduction. So, when evaluating the effectiveness of these diets, take a step back and consider the entire timeline, as initial results may not always predict long-term success. It doesn’t mean you can’t use the diet; it just means you should know what’s happening with your rates of loss. 

Considering potential water issues, how can we use these diets for fat loss?

I want to emphasize that low-carb and ketogenic diets can still have benefits despite my focus on the nuances of fat loss versus water loss. However, it’s important to recognize that the initial rapid weight loss is often due to water loss, which will likely return once carbohydrate intake is reintroduced.

Personal preference plays a significant role in diet and weight loss success. Some studies indicate that low-fat and low-carb diets can produce similar health markers and weight loss results. Therefore, it’s important to be realistic about the narrative that low-carb diets are superior — they don’t appear to be. However, this doesn’t mean that low-carb diets are ineffective for those who prefer them. 

Moderate low-carb diets, such as those on the lower carbohydrate end of a Mediterranean diet, are well-supported by research and offer a great balance for most people. These diets prioritize protein, limit lower-nutrient, higher-Calorie foods, and can improve carbohydrate quality. Adjusting carbohydrate intake and focusing on better-quality carbs can positively impact body composition. This often leads to the inclusion of more vegetable- and fruit-based carbohydrates and fewer processed carbs, even in a caloric deficit. Anecdotally, experimenting with low-carb diets encouraged me to explore various vegetables for the first time and greatly expanded my palate.

However, with stricter ketogenic diets, you do become more susceptible to nutrient deficiencies and potential health risks. This is why I generally advocate modified or short-term ketogenic diets for most people who want to experiment with low-carb diets or simply be mindful of including smart carbohydrate and fat-based fiber choices whenever possible.

Lastly, the more you restrict carbohydrates, the more you need to consider the impact on your training. Low-carb diets can affect training performance, especially for athletes and lifters. Maintaining a moderate carbohydrate intake is a smart choice to support performance for most. This need diminishes as training duration, intensity, and volume decrease (e.g., low-volume resistance training will be less affected than endurance training). Since fat loss phases aren’t ideal for muscle building or peak performance, reducing overall carbohydrates during these periods is generally fine. However, you should still consider timing your intake around training and finding the optimal level for your needs.

Take Home

Varying forms of low-carb and ketogenic diets can be effective for fat loss, but like any diet, they come with challenges. Here are a few key points to keep in mind if you’re considering these diets:

• Low-carb and ketogenic diets have different definitions. The characteristics of “low-carb” and “ketogenic” can vary depending on the source. Examine the details of different diet styles and determine how restrictive in carbohydrates (or higher in fat) you need for your goals.
• A Calorie deficit is still key in fat loss: Regardless of your diet, you must have a Calorie deficit to lose fat. Select a diet that best helps you achieve that deficit.
• Rapid weight loss on low-carb diets is often water: Cutting carbs depletes glycogen stores, and since glycogen binds with water, this results in significant water weight loss rather than fat loss. This is fine, but it should be taken into account when dieting.
• Context matters: These diets aren’t inherently bad for fat loss, but you should understand the variables and adjust your expectations. For example, if you’re also training with higher volume and intensity during a deficit, a keto diet may not sustain performance as well as a moderate-carb diet.
• MacroFactor has a neutral diet stance: The most important thing is finding a diet that works for you and supports your fat loss goals.