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How Keto and Low-Carb Diets Affect Body Composition Results

There is no shortage of information out there about low-carb and keto diets. A quick Google search will yield hundreds of millions of search results (low-carb diets have been some of the most Googled diets since 2014).

Despite their pervasive popularity, a lesser-discussed aspect of these dietary approaches is how they can cause inaccurate results when taking body composition measurements.

In this blog post, we’ll walk you through the basics of low-carb diets, how they can affect your body composition results, and actionable steps to take to ensure your body composition measurements are accurate.

What is a low carb diet?

Low-carb diets are most notably defined by their restriction of carbohydrate intake. For a diet to be considered low-carb, your total daily carb intake should fall between 50–150 grams. Ketogenic diets (a.k.a. very low-carb diets) restrict carbs even further, requiring 50 grams of carbs per day or fewer.

Low-carb diets restrict or limit many high-carb foods:

  • High-starch foods (grains, legumes, some vegetables and fruits)

  • Certain nuts, seeds, and dairy items

  • Refined carbs (sugar, flour)

  • Many low-fat foods

  • Juice

When you initially shift your eating patterns from high to low-carb, two components of fat-free mass—body water and glycogen—are notably affected by the drop in carb intake and will influence your body composition results.

Components of Body Composition Tests

Put simply, body composition is just a way of saying “what the body is made of.” There are four core components of body composition:

  • Water – which is contained in your fat, muscles, blood, and other bodily fluids. Water can be found both within (intracellular) and outside (extracellular) the cells of your body.

  • Protein – which is found in your muscles, organs, and bones (yes, bones do contain protein!).

  • Minerals – which are primarily found within your bones and, to a lesser degree, the bloodstream.

  • Fat – which is found within the fat cells (adipocytes) all over your body.

These four components can be further grouped into two main categories:

  • Fat mass – which is quite literally all the fat in your body.

  • Fat-free mass – which is everything in your body that is not fat. Importantly, this includes body water and your body’s stores of carbohydrate (glycogen).

With a traditional scale, you can only know if you have gained or lost weight but not the cause of the weight change. Did the scale go down because you lost water weight or fat mass? By measuring the above components, body composition tests can provide a more accurate picture of how your body is changing over time or in response to a lifestyle change, such as an increase in physical activity or a decrease in total calorie intake.

How Low-Carb Diets Affect Body Water and Glycogen

Whenever you eat carbohydrates, your body will burn some for energy while the rest will be broken down into glucose and stored in the liver or muscles as glycogen. For every 1 gram of stored glycogen, there are about 3 grams of water. Glycogen actually cannot be stored alone—it must be paired with water.

As you eat fewer and fewer carbs, your body will start to break down and use the glycogen in your liver, releasing it into the bloodstream to be used to generate energy. The glycogen stores in your muscles will also decrease as they get utilized for energy production within the muscles themselves. As your body uses up its glycogen stores, the water once attached to it becomes released and excreted. Thus, as you use up your glycogen, you also lose some water weight.

More drastic carbohydrate restriction, like that seen in ketogenic diets, will lead to a more rapid loss of glycogen and body water. Taken together, your glycogen stores and the water they carry can weigh up to a few pounds. This is why some people experience rapid weight loss in the first week or so when they switch to a low-carb or keto diet.

Aside from glycogen and the requisite loss of body water, a low-carb diet itself can have a mild dehydrating effect. Sodium (a.k.a. salt) intake has a notable effect on regulating how much body water you retain. When your carb intake is sufficiently low (usually 50 grams/day or fewer), your body will begin to make ketones—a by-product produced when your body processes and metabolizes the fat you eat. These ketones can increase your body’s sodium excretion, thus causing further reductions in water weight.

How Low-Carb Diets Can Affect Your Body Comp Test

A sudden shift in glycogen and body water can throw off the tools commonly used to assess body composition, such as dual-energy x-ray absorptiometry (DXA/DEXA) and bioelectrical impedance analysis (BIA).

DXA was originally used to measure bone density to aid in the detection of osteoporosis. Today, it is frequently used to estimate body composition and provides measures for fat mass, lean mass, and bone mineral density. For a DXA scan, you lie down on a table while a robotic arm scans your body by emitting low-level x-rays over a 3–10 minute period.

While considered a “gold standard” in body composition analysis, the accuracy of a DXA scan can be affected by changes in body water and glycogen. A decrease in glycogen stores and hydration status can cause an underestimation of Lean Body Mass.

BIA works by sending a low-level electrical current through your body and measures the resistance (a.k.a. impedance) the current meets as it works its way through your body’s water. The impedance measurements then allow for body composition to be calculated. Unlike a DXA scan, a BIA measurement usually takes less than 60 seconds.

BIA assessments can be sensitive to moderate-to-large changes in hydration status. A short-term sharp increase in hydration can cause an overestimation of Body Fat Mass and underestimation of total body water and fat-free mass (i.e., all the mass in your body that is not fat). Similarly, a short-term sharp decrease in hydration — which is often seen in the first 1–2 weeks of a low-carb or keto diet — can also influence BIA readings. However, after this period body water will largely stabilize until carbs are reintroduced again.

While both low-carb and keto diets can impact common body composition assessment methods, BIA can be precise, and in high agreement with DEXA. Additionally, because BIA provides measures of body water, it may provide more accurate measures to better interpret BC changes.

How to Get Accurate Measures While Low-Carb

Because your body will experience a drop in glycogen and body water when starting a low-carb diet, comparing results before a low-carb diet with those taken while on a low-carb diet is comparing apples to oranges. To ensure you are getting accurate body composition measures, there are a few steps you can take to ensure quality results.

When you first start a low-carb diet, give your body at least 1–2 weeks to fully adjust. This will allow your body to normalise at its new glycogen and body water levels. After this period, take a body composition measurement.

This measurement will be your new baseline. So long as you remain on a low-carb diet, this initial measurement will be an appropriate comparison point to track your body composition changes.

But what if you want to come off of a low-carb diet? The approach is much the same. Eat a high-carb diet for 1–2 weeks and then take a new baseline body composition measurement.

Remember, body water will be reduced with a low-carb diet, and you can expect changes in body water again once carbs are reintroduced. Comparing body composition results taken while on a high-carb diet to those taken while on a low-carb diet may not provide the most accurate picture of how your body composition has truly changed.

Additionally, be mindful of these other dietary factors that can also affect the accuracy and reliability of your body composition test results. Don’t make any major dietary changes at least 3 days prior to testing!

  • Hydration status (keep your fluid intake similar between tests). However, if you test in the morning you may be dehydrated. It is recommended that you drink 1–2 glasses of water at least 1 hour before testing to ensure sufficient hydration.

  • Meal composition (if you ate breakfast before test #1, try to eat the same meal before test #2)

  • Avoid eating right before a test or wait at least two hours after a meal

  • Avoid consuming diuretics such as high doses of caffeine supplements or alcohol. If you regularly consume high caffeine doses, then keep your consumption of caffeine/coffee the same between evaluations.

  • Supplements such as creatine or carbohydrate-loading can also influence test results


Well-formulated low-carb and keto diets can be used as part of an overall approach to supporting a healthy lifestyle. But you should be mindful of how they can affect your body weight and body composition measurements. Correctly timed body composition assessments can prevent you from getting a false result and ensure you aren’t operating off of an incorrect baseline.

By following the strategies outlined in this article, you can better ensure your body composition results will be a reliable reflection of how your body is actually changing.

Thank you very much InBody USA for your contribution and publishing of this article. For more visit

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