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How Do Hydration and Food Intake Change Body Composition Results?

  • How the BOD POD works.

  • How fat mass (FM), fat-free mass (FFM), and their percentages are derived.

  • Areas of potential error.

  • If a woman’s menstrual cycle impacts results.

Now, we’ll walk through the impact that hydration (or dehydration) and food intake occurring prior to a BOD POD have on the test’s results.

If you haven't read part 1:

  • The “BOD POD report”: Click here to view an example printout.

  • FM = Everything in the body that is fat.

  • FFM = Everything in the body that isn’t fat (e.g., organs, bone, muscle).

  • % Fat = Percentage body fat.

Does Hydration Level Alter BOD POD Results?

Study #1 (click to read): What Happens When Wrestlers are Hydrated or Dehydrated?

Study overview: NCAA Division 1 wrestlers were measured in the BOD POD twice: Once before practice (hydrated state) and once after practice (acutely dehydrated state).

Prior to the first BOD POD, hydration was measured via urine specific gravity (USG), with hydrated equating to a result less than or equal to 1.020 g/mL. Wrestlers were defined as dehydrated once confirmed to have lost 2-3% of their body weight and/or produced a USG sample greater than 1.020 g/mL.

Study outcome: On average, hydrated wrestlers had a significantly higher body mass, body density, % Fat, and FFM when compared to their dehydrated states.

This makes sense.

Imagine holding a dry sponge in one hand and a wet sponge that has been soaking in water for an hour in your other hand (or one of those rubbery dinosaurs that expand once dropped in a bowl of water): Once soaked, the wet sponge will weigh more than the dry one.

A hydrated body contains more fluid and will weigh more than a dehydrated one.

Study #2: What Happens When You Drink Water and then Urinate?

Study overview: Ten men and women were measured in the BOD POD 10 consecutive times using different scenarios, including normal testing conditions, while holding a water bottle*, having urinated prior to, and/or having consumed 1 liter of water prior to (i.e., the equivalent of two Grande coffees).

Hydration was measured three times throughout the 10 measurements using bioelectrical impedance analysis (BIA).

Study outcome: Regarding those measurements specific to water and hydration, both the 1-L water consumption and holding the bottle containing water resulted in significant increases in FFM, without a significant change in FM. Urinating did not significantly impact FFM or FM.

*Bottles were filled with the same volume of liquid, but the liquid was either water, oil, or a glucose solution. Just like the densities differ for fat and fat-free tissue, as do the densities of different liquids.

Study #3: Does the Volume of Water Consumed Matter?

Study Design: Twenty-four males had baseline BOD POD tests, then they consumed 0.5 L of water, had another BOD POD test, consumed another 0.5 L of water, had another test, and so on, up until 2 L had been consumed. Overall, five BOD POD tests were conducted per male.

Researchers did not report measuring hydration status at any time throughout the study.

Another arm of this study was to evaluate the impact of creatine supplementation**, which is known to retain water and increase total body water, muscle creatine stores, and body weight.

Study Outcome: With each 0.5-L water increment, body mass and volume significantly increased. However, only after having consumed 1 L of water did body composition significantly change: both % Fat and FM increased whereas FFM remained unchanged.

For the creatine arm, significant increases were seen only for body weight, density, volume, and FFM.

**21 grams creatine monohydrate daily for seven days. Two BOD POD measurements were conducted: One before and one after supplementation.

Study #4: Consuming 1 L of Water Pre-BOD POD

Study overview: Similar to study #3's design, 36 men and women had two BOD POD tests completed: One before and one after having consumed 1 L of water.

Researchers did not report measuring hydration status at any time throughout the study, even though BIA was also used to evaluate body composition (BIA evaluates total body water, too). Total body water values from BIA were not reported.

Study outcome: Similar to the findings from study #3's water arm, consuming 1 L of water resulted in significant increases in body weight, FM, and % Fat, but not FFM.

Does Food Intake Alter BOD POD Results?

Study #5: The Impact of No, Restricted, and Unrestricted Food and Fluid Intake

Study Design: Thirty-two participants were measured five times under three different testing conditions: Normal; after seven hours of unrestricted food and fluid intake, plus exercise; or 15 minutes after consuming either a 500-gram meal*** or the meal plus 1 L of water. Within each of those five times, body composition was measured via the BOD POD, BIA, skin folds, and dual-energy X-ray absorptiometry.

Throughout the five testing sessions, hydration status was measured twice via USG.

Study Outcome: When subjects were allowed to consume food and fluid, plus exercise, in an unrestricted manner in the seven hours leading up to a BOD POD measurement, FM increased by ~0.7 kg (1.5 lb). Granted, in this study “unrestricted” meant that researchers didn’t measure how much participants ate, drank, or exercised.

For the controlled interventions:

  • Consuming the meal plus water increased FFM by ~0.66 kg (1.45 lb) and FM by ~0.82 kg 91.8 lb).

  • Consuming a smaller meal (<500 grams) increased FFM by ~0.63 kg (1.4 lb) and decreased FM by ~0.11 kg (0.25 lb).

***Four slices wholemeal toast (with butter, jam, or Vegemite) and a protein drink.

So What Does This All Mean?

A Quick Loading of Fluid Means a Larger Body Volume

For studies evaluating water loading (i.e., drinking water immediately before a BOD POD test), total body weight will increase (after as little as 0.5 L of water has been consumed) and as should FM and % Fat (after at least 1 L)--mainly due to the body volume having increased.

In speaking with my work's sport scientist, the "quick loading" resulting in higher fat values makes sense: Consuming fluid leads to a small distention (i.e., stomach bulges out) that increases the volume an athlete takes up in the BOD POD. When volume increases, density decreases, and % Fat increases.^

^Body weight will also increase, so the impact on density will depend on how the increases of mass and volume influence one another. Remember: Density = Mass / Volume

A Slow Loading of Fluid Registers as Larger Body Weight and FFM

However, study #3's creatine supplementation arm showed that gradual water retention registers as changes in FFM. The gradual fluid change resulted in a higher body density, meaning a smaller FM and % Fat reading (per the Siri equation reviewed in part 1 of my BOD POD series).

Dehydration Registers as Lower Values Across the Board

For studies evaluating a known dehydrated state, the body will have a reduction in total body water, weigh in lighter when stepping on a scale, and, similar to a dry, shriveled sponge, have a smaller volume. Ultimately, values for body mass, FFM, FM, and % Fat will all be less.

Take-away Message:

Before every BOD POD test, follow the manufacturer’s preparation instructions and enter the machine in a hydrated, fasted, and rested stated:

The more consistent the body is upon entering the BOD POD prior to every measurement, the more comparable the results can be.

Acknowledge the potential impact of menstruation:

As mentioned in part 1, menstruation may lead to weight fluctuations and bloating (i.e., potential for higher results across the board or simply a higher % Fat from the bloating and increased volume), so record if the athlete is having their period and any symptoms they report (e.g., bloating, feeling "heavy" this week).

Include the error when reporting results, rather than assuming any numbers are absolute:

Also in part 1, I wrote about error and how the BOD POD report doesn't include the ranges (i.e., the report lists 15% Fat rather than 13-17% Fat when using a 2% error). Since many factors can contribute to inaccurate results, aim to at least represent those errors when debriefing with an athlete.


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