Session 98

We continue Next Step’s full-length 10, questions 5-10 covering a passage about body density. Follow along, download the handouts, and don’t forget to have fun!

For this episode, we’ve got Physics for Passage 2. But in the real exam, you’re going to be seeing consecutive passages under one section. As always, we’re joined by Bryan from Blueprint MCAT (formerly Next Step Test Prep) as we dive right in.

The podcast is part of the MedEd Media Network, which includes The Premed Years, The OldPreMeds Podcast, Specialty Stories, Ask Dr. Gray: Premed Q&A, as well as The Short Coat Podcast, a production of the Carver College of Medicine at the University of Iowa.

[02:49] Passage 2: Athlete’s Body Fat Percentage

Body fat percentage is a measure of fitness and can be estimated using the hydrostatic weighing method. Archimedes’ principle can be used to calculate the body’s density, which is then used to estimate fat percentage and lean body mass (total body weight minus body fat).

Equation 1 is known as the Hume Formula and is primarily used in adults which tells us lean body mass is equal to… (equation is given). Measurements of dry body weight and the apparent weight of the body and fluid allow for density calculations. This is then used to estimate percent fat using the Siri Formula (equation 2 given).

(Note from Bryan: You should know where the equations are and what they’re about. You don’t actually analyze them until a question forces you to.)

Both formulae depend on three assumptions.

1. The body is made only of fat and fat-free mass.
2. The density values of fat-free mass components, protein and mineral tissue are constant and equal across all individuals.
3. There is a constant non-fat tissue ratio based on average population ratios of bone and muscle.

For health club members participating in the hydrostatic weighing procedure, each participant was struck to a chair supported by a hoist and four cables which were attached to an accurate scale.

Prior to submersion, the participant was asked to fully expel all air from his or her lungs. Next, the chair and the participant were fully loaded into the pull and remained underwater until a stable scale reading was obtained. Finally, the chair and the participant were removed from the water.

[05:10] Question 5

How would the accuracy of the hydrostatic weighing method in a well-muscled athlete compare to the accuracy of the test in an obese person?

• (A) In the athlete, the true percentage would be equal to the calculated percentage while in the obese person, the true percentage would be lower than a calculated percentage.
• (B) In the athlete, the true percentage would be higher than the calculated percentage while in the obese person, the true percentage would be lower than a calculated percentage.
• (C) In the athlete, the true percentage would be equal to the calculated percentage while in the obese person, the true percentage would be higher than a calculated percentage.
• (D) In the athlete, the true percentage would be lower to the calculated percentage while in the obese person, the true percentage would be higher than a calculated percentage.

[06:10] True Percentage vs. Calculated Percentage

True percentage refers to the actual body fat the person has in reality, while calculated percentage is the result when you dump them underwater and do all this math.

First, you always want to conceptually get your head around the question. Most students make the mistake of immediately doing the math, given the equations in the passage. But back up a second and focus on the question.

So here, the question indicates a well-muscled athlete, and as you know, an athlete would have denser bones and muscles. So when you dunk him in the water, they’re just going to sink right into the bottom of the pool. The calculation we do with someone that has abnormally dense body means that when you do the math, the formula is going to tell you the guy has literally no fat at all, since fat floats.

But in reality, even your most high end of Olympic athletes will still have body percentage, but very low. So the answer here is (B). Reading the answer choices provided, where they mentioned the athlete first, you don’t even have to consider the obese person anymore.

[09:38] Full Credit for Partial Knowledge

Bryan calls this full credit for partial knowledge where you get full credit for the question just by figuring out the super dense guy who sunk right to the bottom of the pool. So just take a second about the question that was just asked and think about where you want to go before you start talking.

Take a second and think about what is asked of you before start diving in and doing equations because you may just be able to answer it without doing any equations.

[10:15] Question 7: Specific Gravity of Fat

If the average bone mineral density is 3.88 grams per cm3, which of the following is a reasonable estimation of specific gravity of fat in the body?

• (A) 0.09
• (B) 0.8
• (C) 4.2
• (D) 9.1

So here, you have to know what specific gravity is, which is density relative to water. So water has a density of 1 gram per cc. Anything less than 1 is going to float and anything bigger than 1 is going to sink. You want to know that fat floats. So right away, you can eliminate C and D. And you’re left with A and B. 0.09 means 9% density is like a life jacket or a pool noodle. It’s so airy and light and non-dense. Then you’d start floating away. So logically, you’re left with 0.8. Fat floats a little bit and it slowly floats to the top so it’s a little bit less dense than water.

As you can see here, all the choices are numbers and no math has to be done. You just have to use reasonable everyday common sense.

[12:40] Question 9: Buoyancy

What is the most likely reason that the subject’s required to exhale completely prior to immersion in the water tank?

• (A) Air in the lungs decreases the apparent weight of the body in the water and increases the percent fat estimation.
• (B) Air in the lungs decreases buoyancy and decreases the apparent weight of the body in the water.
• (C) Air in the lungs decreases buoyancy and increases the body density estimation.
• (D) Air in the lungs increases buoyancy and decreases the percent fat estimation.

Breaking down the question:

When you take a deep breath, you float a little bit. So it’s going to increase buoyancy. Hence, choices B and C are out. The answer here is A. Anything that makes you floatier is going to make the machine call you fatter because fat floats. And anything that makes you sinkier is going to make you less fatty because everything else sinks. So air which makes you floatier is going to make you read as having more fat.

[15:11] About Blueprint MCAT (formerly Next Step Test Prep)

If you’re looking for the best full length MCAT practice exams, check out Blueprint MCAT (formerly Next Step Test Prep). Save 10% off those by using the promo code MCATPOD.

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