EXPLANATION FROM Fox LSAT

The first problem I see with the logic here is that the conclusion goes way off the rails with “the most effective way.” The study only compared two groups: high-carb/low-protein vs. low-carb/high-protein. How can the conclusion possibly say “the most effective way,” if it only looked at two ways? In order to make that conclusion, you would have had to study every possible method of losing body fat!

We’re asked to weaken the argument, and the above features such a huge problem with the reasoning that we should start by looking for that particular issue in one of the answer choices. If we can’t find it, then we’ll reconsider and look for another flaw.

A) This isn’t what we’re looking for, but it still might be worthy of consideration. Let’s get through the rest of the answer choices and if something awesome doesn’t jump out at us, we’ll come back to this one.

B) Well, yeah. Of course you can ruin your diet by “consuming large quantities of protein” alongside large quantities of carbs, fat, alcohol, and everything else. If you do that, you’re going to gain body fat. But that’s not the point of the study, because the study was about two groups that were on a fixed total caloric intake. No way is this it.

C) This would only strengthen the idea that the high-protein/low-carb diet is good for lowering body fat. We’re looking for a weakener, so this is out.

D) This would also strengthen the argument, not weaken it. If the high-carb people were also exercising, and they didn’t lose weight, then this would support the idea that low-carb is superior. We’re looking for a weakener, so this is out.

E) Regaining the weight is simply irrelevant. The conclusion of the argument wasn’t about keeping fat off, it was only about losing fat in the first place. No way.

I didn’t find exactly what we were looking for, but I really hate B through E. So now we can reconsider A. Here’s my case for it: If it’s true that the low- protein/high-carb diet causes you to retain water, then it’s possible that your body did lose fat even if you didn’t lose weight. The conclusion of the argument wasn’t about losing weight, it was about losing fat. If A is true, then it’s possible that the low-protein/high-carb diet was actually better than the high-protein/low-carb diet for losing fat. That’s the best weakener of the bunch.

So A is our answer
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Correct Option A

In a study,
Group 1 - high-protein, low-carbohydrate diet;
Group 2 - low-protein, high-carbohydrate diet.
Both diets contained the same number of calories, and
each volunteer’s diet had contained moderate levels of proteins and carbohydrates.
After ten days,
- those on the low-carbohydrate diet
had lost more weight than
- those on the high-carbohydrate diet.
Conclusion: the most effective way to lose body fat is to eat much protein and shun carbohydrates
Assumption: Fat loss is weight reduction.

Which one of the following, if true, most weakens the argument above?

(A) A low-protein, high-carbohydrate diet causes
(The human body to retain water),
the added weight of which largely compensates for the weight of any body fat lost,
whereas a high protein, low-carbohydrate diet does not.
Correct: Gives details related to Deit and highlight Despite weight loss by fat burning or reduction, but high-water intake which might have maintained the weight, this counters the conclusion.

(B) Many people
who consume large quantities of protein
nevertheless, gain significant amounts of body fat.
Flaw: gives details about Diet, and more emphasis on protein intake, this stem actually fails to address about weight reduction or weight gain.

(C) A high-protein, low-carbohydrate diet will often enable
the human body to convert some body fat into muscle,
without causing any significant overall weight loss.
Flaw: this gives details about Diet, but gives more details opposite to conclusion (low carbohydrate = fat loss = no weight reduction)

(D) In the experiment,
the volunteers on the high carbohydrate diet engaged
in regular exercise of a kind known to produce weight loss, and
those on the low-carbohydrate diet did not.
Wrong: gives no details about Diet or part of experiment, but try to adds another one part of experiment about excercise, and comparison between two groups, fails to address conclusion relate through diet about weight reduction.

(E) Many of the volunteers
who had been on the low-carbohydrate diet
eventually regained much of the weight they had lost on the diet
after returning to their normal diets
Flaw: gives details about post experiment and regain of normal past diet of volunteer, it actually strengthens the conclusion, saying it was low carbohydrates and high protein, which helped to reduce significantly reduction in weight

Group1: HP LC
Group2: LP HC
Group1 lost weight.

Conclusion: Thus, the most effective way to lose body fat is to eat much protein and shun carbohydrates.

We need to weaken the conclusion over here.

(A) A low-protein, high-carbohydrate diet causes the human body to retain water, the added weight of which largely compensates for the weight of any body fat lost, whereas a high protein, low-carbohydrate diet does not. - Correct. This weakens the conclusion stating why group 2 did not lose weight.

(B) Many people who consume large quantities of protein nevertheless gain significant amounts of body fat. - Irrelevant.

(C) A high-protein, low-carbohydrate diet will often enable the human body to convert some body fat into muscle, without causing any significant overall weight loss. - Irrelevant. We are not concerned about the muscles here.

(D) In the experiment, the volunteers on the high carbohydrate diet engaged in regular exercise of a kind known to produce weight loss, and those on the low-carbohydrate diet did not. - Incorrect. This doesn't weaken the conclusion in any sense.

(E) Many of the volunteers who had been on the low-carbohydrate diet eventually regained much of the weight they had lost on the diet after returning to their normal diets. - Incorrect. We are not concerned about the post diet weight gain/loss.

IMO A

Yes, the answer is A, because the evidence supports the idea that one diet helps to lose weight, but the conclusion of the argument is not about losing weight, but instead talks about how "to lose body fat". Those aren't necessarily the same thing, and if A is true, they definitely are not the same thing, and the conclusion doesn't follow from the facts.

There's something tempting about answer D here, though it's obviously not the right answer to a test question like this. D at first appears to strengthen the argument rather than weaken it -- even though the high-carb dieters were exercising, they still didn't lose as much weight as the high-protein dieters, so that seems to reinforce the idea that a high-protein diet helps to lose weight or body fat. So at first glance D seems a very bad answer to the question. But if the researchers, studying differences in diet, weren't controlling factors like exercise that you'd obviously need to control in a study like this, what other methodological errors were they making? Answer D suggests to me that this was an incompetently conducted study, which means its conclusions really aren't reliable. So in the real world, D seems like a good answer to a question like this. On the GMAT or LSAT, however, I don't think I've ever seen a CR question where you weakened an argument by attacking the competence of the people making the argument or collecting the supporting evidence, so this line of reasoning isn't likely to matter on the test.
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This a tricky CR question.
The idea is to read the argument and the choices very carefully.

Study: Those on high-protein, low-carbohydrate diet - lost more weight than - Those on the high-carbohydrate diet.
Conclusion - The most effective way to lose body fat is to eat much protein and shun carbohydrates.

Prethink:
The argument assumes that the body weight is primarily due to body fat.
What if body weight is reduced due to dehydration? - The argument won't stand anymore.

Let's look at the choices

(A) A low-protein, high-carbohydrate diet causes the human body to retain water, the added weight of which largely compensates for the weight of any body fat lost, whereas a high protein, low-carbohydrate diet does not.
In case of high-carbohydrates, the body weight is NOT reduced because body fat lost is compensated by water retained.
In other words, body weight remains same despite the fat is reduced.
i.e. body weight is NOT primarily due to body fat.
The choice attacked the assumption (a mandatory consideration) on which conclusion lies.
Thus, the argument is weakened.
(B) Many people who consume large quantities of protein nevertheless gain significant amounts of body fat.
If so, the choice seems to challenge the argument's conclusion that consuming proteins will help losing body fat.
We must be aware of half-information choice.
Are these people the part of of the study? If not, they might not have had low-carbohydrate diet.


(C) A high-protein, low-carbohydrate diet will often enable the human body to convert some body fat into muscle, without causing any significant overall weight loss.
The argument assumes that the body weight is primarily due to body fat.
The choice assists on the same.
Strengthening the argument.
(D) In the experiment, the volunteers on the high carbohydrate diet engaged in regular exercise of a kind known to produce weight loss, and those on the low-carbohydrate diet did not.
The conclusion implies that the volunteers on the high carbohydrate diet could not match those on low-carbohydrate diet in weight losing (irrespective of the exercises mentioned here).Thus, the choice adds nothing to the argument.
(E) Many of the volunteers who had been on the low-carbohydrate diet eventually regained much of the weight they had lost on the diet after returning to their normal diets.
The choice is tempting. If the effect is temporary, then the conclusion is weakened.
However, note the phrase "after returning to their normal diets" . The conclusion does not say anything about "quitting" the low-carbohydrate diet.
If this phrase were not there, this choice would have weakened the conclusion.


Takeaways -
Note 1: In such study-based conclusions, we must focus on the key words that the conclusion derive.
Note 2 : In case there is a conclusion that says "doing X will do Y" , ensure that Y is not a "temporary" impact of X.

IMO A

In a study, one group of volunteers was fed a high-protein, low-carbohydrate diet; another group was fed a low-protein, high-carbohydrate diet. Both diets contained the same number of calories, and each volunteer’s diet prior to the experiment had contained moderate levels of proteins and carbohydrates. After ten days, those on the low-carbohydrate diet had lost more weight than those on the high-carbohydrate diet. Thus, the most effective way to lose body fat is to eat much protein and shun carbohydrates.

Which one of the following, if true, most weakens the argument above?

Last line is the conclusion. HPLC is required to help lose body fat.

(A) A low-protein, high-carbohydrate diet causes the human body to retain water, the added weight of which largely compensates for the weight of any body fat lost, whereas a high protein, low-carbohydrate diet does not. - CORRECT. Initially, it looked what good use of water is there. But this option does explain that body fat is lost even if weight is not.

(B) Many people who consume large quantities of protein nevertheless gain significant amounts of body fat. - WRONG. If 'many' stands for exceptions than this option is a waste and if not then this is good candidate. But such an open option is not worthy generally.

(C) A high-protein, low-carbohydrate diet will often enable the human body to convert some body fat into muscle, without causing any significant overall weight loss. - WRONG. How much is 'SOME' ?. It did waste my time in deciding whether it is good enough.

(D) In the experiment, the volunteers on the high carbohydrate diet engaged in regular exercise of a kind known to produce weight loss, and those on the low-carbohydrate diet did not. - WRONG. First, an external factor i.e. 'exercise' and second 'weight loss' instead of body fat loss.

(E) Many of the volunteers who had been on the low-carbohydrate diet eventually regained much of the weight they had lost on the diet after returning to their normal diets. - WRONG. Not normal diets but HPLC.

Answer A.
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