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In Tim Ferriss' The Four Hour Body, the claim is made that lactose intake seems to inhibit fat loss. Note that it is not claimed that it causes fat gain. Like many claims in this book, it is (and the author is upfront about this) anecdotal and he does not provide an explanation for why it might be so.

So my question: can anyone verify that this is or isn't the case?

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According to the 2007 study, Effects of Dietary Lactose on Long-term High-fat-diet-induced Obesity in Rats, "the addition of lactose to a high-fat diet decreased body weight, body weight gain, fat accumulation, and the level of serum leptin."

In the study, four groups were observed over 84 days: A control diet group (Cont), a lactose diet group (Lac) in which corn starch in the control diet was replaced by lactose at 10% of the diet weight, a high-fat diet group (Fat) in which lard was added to the control diet to adjust the lipid energy ratio to 40% , and a lactose-added high-fat diet group (Fat+Lac) in which 10% lactose was added to the high-fat diet. The graphs below compares the weight of fats and the amount of serum leptin (a hormone that regulates long-term appetite) between the four groups at the end of the observation.

The graphs show there is no significant difference between the fat accumulation and serum leptin levels between the Cont and Lac group, but the Fat+Lac group does have decreased fat accumulation and serum leptin from the Fat group.

enter image description here

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Hmm, very interesting. Especially that there was no difference between the control and lactose-only groups. I'm wondering if the galactose is responsible, since it doesn't stimulate insulin production and helps regulate the glucose. –  Matthew Read Mar 10 '11 at 21:48
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From your reference, I would think that is an appropriate hypothesis. As for the difference between Fat and Fat+Lac, it seems my reference suggests the lactose helps with absorption of calcium, which other research shows to have anti-obesity effects. –  Andrew Ferk Mar 10 '11 at 22:08
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I think this experiment is slightly flawed in that the Fat group has a higher caloric intake than Cont or Lac and Fat+Lac has a higher intake yet. What's interesting is that Fat+Lac = less fat despite have more calories (probably per @Andrew Ferk's comment and @md5sum's answer about calcium). For me personally, this question was about drinking (whole) milk or eliminating it when I want to lose fat, and I think it seems as though it would be something to avoid (depending, of course, what I replace it with). –  Greg Mar 12 '11 at 6:04
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This is an interesting question, as "Dairy Products" will help you lose weight due to their high calcium concentration (Source). "Lactose" specifically, though, is where the carbohydrates in milk come from, and complex carbohydrates are converted into glycogen in the liver and used for the body's energy source, thus inhibiting the body from burning it's fat as a resource.

Lactose is a disaccharide derived from the condensation of galactose and glucose - Wikipedia

Intake of excessive carbohydrates leads to an uptake in the body's blood sugar levels, triggering the body to produce more insulin. Insulin promotes and aids in fat storage in the body.

In summary, "Milk" and "Dairy Products" in moderation aren't going to have a major effect on your fat loss, but excessive amounts of "Lactose" will. Note that the lactose is strained from curdled milk, leaving almost lactose-free curd for cheese production, which is why your hard cheeses have little to no carbohydrates in them, although there is usually enough lactose to trigger a reaction in those allergic to lactose.

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First you write that "Dairy Products will help you lose weight due to their high calcium concentration", and then you write "In summary, "Milk" and "Dairy Products" in moderation aren't going to have a major effect on your fat loss". Which one is it? –  Cray May 8 '12 at 16:51
    
It's entirely dependent upon which dairy products you consume, and how those products are processed, but overall, generally speaking, the effect of the extra calcium for weight loss will be about equally negated by the amount of lactose for weight gain. If you have dairy products without as much lactose, such as cheese, then you take in more calcium compared to the amount of lactose you take in. –  Nathan Wheeler May 8 '12 at 21:21
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Lactose is broken down by the lactase enzyme into galactose and glucose1. Glucose, of course, stimulates insulin production2, which stimulates fat retention3. So yes, lactose should inhibit fat loss. (Edit) But, as per my comment on Andrew's answer, galactose has the opposite effect as glucose4. I am inclined to believe, due to the results of the study posted in his answer5, that the galactose actually outweighs the glucose and thus lactose actually aids fat loss (at least at <10% energy intake from lactose).

(1): Wikipedia - Lactase
(2): Wikipedia - Insulin
(3): UNU.edu - "The inhibitory effect of insulin on lipolysis in adipose tissue, and the subsequent decline in plasma free fatty acid levels leads to the postprandial inhibition of fat oxidation."
(4): American Journal of Clinical Nutrition
(5): Nature

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+1 for simple :D Sometimes I wish they allowed just "Yes" or "No" answers –  Nathan Wheeler Mar 10 '11 at 21:29
    
Thanks :) I considered talking about milk & calcium but then I would have wanted to talk about organic milk and making sure the cows are fed grass so that there's more Omega-3 and less Omega-6 and maybe we should just all drink almond milk or eat more vegetables with calcium .. etc. :P –  Matthew Read Mar 10 '11 at 21:44
    
Aaaand now it's more complicated ... –  Matthew Read Mar 10 '11 at 21:55
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Tim cites this study: http://www.ajcn.org/content/74/1/96.full, which basically concludes that while milk products have a low glycemic index, they have a high insulinemic index, meaning they cause insulin concentrations to be higher in the blood.

Regarding the study cited by Andrew Ferk, I'm curious about the experiment design, especially the fact that the control group is being fed corn starch, and the experimental group is getting lactose.

It's of concern that the testing isolates lactose instead of feeding the rats milk products, which would include fats and proteins. It's also of concern that the protein:fats:carbs ratios are very low in protein, either 11 or 13 percent.

I think this all adds up to Tim's main point, which is this book is a record of a number of experiments he undertook on himself, and if you aren't approaching it the same way, you might not have good results.

So, maintain a steady diet, and try a month without milk, and a month with, keeping the other balances the same, and see how it affects things.

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You got the key point about lactose from the book: it's insulinemic -- not good for insulin production. My personal experimentation has found that removing milk from my diet has improved my energy levels and made my blood sugar more conistent. Anecdotal, but there you go. –  jcollum Dec 31 '11 at 20:41
    
I'm beginning to come around to this idea. My own personal experiment seems to indicate a severe slow down in weight loss with the inclusion of even moderate amounts of dairy. This makes me sad, 'cause I love me some cheese. –  Hack Saw Feb 28 '12 at 20:22
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