Tuesday, February 21, 2006

Low-Carb - High-Carb - Protein...What Does the Data Tell Us?

One of the largest meta-regressions to review the effects of low-to-moderate carbohydrate diets was published this month in the American Journal of Clinical Nutrition, Effects of variation in protein and carbohydrate intake on body mass and composition during energy restriction: a meta-regression. This particular review has received no mention in the press - most likely because the null findings from the WHI Dietary Modification Trial continues to dominate the headlines.

It's an important review however, bringing together a comprehensive and impressive set of data - 87 studies made the cut and were included in the analysis. Oh, and I think it's important to note, the researchers in this study - from the Department of Food Science and Human Nutrition and of Statistics, University of Florida - none have previously published studies investigating low-carb diets or low-fat diets. Noteable too is the conflict-of-interest disclosure - the paper was not the product of any outside funding - in fact, the paper states the lead researcher, James W Krieger, funded the effort.

So, then, let's get to the good stuff - what the researchers wanted to know and what they found when they started looking at all this data! They reviewed a number of factors:
  • Body mass change
  • Fat-free mass change
  • Percentage changes in body fat
  • Fat mass changes

After this comprehensive review and analysis, their conclusion was "Low-carbohydrate, high-protein diets favorably affect body mass and composition independent of energy intake, which in part supports the proposed metabolic advantage of these diets."

Body mass change

Diets with carbohydrate intake in the lowest quartile were associated with a 1.6–1.7kg greater body-mass loss than were diets with carbohydrate intake in the highest 3 quartiles. When carbohydrate intake was categorized as low or high, the significant effect in the low-carbohydrate intake group remained.

Fat-free mass (FFM) change

The amount of FFM retained tended to increase with each successive quartile of protein intake, with a significant difference existing between the upper 2 quartiles (greater than 1.05g/kg) and the first quartile (0.7g/kg). Specifically, the third quartile ( greater than 1.05 and 1.2g/kg) was associated with 0.78kg additional FFM retention and the fourth quartile (greater than 1.2g/kg) was associated with 0.96kg additional FFM retention.

Compared with carbohydrate intake in the lowest quartile, the carbohydrate intake in the highest quartile was associated with 0.98kg greater FFM retention. Carbohydrate intake in the second and third quartiles tended to be associated with 0.62–0.65kg more FFM retention.

In their discussion, the researchers state "Protein intake was a significant predictor of FFM retention. A daily protein intake of greater than 1.05g/kg was associated with a greater FFM retention than was a protein intake closer to the RDA. The magnitude of this effect increased when studies of greater than 3-mo duration were analyzed. Thus, the protein RDA may not be optimal for FFM retention during energy restriction, particularly during prolonged periods of dieting. Energy restriction can decrease nitrogen balance and thus decrease the amount of protein and FFM retained by the body. An increase in protein intake would increase nitrogen balance and thus increase the amount of FFM retained."

Percentage changes in body fat (BF)

Diets with a carbohydrate intake in the lowest quartile were associated with a 1.32–1.48% greater decrease in percentage BF than were diets with carbohydrate intake in the highest 3 quartiles. When carbohydrate intake was categorized as low or high, the significantly greater decrease in percentage BF in the low-carbohydrate intake group remained.

Fat mass (FM) changes

Diets with carbohydrate intake in the lowest quartile were associated with a 1.79–2.32kg greater loss of FM than were diets with carbohydrate intake in the highest 3 quartiles. When carbohydrate intake was categorized as low or high, low-carbohydrate diets were associated with a greater loss of FM than were high-carbohydrate diets.

One claim often made by those who insist that low-carb diets are not effective is the contention that weight loss is just water loss. Here, the researchers are clear - "Compared with higher carbohydrate intakes, low-carbohydrate diets increased the loss of body mass, BF, and percentage BF, even after control for energy intake as a covariate in the regression analyses. The mean total carbohydrate intake in the low-carbohydrate studies ranged from 79g–97g, depending on the analysis. Typically, a carbohydrate intake of less than 100g will cause ketosis. These results support the apparent metabolic advantage of low-carbohydrate, ketogenic diets. The additional body mass change is not likely due to water loss, because the duration of the diet periods (6–24 wk) was too protracted and estimations of total body water tend to be similar between low-carbohydrate and low-fat diets after 2-wk. The similar results of the analyses on body mass and BF also supports the concept that the effect on body mass of low-carbohydrate diets is an effect on FM rather than on body water."

At the end of their discussion, the researchers - a second time - make a bold statement - In conclusion, low-carbohydrate diets may increase the loss of body mass, FFM, FM, and percentage BF during weight reduction compared with traditional diets. The RDA for protein may be insufficient for optimal FFM retention during weight loss; high protein intakes (greater than 1.05g/kg) may improve FFM retention.

The researchers did a good job of selecting studies and weeding out those that did not meet their inclusion criteria. If you recall, last week I wrote about how poor data results in useless analysis in my article Comparing Low-Carb to Low-Fat: Analyzing a review. A quality review requires quality data - quality data comes from participants actually sticking with a particular dietary approach.

More importantly, this particular review highlights the importance of protein intake. The researchers took the additional step to analyze not percentage of calories, but actual gram intake in each study. This approach allowed us to gain a better understanding of how critical protein is in our diet. Those consuming protein at the RDA did much worse than those consuming a higher protein intake.

The review puts the spotlight on just how important protein consumption is when you're following a weight loss diet - higher in carbohydrate or lower in carbohydrate - protein is a critical, essential nutrient and if you reduce your intake of protein, you'll lose more lean body mass. This fact is all the more obvious in this review and calls into question, once again, the standard recommendation to consume protein as a percentage of calories (10-15%) while trying to lose weight. As I've pointed out a number of times - following that advice reduces protein intake dramatically - something this paper clearly shows is detrimental to lean body mass in weight loss regardless of carbohydrate intake.

Let's not forget, protein isn't just an essential nutrient - it also offers a higher degree of satiety when one is trying to lose weight too!Take away message from this review - low-carb diets work, they do have a metabolic advantage and it's critical to make sure you're eating enough protein each day while keeping your carbohydrate intake low!


  1. I have long wondered why low carb only seems to work below some level. Finally Dr. Mike Eades gave me the answer in his blog "We never failed" It would be nice to have the scientific papers that support his blog. Perhaps you could get him to tell you so you could post them here - along with a "reprint" of the blog text.


  2. I'm wondering - are you asking for research that documents the process in the metabolism when carbs are restricted or research that clearly shows carbohydrate needs to be restricted to a particular level to maximize effect?

  3. Being an engineer my writing skills are somewhat lacking - almost kept me from graduating. :-)

    I should have said, "How do you know that?" in reference to these two sentences [quote] Those glucose-only cells are some of the cells in the brain, the red blood cells, cells in the kidney and cells in the retina. All these glucose-using cells consume about 120-150 grams of glucose per day [unquote]

    I have found in my 8th addition of Guyton (Textbook of Medical Physiology) enough on red blood cells to see that they can only use glucose. But I would like to find more on the other "glucose-only cells" to understand why they can only use glucose. A pointer to a better text or web site with that information would be fine.

    The research or other study that determined the consumption rate of those "glucose-only cells" would then complete the picture. I have full confidence in the Drs. Eades as to the accuracy of their statements. But it is nice to be able to tell some one else, here is the study that shows this. I often tell people about the PPLP bibliography and point out that there are over 400 journal citations in it. Moreover I have checked the abstracts, where there were ones, for all of the citations listed in the PubMed database.

    If you are interested in the bibliography with the PubMed reference numbers for those I found, I can provide them to you.


  4. I would edit my comment of yesterday if I could as I thought of a better way to ask my question.

    Engineers have to measure things when they design build and repair their systems. That allows them to quantify the performance so an evaluation can be made. When I see a measurement quoted, I wonder; Who made it? How was it measured? What was the accuracy of the measurement? Did someone else verify the measurement?

    The statement, "All these glucose-using cells consume about 120-150 grams of glucose per day" is a VERY significant one in terms of, does low-carb work or not. In fact it could be considered the "corner stone" of the low-carb diets. When you go above the requirements for glucose by those cells, then the excess must be used by other cells OR stored as fat. Everything I read indicates that most cells would rather "burn" fats then carbs.


  5. The statement, "All these glucose-using cells consume about 120-150 grams of glucose per day" is a VERY significant one in terms of, does low-carb work or not. In fact it could be considered the "corner stone" of the low-carb diets. When you go above the requirements for glucose by those cells, then the excess must be used by other cells OR stored as fat. Everything I read indicates that most cells would rather "burn" fats then carbs.

    I looked for an email address to reply, but your blogger profile didn't have one, so I'll reply here.

    Metabolically, yes there are certain cells within the body that exclusively use glucose. Those within the brain we know can and will use ketones instead when glucose is limited though.

    Let's first tackle brain requirements since these are the ones most often cited as reason to avoid low-carb diets.

    Recently it's come to be understood that those undergoing open-heart surgery have less nuerological complications if their brain is provided with ketones during surgery instead of depending on glucose infusions. I'd learned about this a couple of years ago and the use of ketone infusion has just completed Phase I Trials - an abstract about the results is here:

    In the abstract we find: KTX 0101 is the sodium salt of the physiological ketone, D-beta-hydroxybutyrate (betaOHB). This neuroprotectant, which has recently successfully completed clinical Phase IA evaluation, is being developed as an intravenous infusion fluid to prevent the cognitive deficits caused by ischemic foci in the brain during cardiopulmonary bypass (CPB) surgery.

    If you're not aware, those undergoing bypass surgery often are left with cognitive deficits afterward. This new approach to protect the brain during the surgery is quite promising and offers a good look at how important ketones may actually be to our brain. As the researchers note, the ketones act as a "superfuel" for the brain during the bypass surgery.

    But let's put that aside for a moment - the IOM has stated that the body requires a minimum of 130g of carbohydrate a day to ensure adequate glucose is available to those cells that require glucose.

    This assumes that ONLY carbohydrate is converted to glucose and fails to acknowledge that metabolically, protein is also converted to glucose. Depending upon the type of protein consumed the range is 42% to 80% conversion. This conversion to glucose from protein is one reason why protein also stimulates an insulin response when we eat it.

    If dietary protein consumption is adequate - and adequate is higher than the current RDA here - than one is able to meet their essential amino acid requirements and still have protein to convert to glucose on an as-needed basis through the metabolic process of gluconeogenesis.

    Gluconeogenesis has long been understood in the human metabolism, although some contend that it cannibalized lean body mass and therefore is undesirable. To date the research offers a contradictory view of this since those on low-carb diets, who are eating an adequate amount of protein each day, often experience a "sparing" of lean body mass and lose more body fat instead.

    So far as glucose requirements being a corner-stone of low-carb diets - quite honestly, I think it's already there without being stated - each diet out there emphasizes the need for adequate protein, with some like the Eades carefully providing a formula to determine how much you need. Could it be stated more clearly why? Sure. But, as I've learned, no matter how well you articulate it to those who think you're doing harm by not eating carbohydrates in a high quantity - it doesn't matter if their mind is already made up on the issue. No amount of metabolism basics can convince them that the body is capable of meeting glucose requirements even with less carbohydrate.

    What I think is telling is - and I think you'll appreciate this as an engineer - that over our evolutionary history, we've developed five different redundancies to raise blood sugars - glucose. Yet, we have just one pathway to lower blood sugar. Redundancy highlights how important a process is/was to us and our survival - excess blood sugar was not a threat, low blood sugar was - because we simply did not eat excessive carbohydrate. I'd even suggest carbohydrate in any really high quantity was difficult for us to come by - protein, on the other hand, much more readily available for easy consumption and it played a significant role in meeting our glucose needs, as it does now if we restrict carbohydrate - without causing the spikes in insulin or blood sugars we experience when we consume excessive carbohydrate.

    What's excessive? Any amount that exceeds our requirement. So even if the IOM is correct - let's pretend that protein doesn't come into play here - why would anyone say we have to eat more than 130g a day?