No evidence MSG makes you fat

Another spurious correlation is already well on its way to becoming an urban legend as a cause of obesity. A new study in the current issue of the journal of the Obesity Society was reported as having shown for the first time that MSG (monosodium glutamate) is associated with overweight in Chinese adults. Incredibly, it would appear no one had read the study because its data actually revealed that glutamate had absolutely no relationship to their BMIs.

Overview

The authors of this paper, led by Dr. Ka He, M.D., assistant professor of nutrition and epidemiology at the University of North Carolina at Chapel Hill School of Public Health, used data on 752 of the healthy Chinese men in INTERMAP. The INTERMAP (International collaborative of macronutrients and blood pressure) database is an epidemiological investigation designed to look for links between diets and blood pressure among 4,680 middle-age adults from four countries (UK, US, China and Japan). Beginning in 1995, INTERMAP researchers conducted four 24-hour dietary recall interviews, two 24-hour urine collections, and took eight blood pressure readings on each adult. All foods and drinks consumed in the previous 24 hours were recorded during each interview and coded into its computer database.

As Dr. Ka He and colleagues noted, most of the Chinese participants were from rural areas and rarely ate processed or restaurant foods, meaning the MSG in foods they prepared at home could be more precisely measured and would most accurately reflect the total amount of MSG in their diet. MSG was used by 82.4% of the rural Chinese in their homecooking. For this analysis, the authors used the average MSG intakes from the four interviews for each person.

They then applied computer modeling to look for correlations between MSG usage, as divided into tertiles, and the percentage of those with BMIs in the overweight categories.

Findings

There was little real difference in the actual average BMIs associated with those not using MSG and those using the most: an actual difference of only 1.2 units. But when expressed in terms of the percentage of those whose BMIs crossed the threshold of BMI ≥25, it led to the report that 30% of the highest users of MSG were associated with overweight compared to 12% of non MSG users — an adjusted odds ratio of 2.75.

Although this might sound like a big correlation, given the poorly tenable odds ratios for this type of study, it’s another example of a data dredge statistical coincidence. As readers remember, when a computer culls through enough data, the law of truly large numbers means the probability increases that it will find all sorts of amazing, and meaningless, correlations.

This week’s news stories have uncritically reported on this study from the UNC press release and the headlines quickly moved from “MSG use linked to obesity” to turn the correlation into a causation with “MSG May Lead To Obesity” and then “MSG Spice Causes Obesity?”

And another food fear myth is born.

A closer look

But knowledge of biochemistry — as well as the body of evidence and every expert review of the effects and safety of MSG as consumed by humans — demonstrates this reported link between BMI and MSG is spurious.

It is biologically implausible that this correlation has anything to do with anything in the MSG. Here’s why.

As readers may remember in the recent evidential overview, MSG is simply glutamate, water and sodium and dissolves into these three things the moment it hits our tongue. Glutamate is one of the most common amino acids (building blocks of protein) in nature and is found in virtually everything we eat. [It’s even sold as high-priced all-natural dietary supplements, for those who prefer to get their chemicals in pill form.] It’s so essential to our metabolism, growth and brain function, it’s even produced by our own bodies, about 50 grams of free glutamate a day. Our muscles, organs and tissues naturally contain about 4 pounds of glutamate and it’s also abundant in breast milk (at levels ten times that of cow’s milk).

Remember, the umami flavor cherished in Asian cooking comes not only from MSG, which has been made from fermented sugar beets, sugar cane or corn for more than a hundred years. MSG is used in very small amounts, about 0.2% to 0.8% of food by weight on average worldwide. In this study, the Chinese averaged 0.33 g/day of MSG. But for more than a thousand years, Asian cooks have used other special glutamate flavor enhancers, such as soy sauce made from fermented soybeans and grains, fish sauces from fermented fish broth, seaweed stocks, mushrooms and other sources of free glutamate to achieve the perfect balance of umami. Soy sauce alone as 1,090 mg/ml of free glutamate. Free glutamate is also found in nature and in many foods, and it’s formed as produce naturally matures and ripens, such as mushrooms, broccoli, seaweeds, tomatoes, peas, walnuts, eggs, potatoes, children, grapes, beers, etc.

What hasn't been reported in the news, is that there was no difference in the total amount of free glutamate or glutamic acid consumed by the Chinese who used no MSG in their cooking and those who used the most! The sources simply differed. The total intakes were 2.9% of kcal in non-MSG users, 2.9% in Tertile 1 (lowest third of MSG users), 2.9% in Tertile 2, and 3.0 in Tertile 3 (highest third of MSG users). There was no statistical difference in the dietary intakes between any of the Chinese in this study.

With the identical chemicals consumed in identical amounts, to believe that there is something mystically, magically fattening about the MSG label is woo.

It was a spurious correlation. The bottom line is that the INTERMAP population study wasn’t designed to identify factors relating to weight status; it was studying blood pressures. More importantly, it didn’t look at a single health outcome, which is the most important outcome in medical research.

This secondary analysis study failed to derive any tenable correlations between MSG and overweight among these 752 Chinese adults. It was a null study.

© 2008 Sandy Szwarc

For fun. You, too, can be an epidemiologist. If you wanted to create other explanations for the spurious correlation of the 5% difference in average BMIs between non-MSG users (BMI 22.3) and the highest MSG users (BMI 23.5), you could.

There were no statistical differences in their activity levels (heavy activity varied from 1.9 to 2.0 hours/day); the sugars they ate or caloric intakes (all within 251 kcal/day). There was, however, an 11% difference in smoking status, with the smallest BMIs among the smokers.

Next, make up some explanation for a correlation between not cooking with MSG and smoking.