Junkfood Science: Carbs humbug? — Are carbs really fattening?

December 13, 2007

Carbs humbug? — Are carbs really fattening?

If visions of sugarplums ... and cookies and fudge and all of those puff pastry holiday nibbles... give you a fright, a recent randomized trial examining if “bad” carbs make us fat, will be welcome news. Since this study didn’t make the news, we'll look at it now, since this is the season for good cheer.

This was a small crossover intervention trial by researchers with MRC Human Nutrition Research, Elsie Widdowson Laboratory in Cambridge, UK, to compare diets with carbohydrates that have high glycemic indexes — refined foods popularly feared as being fattening, like white breads, breakfast cereals, rice, pasta and potatoes — to diets with low glycemic index (GI) carbohydrates, like wholegrain breads and grains. How often have we heard that we need to avoid refined flours, starches and sugars lest we get fat?

The trial subjects were all healthy women, age 34 to 65 years, who were “overweight” or “obese” and had had stable weights for at least 2 months to ensure they weren’t on the rebound from a diet. They were provided the carbohydrate foods to include ad lib in their usual diets and were told to eat normally. They maintained about 50% of their calories from carbs (within the 45-66% recommended in the Dietary Guidelines). The women were blinded as to what the study was testing and they didn’t know the GI of the carbs they were eating. They were randomized, and for the first 12 weeks, half received low-GI carbs and the others high-GI carbs (same calorie counts); then for the final 12 weeks, the GI of their carbs were switched. The women kept careful food diaries which were later analyzed.

The researchers also conducted exhaustive appetite-satiety tests during the final week of each intervention period to examine the changes in their satiety and the amount of calories they ate the rest of the day after high- and low-GI breakfasts (of identical calories, fats, protein and carbs). They had fasting blood samples drawn before breakfast and every 15 minutes for the first hour, then at 90 and 120 minutes after breakfast, to measure their glucose, insulin and fatty acid levels. The women were provided with ad-lib snacks in the morning and a lunch 4 hours after breakfast and the researchers secretly weighed the uneaten food to determine precisely how much each woman had eaten. The women graded their hunger and fullness levels at 30 minute intervals throughout the investigation.

All women had reduced their dietary GI on the low-GI diet, with a mean difference of 8.4 units, which the researchers felt reflected doable changes for people to maintain in real life. While on the low-GI diet, their carbohydrates and fiber levels were slightly higher compared to high-GI diets, although absolute differences proved small. At the end of the 24-week study, which the researchers note was longer than most previous studies of GI, there were no statistical differences in body weight, waist circumference or fatness between the intervention periods — no differences between the low or high GI diets. There were no differences in the total calories eaten over the day on either diet, nor was there any differences in the women’s ratings of appetite at any time.

Despite oft-repeated fears that refined “bad” carbs send our glucose and insulin levels soaring, these researchers found “no differences in glucose, insulin and non-esterfied fatty acid responses to the lower versus higher GI breakfasts, with no differences in either total area under the curve or single measurements at any time point.” Concerns that carbohydrates in the diet stimulate insulin production and are responsible for obesity and illness were recently examined here.

As these researchers noted, previous studies suggesting that GI affected fullness/hunger or calorie intakes in the short term often also had differences in macronutrients. “It is notable that the fixed breakfasts incorporating lower and higher GI foods did not result in differing postpandial glucose responses,” they wrote, even though each of the foods had differing GI when tested separately. This illustrated, they explained, that a variety of foods and ingredients (fats, protein and carbs) are normally eaten together, so effects from GI aren’t predictable, and all sources of calories affect glucose and insulin responses. As they concluded:

[T]his controlled laboratory investigation and free-living dietary intervention study has found no evidence to support a beneficial effect of a reduction in GI of the diet on satiety, energy intake, body weight or fatness through simple substitution of staple carbohydrate foods. It does not preclude an impact on weight from a greater reduction in dietary GI, but this is likely to involve wider dietary changes. Claims that the GI of the diet per se may have specific effects on body weight may therefore be misleading.

Of course, this is just one study. To put it into perspective, what does the body of research evidence tell us? Are carbs and high glycemic foods especially fattening?

“Most epidemiologic studies show an inverse association between carbohydrate consumption and BMI,” according to Glenn A. Gaesser, Ph.D., a professor of exercise physiology at the University of Virginia, Charlottesville, in a medical literature review for JADA. The BMI differences between those eating the most and least carbohydrates in observational studies are modest, but hold in the most carefully-done studies controlling for calorie and fiber intakes and physical activity.

And, working backwards, we know that clinical studies have also demonstrated that fat and thin adults and children eat no differently to explain the differences in our sizes. When not trying to restrict our eating, virtually all people will naturally eat a variety of foods over time, too, minimizing concerns over any single food.

The relationship between BMI and glycemic index and glycemic load (which reflects the total carbohydrates eaten and their GI) is a contentious topic, even down to the GI and GL values attributed to foods. But most epidemiological studies don’t support that glycemic load adversely affects BMI, with most showing no relationship when adjusting for total calorie intakes and other confounding factors, said professor Gaesser.

What about refined white flour products that many believe are fattening? The Framingham Offspring Study, Women’s Health Study and Physician’s Health Study, for instance, revealed no relationship between refined grains and BMI, and even suggested inverse relationships. To help allay health concerns, professor Gaesser added that “a number of reports indicate no association between refined grain intake and risk of type 2 diabetes, insulin resistance, cardiovascular disease, ischemic stroke, breast cancer, and all-cause mortality.” Sounds just like the half century of evidence to date on sugars.

Observational studies cannot demonstrate cause and effect, of course. And weight loss studies are sure to find short-term weight loss with whatever diet or food is being tested. [All diets work ...wait for the punchline.] So we can look at large intervention trials where weight loss wasn’t the main goal to see if dietary changes did anything for BMI in the long run, like that Women’s Health Initiative study. JFS readers know what it found were the results of years of conscientiously eating “healthy.”

Science really is our friend — it can even help make for a worry-free, guilt-free and delicious holiday as we enjoy the special treats the season brings.

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