A new study is leading people to think it does. But it doesn’t. Eating more carbohydrate does not raise the risk for disease if it is done within the context of a low-fat diet. This attention-grabbing study was small (16 people), short in duration (18 weeks), funded by the Dairy Institute, the Beef Institute, the Egg Nutrition Center, and the Atkins Foundation. It is not applicable to anyone who is not morbidly obese with metabolic syndrome, as describes its participants.
The author of the journal article in my previous post said that under conditions of excess energy (calories), especially excess dietary fat, glucose from carbohydrate we eat is progressively blocked from entering muscle and fat cells (in part by lower availability of the GLUT4 glucose transporter which I’ve discussed previously). These cells become resistant to insulin. The pancreas releases more insulin to counter the resistance. In relatively healthy people, this extra insulin will work; it will cause blood levels of glucose to fall back into a healthful range. After a while (of eating a high energy/high fat diet), the beta cells in the pancreas struggle to keep up. They release less insulin; blood glucose levels rise and stay elevated for longer periods.
Where does all this blood glucose go? It can’t stay in the blood. Some goes out in the urine. (Years ago, physicians checked for diabetes by tasting urine for sweetness). Since it can’t easily get into muscle cells where mitochondria reside, it has difficulty being oxidized for energy. But it also has difficulty getting into fat cells, so it isn’t easily used to make body fat, not directly. A lot of it goes back to the liver where it can be used to construct fatty acids, some of which stay in the liver (and contribute to NAFLD, which is non-alcoholic fatty liver disease, 20% of the population and rising), and some are released to the blood stream.
In sum: High-fat diet –> insulin resistance –> glucose build-up –> fat production.
This implies that if you put someone on a low-fat diet, you can fix their insulin resistance, such that their blood glucose won’t go up when they eat glucose. Indeed, that is what happened in this study. See also: It’s The Fat In The Diet, Not The Carbohydrate, That Drives Diabetes.
This new study recruited 16 obese adults with metabolic syndrome, a condition that generates more fatty acids from dietary carbohydrates than it does in healthier people.
The researchers found two things:
- That the fat we eat is not necessarily the fat that ends up in our blood. “The results show that dietary and plasma saturated fat are not related.” That makes sense from other things I’ve read.
- That “increasing dietary carbohydrate across a range of intakes promotes incremental increases in plasma palmitoleic acid.” I think they should have added, “in obese people with metabolic syndrome.” Palmitoleic acid, which is a marker for de novo lipogenesis, increases when insulin resistance is present as it would be in these subjects. … for reasons above, reasons the authors reiterated in their literature review:
“High-carbohydrate intake in individuals with an impaired ability to oxidize glucose stimulates DNL [de novo lipogenesis or fat production] and secretion of saturated fatty acid- and monounsaturated fatty acid-enriched VLDL particles; a serum profile associated with insulin resistance.”
They tested 6 isocaloric diets. All were high-fat (28.7% to 73.7% of calories), high-saturated fat (11% to 30%, AHA recommends less than 7%), and high-protein:
Baseline energy 3028 kcal, 44% carb, 38.6% fat
c1 2553, 7.4%, 73.7%
c2 2528, 13.1%, 68.7%
c3 2585, 20.3%, 62.3%
c4 2506, 28.6%, 54.6%
c5 2517, 39.9%, 43.3%
c6 2509, 55.2%, 28.7%
None of these diets changed levels of circulating saturated fat. “Plasma saturated fat remained relatively stable.” However, the more total fat and saturated fat participants ate, the higher their total cholesterol and LDL. The diets did change the type of fatty acid in circulating fat. As carbohydrate increased, palmitoleic acid increased. And, “Increases in palmitoleic acid indicate that more carbs are being converted to fat instead of being burned as fuel,” states senior author Jeff Volek.
Some other outcomes:
- Lower serum cholesterol from lower fat diets: From C2 (83 grams carbs) to C6 (344 grams carbs) there was about a 12 point drop in LDL cholesterol, and about a 10 point drop in total cholesterol. So, as they progressively increased carbs and decreased fat, their cholesterol numbers trended better. (“Serum cholesterol responses were variable but consistent with the known effects of carbohydrate restriction to increase, on average, total cholesterol, HDL-C and LDL-C relative to low-fat diets.”)
- Lower systolic blood pressure from lower fat diets: The lowest systolic blood pressure reading occurred at C6, the highest carb intake (344 grams), and lowest saturated fat intake.
- Fat mass and body mass both dropped throughout the study, but this could have been the result of their lower calorie diet (they ate about 500 fewer calories a day).
The following was a red flag for me … The 16 individuals were put on a low-carb diet prior to the start of the study, “to initiate metabolic adaptations to carbohydrate restriction.” Those adaptations included increased fat burning and decreased glucose burning/oxidation. That could have favored their results:
Whole body fat oxidation increases markedly when dietary carbohydrate is restricted, and it is likely that SFA become preferred substrates for beta-oxidation in low-carbohydrate-adapted individuals.
It’s almost as if they set this study up to show that eating carb was bad for you (it gets converted to fat), and eating saturated fat was good (it gets burned for energy). I mean, they chose metabolically impaired individuals who already have glucose intolerance, placed them on a low-carb diet prior to the study to prime them to burn fat, and then fed them high fat which would maintain their insulin resistance.
Maybe they were influenced by their sponsors, who would benefit from results that placed saturated fat consumption in a positive light:
Funding: This work was funded by a grant from Dairy Research Institute, The Beef Checkoff, the Egg Nutrition Center, and the Robert C. And Veronica Atkins Foundation.
“Beef, eggs, and dairy were used daily throughout all diet phases.”
Carbohydrate consumed against a backdrop of high fat has been shown to increase risk for diabetes, heart disease, and cancer. Carbohydrate consumed against a backdrop of low-fat (something below 15%) has been shown to be protective against diabetes, heart disease, and cancer.