Prevalence of Fat Gain from Carbohydrates
“Do not eat carbohydrates, or they will be turned to fat.”
Is the demonization of carbohydrates warranted? In this article, we will discuss what the science has to say, as well as understand the physiological implications of the science. Let us begin.
Are Carbohydrates converted to Fat?
The answer is yes. The process is called de novo lipogenesis and involves the conversion of carbohydrates to lipids (fat) in the hepatocytes (liver cells) and adipocytes (fat cells), primarily .
Circumstances of De novo Lipogenesis
High Fat Diet
In a condition in which you consume high amounts of fat, consuming substantially less carbohydrates (like a ketogenic or near ketogenic state), the amount of de novo lipogenesis is just about zero .
High Carbohydrate Diet
This is the applicably more useful scenario, because it is exactly what we are trying to answer: Are carbohydrates converted to lipid? This depends.
In a situation in which you consume large amounts of carbohydrates (700, 800g or more), and overconsume your calories for a day or several days, de novo lipogenesis will increase and lead to noticeable fat gain . However, in an experiment in which adults consumed 7000 calories a day for two and a half months, of which 4900 of those calories came from carbohydrates (70% of the diet, > 1200g of carbohydrates), there was a gain of 12 kilograms (26.4 lbs) of fat that could not be accounted for by the lipid consumed in the diet . This may not say much until we input their average total daily energy expenditure, which was 3370 calories a day – meaning, they were overconsuming roughly 3600 calories a day, which would equate to fat gain nearer to the 19kg (42 lbs) mark . We’ll uncover why there is a discrepancy later.
So, in the massive overconsumption of carbohydrates over time, there is a noticeable gain in fat due to de novo lipogenesis.
However, if you equate calories with total daily energy expenditure (your maintenance energy/calorie intake to neither gain or lose weight), de novo lipogenesis is essentially inert – leading to only a few grams of lipid produced .
There are, however, situations in which de novo lipogenesis will change, although a person is not overconsuming carbohydrates.
For example, under consuming lipids (fat), leads to an increase in de novo lipogenesis . Meanwhile, drinking alcohol can also increase de novo lipogenesis, although this effect is ameliorated over a 48 hour period . Women also have increases in de novo lipogenesis, depending on which phase of the menstrual cycle they are in . De novo lipogenesis may also become more active in pathologic hyper insulinemic situations, and even with the type of carbohydrate ingested .
Understanding the Physiology
Are carbohydrates converted to fat? Yes. However, in high fat intake or equated caloric intake, carbohydrate induced lipogenesis is extremely low. In repetitive high carbohydrate intake, de novo lipogenesis occurs more readily, but not before increasing carbohydrate utilization and simply storing more lipid (fat). Among other scenarios, these are the primary, but de novo lipogenesis is likely not a primary contributor to fat gain unless in extreme situations that are unlikely to occur.
Writer: Nicolas Verhoeven
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 Hellerstein, M. (1999). De novo lipogenesis in humans: metabolic and regulatory aspects. European Journal of Clinical Nutrition, 53(S1), s53-s65. doi:10.1038/sj.ejcn.1600744
 Pasquet, P., Brigant, L., Froment, A., Koppert, G. A., Bard, D., De Garine, I., & Apfelbaum, M. (1992). Massive overfeeding and energy balance in men: the Guru Walla model. The American Journal of Clinical Nutrition, 56(3), 483-490. doi:10.1093/ajcn/56.3.483
 Schwarz, J., Linfoot, P., Dare, D., & Aghajanian, K. (2003). Hepatic de novo lipogenesis in normoinsulinemic and hyperinsulinemic subjects consuming high-fat, low-carbohydrate and low-fat, high-carbohydrate isoenergetic diets. The American Journal of Clinical Nutrition, 77(1), 43-50. doi:10.1093/ajcn/77.1.43
Now that we have a good grasp on the various situations, why might de novo lipogenesis (DNL) be so selective in its occurrence?
In a high fat situation, there is a carbohydrate “deficient” scenario, in which glucose is not readily available, so why spend it further on a molecule, lipid, you have abundant levels? It makes no sense, hence the decrease in lipogenesis.
In a high carbohydrate state, in which fat is still adequately consumed and energy intake is balanced (at maintenance), the cells do not increase their lipogenic features by much, because again, if everything is adequate, there is no drive for the creation of new lipids.
In a high carbohydrate state, in which fat is adequate, yet you are overconsuming carbohydrates drastically, de novo lipogenesis increases, but only after mass quantities of carbohydrates are taken in – why?
Because the cells become highly aerobic glycolytic, shifting their reliance on glucose (carbohydrate) for energy, over lipid. Their oxidation of glucose increases incredibly to a point that lipids remain stored, decreasing the need for lipogenesis; however, once the glucose oxidation capacity of all the cells has reached maximum, the remaining glucose must be disposed and is then shuttled to de novo lipogenesis. This process can be delayed if glycogen stores are low upon the ingestion of high levels of glucose .
In other scenarios, like the underconsumption of lipids in the diet, regardless of the overall energy intake, carbohydrates are shuttled to de novo lipogenesis, presumably, in an attempt to recoup some of these lipids that would otherwise enter in diet.