Metabolic Pathways of Proteins, Carbohydrates and Fats in Humans

The next time someone tells you that carbohydrates are essential for life, or that you’ll die if you don’t eat carbohydrates, point them to the metabolic pathways diagram and ask them to identify which metabolic needs go unmet without carbohydrate ingestion.

Diagram of metabolic pathways for fats, carbohydrates and proteins.

See a full-size image of the metabolic pathways. Click terms in the image for additional details.


Hard to do, right? That’s because there are none.

Of course, understanding that “you don’t need to eat carbohydrates to support life” is not at all the same thing as thinking, “carbohydrates are not required to support life.” They are. Alice and Fred Ottoboni put it well, “Carbohydrate metabolism is, for all practical purposes, the metabolism of glucose (84)” and it is well understood that certain types of cells required to support life need glucose to survive.

The question is, of course, how much glucose is necessary to sustain life? Some estimates put it as low as 20g a day. Phinney and Volek indicate that the brain needs “less than 50 grams [of glucose] a day” (The Art and Science of Low Carbohydrate Living (44)). They also assert that your liver can produce up to 50 grams a day of glucose via gluconeogenesis.

So there you have it: you don’t need to eat carbohydrates in order to meet the minimal glucose demands of a limited number of cells that need glucose to survive. Anyone who tells you that you otherwise is propagating fear, uncertainty and doubt.

What then of the rest of your cells that don’t need glucose? They have the ability to use ketone bodies and/or free fatty acids for energy. So, in the absence of exogenous glucose intake, where does your body get the required amount of glucose?

Gluconeogenesis is the combination of pathways through which our metabolism collects and revises the carbon skeletons of amino acids, the glycerol backbones from triglycerides, and even lactic acid to make new glucose to feed those few tissues (like the lens of the eye and red blood cells) that can’t burn either fat or ketones.

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So what does this mean? It means that we should not confuse our body’s ability to maintain a normal blood glucose with our dietary carbohydrate intake. When humans are adapted to a low carbohydrate diet, blood sugar levels and one’s carbohydrate intake are completely independent of one another. In fact, keto-adapted humans maintain better glucose levels across feeding, fasting, and extremes of exercise than when fed a low fat, high carbohydrate diet[23, 27]. (Phinney and Volek, The Art and Science of Low Carbohydrate Living).

Dr. Ron Rosedale
is clear on this point. “Even if you are starving, and eating no carbohydrates, or fat, or protein, there is no such thing as “glucose deficiency” (unless insulin toxic or relatively rare conditions where glucose cannot be made sufficiently, such as cortisol deficiency). The body can easily make what it needs.”

And of course, the other central question for this discussion is about optimal health. Once we understand that you don’t need to eat carbs to survive, you should be asking about whether or not eating carbs helps you achieve optimal health, or better health than you would have without them. This, of course, is a much bigger discussion that I’ll be getting in to another time. For now, I’ll just end with an additional remark by Dr. Rosedale:

When you eat the glucose, there are different effects than if your liver makes it, namely it circulates for hours and leads to a spike in insulin and leptin, that circulates for hours, that over time will contribute to insulin and leptin resistance…that ultimately contributes to metabolic chaos resulting in chronic diseases of aging including obesity, diabetes, cardiovascular disease, osteoporosis, autoimmune disease, cancer, and others.

Many thanks to Alice and Fred Ottoboni for giving me permission to reproduce their fantastic metabolic pathways diagram from their excellent reference, Modern Nutritional Diseases.

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