8.9 Metabolic Conditions

You have learned about the metabolic pathways, so now we’re going to apply that knowledge to three conditions: fasting, the Atkins diet, and the Ornish/Pritikin diet, as ways to illustrate how you can use this knowledge to understand what happens after you eat.

In fasting, we’re going to be considering what is happening metabolically during a prolonged period without food. This is a catabolic condition. The Atkins diet is a carbohydrate-restricted diet, so we are going to consider what happens metabolically when someone is eating a diet that essentially only contains protein and fat over an extended period of time. This is an anabolic condition. Finally, the Ornish/Pritikin diet is a very low-fat diet, so we’re going to consider what happens metabolically when someone is eating a diet that is essentially only carbohydrates and protein over an extended period of time. This is also an anabolic condition.

Tissue Specific Metabolism^[1]

For each condition, we’ll be discussing what happens in the liver, in muscle tissue, and in adipose tissue.  All cells in the body can make ATP and proteins, but these three tissues have specialized metabolic functions.

The liver is the most metabolically flexible organ; it can store and breakdown glycogen and store and break down triglycerides. It can also use amino acids and other molecules to produce glucose that can then be released into the blood for other parts of the body to use.  The liver also makes ketone bodies, an energy source that can be used by the brain when glucose is not available.  The liver is the only tissue that makes ketone bodies and is the main site of gluconeogenesis (the kidneys also do small amounts).

Muscle tissue stores and breaks down both glycogen and triglycerides.  Muscle tissue can also produce lactate under anaerobic conditions (see section 8.7). This lactate circulates back to the liver for gluconeogenesis. Muscle cells cannot secrete glucose into the blood the way that the liver does.  They use the glucose they produce for themselves.  You can think of muscle as being selfish with glucose and the liver as being generous.

The major function of adipose tissue is to store energy in the form of triglycerides.  Therefore the major metabolic activity of adipose tissue is the synthesis and breakdown of triglycerides.

Metabolic Hormones: Insulin and Glucagon

Now that you have an understanding of the glycemic response (Chapter 5) and macronutrient metabolism, you should be able to understand the broader effects of insulin and glucagon that are summarized in the following tables. Knowing what hormone is elevated in the different conditions helps you to understand the metabolism that occurs in different conditions.

Table 8.91 Insulin’s effects on targets in tissues^[2]

Table 8.92 Glucagon’s effects on targets in tissues^[3]

Fasting

In this condition a person has been fasting for an extended period of time (18 hours or longer). As a result, the person is in a catabolic state with low blood glucose levels, which leads the pancreas to secrete glucagon.

The liver will break down glycogen to secrete glucose for other tissues to use until its stores are exhausted. Amino acids and lactate from muscle will be used for gluconeogenesis to synthesize glucose. Glycolysis will not be occurring to any great extent in an effort to spare glucose for use by other tissues. Fatty acids that are received from adipose tissue will be used to synthesize ketone bodies that can be used by tissues that cannot directly use fatty acids as a fuel, such as the brain..

Muscle tissue will break down glycogen to glucose until glycogen stores are exhausted. Glucose will be used to make ATP. Once there isn’t enough glucose for the muscle to use, fatty acids taken up from adipose tissue, and from the breakdown of muscle stores, will be used for ATP. Amino acids from protein breakdown and lactate will be used by the liver for gluconeogenesis.

Adipose tissue will break down triglycerides to fatty acids and release these for use by the muscle and the liver. It will not be taking up anything.

Atkins Diet

In this condition, assume a person just started into Phase I of the Atkins Diet and he/she has just consumed a meal of all protein and fat with no carbohydrates of any kind. As a result, this person is in an anabolic state, but blood glucose levels are low, meaning the pancreas will secrete glucagon.

Liver glycogen stores will be broken down to secrete glucose for other tissues. The liver will also be using amino acids from digestion and lactate from muscle for gluconeogenesis.

Amino acids will also be used for protein synthesis. Some triglycerides absorbed from the meal will be broken down to fatty acids and used to synthesize ketone bodies. Other triglycerides will be packaged into VLDL and secreted from the liver.

Muscle tissue is going to break down glycogen to glucose, and receive glucose from the liver. This glucose will be used to make ATP until it runs out. Once there is not enough glucose for the muscle to use, fatty acids from multiple sources will be broken down for energy. Amino acids taken up will be used for protein synthesis, and lactate will be secreted for the liver to use for gluconeogenesis.

In adipose tissue, fatty acids are also going to be taken up. These fatty acids will be used to make triglycerides for storage.

Ornish/Pritikin Diet

In this condition, assume a person is on the Ornish/Pritikin Diet and just consumed a meal containing carbohydrates, with minimal but adequate amount of protein, and no fat. As a result, this person is in an anabolic state with high blood glucose levels, meaning the pancreas will secrete insulin.

The liver will take up glucose and synthesize glycogen until its stores are filled. After these stores are full, glucose can be used to make triglycerides for storage.  These triglycerides will be packaged into VLDL and secreted from the liver. Amino acids will also be taken up and used for protein synthesis as needed. Because there is plenty of glucose, gluconeogenesis and ketone body synthesis will not be operating to any great extent.

Muscle tissue will take up glucose and synthesize glycogen until those stores are filled. Some glucose will be used to make ATP. Fatty acids that are cleaved from low-density lipoproteins are also going to be taken up. These fatty acids will be used to synthesize triglycerides for storage. Whatever amino acids are taken up will be used for protein synthesis. The muscle will not be secreting anything in this condition.

The adipose tissue is going to take up glucose to produce triglycerides for storage.  The adipose tissue won’t be secreting anything under this condition.

The brain will have plenty of glucose available for its use, so it is not going to have to use ketone bodies like it would during fasting and during prolonged Atkins diet consumption.