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Joslin’s Diabetes Deskbook, Updated 2nd Ed, Excerpt #5: The Body’s Fuels

May 7, 2012

By Richard S. Beaser, M.D.

Can insulin injections make you fat…? Find out when you read our latest excerpt from the Joslin Diabetes Deskbook:



The two main fuels that the body relies on for energy are glucose from carbohydrates and fatty acids from fats. Protein can also provide an alternative source of energy if glucose is in short supply. Because humans eat episodically during the day, with the fuel supply coming in sporadically, mechanisms exist for the fuel to be stored for use between feedings. It is useful to review why each of these energy sources is needed and how it is used

Carbohydrate is found in most foods (vegetables, pastry, bread, potatoes, pasta, etc.) and is the primary source of energy for the body. As food is consumed, the carbohydrate is digested into its basic constituent, glucose, and absorbed into the bloodstream. At this point, the glucose may be used to provide for either immediate energy needs or can be stored in muscle or peripheral tissues (primarily muscle) for later use.

When stored, carbohydrate is usually found within cells in the form of glycogen. Insulin is required for this storage process to occur. The largest depot of glycogen is in skeletal muscle, while the most accessible storage source is the liver, which provides glucose in the event that it is needed rapidly to raise the blood glucose level.

The availability of glycogen as a source of “quick” energy is quite important. Suppose someone with diabetes has taken too much insulin and as a result experiences a hypoglycemic reaction. Even if the person does not eat extra food immediately, the blood glucose level will usually rise subsequently. This is called “rebound hyperglycemia” caused by the release of glycogen from storage in the liver. This release is triggered by the body’s hormonal “counterregulatory” mechanism made up of epinephrine (adrenaline) and glucagon. Although all people without diabetes, and most people with diabetes, have this ability to bring blood glucose levels back up to a safe range, people with diabetes still should treat low blood glucose reactions by consuming concentrated carbohydrates. They shouldn’t depend on the usual automatic mechanism always functioning well enough to keep out of trouble, as a significant hypoglycemic event can overwhelm the counterregulatory function.

Glycogen is also released when muscular exertion requires it for energy. Long-distance runners often eat large quantities of carbohydrate prior to a race to “load” their glycogen stores for the long run. Unfortunately, while the available glycogen storage space in muscle and liver is limited, the unlimited storage depot for all of the excess glucose that is consumed is the adipose (fat) cells! Insulin is involved in this mechanism as well, helping promote the change of the excess glucose into fat, which is deposited for long-term storage. As a result, there is a myth among patients and some healthcare professionals that insulin injections make you fat. Insulin doesn’t make you fat — excess calories make you fat. Insulin only helps by doing its job, and this underscores the importance of proper balance between insulin, food consumption, and activity.


Dietary sources of protein (e.g., meat, cheese, and fish) are digested into amino acids, which then enter the circulation. Circulating amino acids can also stimulate pancreatic insulin secretion. While proteins provide structural integrity and are part of important body constituents such as enzymes, they can also be metabolized into energy in a process similar to the metabolism of glucose. Amino acids, converted to glucose in the liver and stored as glycogen, can eventually provide energy, or can be stored as fat for later use.


The third fuel for the human metabolism is fat, (e.g., oils from corn, peanuts, and olives, as well as fat in meat, fowl, butter, and other dairy products such as milk, cream, and cheese). These foods are digested into fatty acids in the intestine and absorbed into the bloodstream where they are used by the body. Some fats are called “essential fats.” The body needs these for metabolism and they must be in the diet. Most fats, however, are either used for energy or stored in cells as triglycerides for later use. When other fuels are in short supply, such as during prolonged fasting, the level of insulin in the blood falls. The reduced level of circulating insulin promotes the removal of the fat from the storage depots and helps its entry into the circulation, where it is accessible by muscle and other organs for use as energy.

Fat is a very efficient form of energy storage, providing 9.5 kilocalories (kcal)/gram of triglyceride, as compared to 4 kcal/gram of carbohydrates or proteins, either directly or from glycogen. The byproducts of fat metabolism are ketones (including acetone).


Copyright © 2010 by Joslin Diabetes Center. All rights reserved. Reprinted with permission. Neither this book nor any part thereof may be reproduced or distributed in any form or by any means without permission in writing from Joslin. Note: Joslin does not endorse products or services.

For Excerpt #1 from the Joslin Diabetes Deskbook, 2nd Ed, in this series, just click here.

For Excerpt #2 from the Joslin Diabetes Deskbook, 2nd Ed, in this series, just click here.

For Excerpt #3 from the Joslin Diabetes Deskbook, 2nd Ed, in this series, just click here.

For Excerpt #4 from the Joslin Diabetes Deskbook, 2nd Ed, in this series, just click here. 

You can purchase the updated 2nd Edition of JOSLIN’S DIABETES DESKBOOK at:


Please Note: Reasonable measures have been taken to ensure the accuracy of the information presented herein. However, drug information may change at any time and without notice and all readers are cautioned to consult the manufacturer’s packaging inserts before prescribing medication. Joslin Diabetes Center cannot ensure the safety or efficacy of any product described in this book.

Professionals must use their own professional medical judgment, training and experience and should not rely solely on the information provided in this book to make recommendations to patients with regard to nutrition or exercise or to prescribe medications.

This book is not intended to encourage the treatment of illness, disease or any other medical problem by the layperson. Any application of the recommendations set forth in the following pages is at the reader’s discretion and sole risk. Laypersons are strongly advised to consult a physician or other healthcare professional before altering or undertaking any exercise or nutritional program or before taking any medication referred to in this book.