Carbohydrates and Insulin

When we consume carbohydrates, most are broken down to glucose and absorbed into the blood. The rise in blood sugar immediately triggers the release of the hormone insulin from the pancreas. Insulin is a very important hormone but too much adversely affects one’s endurance and health, especially by reducing the ability to burn body fat.

Insulin allows us to use and store glucose, and as this happens, blood sugar is lowered. Insulin works through three different mechanisms:

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About 50 percent of the blood sugar (about half the carbohydrates you eat) is quickly used throughout the body for energy, especially in muscles and the brain.

Up to about 10 percent of the carbohydrates you eat are converted to glycogen, a storage form of sugar. The glycogen is stored in the muscles and liver. The amount depends on the level of your glycogen stores. (Muscle glycogen, for example, is converted back to glucose for energy, and liver glycogen helps maintain blood sugar levels between meals and during nighttime sleep.)

About 40 percent or more of the carbohydrates you eat is converted to fat and stored as body fat. This is the source of fat used by the aerobic muscles for energy, but if the fat-burning mechanisms are not working well, or if too much carbohydrate is consumed, fat stores can get larger.

Insulin is produced whenever you eat carbohydrates, except when you consume them during training or competition. Insulin levels are reduced during physical activity, helping the body burn more fat for energy. Smaller amounts of insulin may also be produced if you consume a protein-only meal, and in some people, a high-protein meal can stimulate significant amounts of insulin. But for most people, it’s predominantly carbohydrates that trigger the insulin mechanism. As noted, too much insulin causes problems for endurance athletes, as can too little (a problem for diabetics).

In the past several decades, the high consumption of highly refined carbohydrates has contributed significantly to the obesity epidemic and other chronic illness such as diabetes and heart disease. In addition, there is an “overfat” (as opposed to “overweight”) problem, even among athletes. The trend in carbohydrate over-consumption continues today, propelled by companies selling refined carbohydrates and sugar. This is quite prevalent in the endurance sports market.

During most of our evolutionary history, humans lived near the sea and consumed significant amounts of fish, seafood, and other land-animal proteins. More importantly, large amounts of plant foods were also part of our diet. These included vegetables, fruits, nuts, and seeds. In addition, our ancestors were very active physically, even more than most endurance athletes today. Only in the last five-thousand years has this changed. The agricultural revolution brought a dramatic increase in carbohydrate foods and the industrial revolution brought highly refined carbohydrates to the table. The intake of refined carbohydrates by humans has never been as dramatically high as in just the last one hundred years. This relatively short period of significant dietary change has contributed to the overfat problem, and to many problems leading to heart disease, cancer, obesity, and other diseases. This is due, in part, to the over-consumption of refined carbohydrates, higher levels of insulin production, higher fat stores, and chronic inflammation.

The more carbohydrates consumed, the more insulin produced by the pancreas. Even healthy people can overproduce insulin by eating excessive amounts of carbohydrates. While this problem is thought to occur in those who are overfat, it can also occur in lean people as well.

This potentially leads to a condition referred to as “insulin resistance,” associated with the inability of insulin to efficiently fuel the muscles with glucose. As a result, energy is reduced, along with fat burning. Hunger is another result—because the cells don’t get enough energy, each time these people eat carbohydrates, the brain gets the message that the cells don’t have enough sugar and the brain tells the pancreas to make more insulin. Finally, insulin is produced beyond normal limits, a condition referred to as “hyperinsulinism.” While it takes more insulin to get glucose into the insulin-resistant cells efficiently, this hormone continues to perform its other tasks, including turning carbohydrates into fat for storage.

In addition to causing even more carbohydrates to convert and store as fat, excess insulin can continue lowering the blood sugar—because that’s one of its functions, including converting glucose to fat. Since the brain exclusively relies on glucose for fuel, periods of reduced blood sugar can result in impaired mental function, including loss of memory, reduced concentration, and other cognitive impairments. Low blood sugar also results in hunger, sometimes only a couple of hours or less after the meal. Cravings for sweets are typically part of this cycle and resorting to snacking on more carbohydrates maintains the vicious cycle. And if you don’t eat, you just feel worse. Eventually, the fat-storage deposits get bigger. While this problem can be relatively minor, in some athletes it can cause a more serious condition, such as higher body fat or even diabetes, the full spectrum of this carbohydrate problem is a condition I term carbohydrate intolerance, discussed below.

High insulin levels also suppress two important hormones: glucagon and growth hormone. Glucagon has the opposite effect of insulin and is produced following protein consumption. While insulin promotes fat storage, glucagon promotes the use of fat and sugar for energy. Growth hormone helps provide many of the endurance benefits we obtain through training, including muscle development, sugar and fat burning, and the regulation of minerals and amino acids.