Insulin, Diet, Disease and Athletes

The 40/30/30 diet has been reputed to be healthful and to improve athletic performance in endurance athletes. Despite the lack of credible scientific evidence, consumer products exist and some individuals recommend or follow this diet.The following is from an article posted on the public usenet newsgroup:, and is part of a discussion with Dr. Phil Maffetone with regard to the biochemistry of insulin and carbohydrate metabolism -- central to the dietary theme -- as it applies to healthy athletes, as well as individuals suffering from diabetes or hypertension.

Subject: Insulin, diet, disease, and athletes (long)

From: Mark A. Jenkins, M.D.

In article <3ba060$>
GILBOA, (Dr. Phil Maffetone) writes:

Problems casused by excess carbo (which produces excess insulin) include heart disease, hypertension, stroke, high blood cholesterol and triglycerides, and diabetes. This information is well known in the scientific literature but is rarely mentioned in the popular press. The correct term for the problem in it's early stages, before these diseases apprear, is insulin resistance, carbohydrate intolerance or any number of other names. In my opinion, perhaps half the population (athletes too) is insulin resistant to some degree.


Mark A. Jenkins, M.D. writes,

As promised I will begin discussing insulin and its role in disease. Why is this relevant to triathlons? I'm following a thread that was started in this newsgroup about the optimum ratio of carbohydrate, fat, and protein in the diet. I was going to sit on the sidelines and watch, but I was intrigued by the paragraph above and decided to review the literature on the subject.


Insulin is a polypeptide hormone that is intimately involved in the regulation of fuel metabolism. It is manufactured and released from the pancreas. There are many factors that govern insulin release -- glucagon, amino acids, gut insulinogenic hormones, glucose -- but glucose appears to be the most important stimulus. After a brief journey in the blood stream (t 1/2 = 4 - 8 minutes), insulin binds to cell surface receptors. Once insulin binds to these specific receptors, various regulatory enzymes are modified, and this accounts for insulin's biological effects.

Insulin promotes anabolic (storage) processes and inhibits catabolic (breakdown) ones. Thus, it signals a "fed state" and instructs the body to store fuels for use later. Insulin promotes uptake and storage of glucose in muscle and fat cells. In the muscle, glycogen is built from glucose molecules. In fat cells, the glucose is used to make fat. Once assimilated into fatty acids the glucose is trapped (i.e. animals cannot break down fatty acids to yield glucose). However, the fatty acids can be exported into the blood stream for use as fuel. Additionally, insulin inhibits the production of glucose by the liver. The body has a series of checks and balances to all of it's processes and insulin is no exception. There are counter-regulatory hormones (glucagon, catecholamines, and cortisol) which give the opposite signal to insulin.


When there is insufficient biological action of insulin then diabetes mellitus occurs. There are two types of diabetes: insulin dependent (IDDM) and non-insulin dependent (NIDDM). In the former, the pancreas does not produce sufficient insulin and there are low levels of insulin in the bloodstream. In NIDDM, the pancreas is producing enough insulin, but the problem is the insulin receptor on the cells elsewhere in the body. In this circumstance, the insulin receptors are insensitive to insulin. This may occur because genetic material has coded for the manufacture of an abnormal receptor, or because events have caused the receptor to be down-regulated. These individuals often have high insulin blood levels (hyperinsulinemia). 80% of individuals with NIDDM are obese. High fat stores down-regulate insulin receptors and cause a resistance to circulating insulin. The other 20% of insulin resistant individuals -- who are not obese -- have a genetically inherited insulin receptor that does not work properly. Insulin resistance has also been linked to high blood pressure. The mechanisms are complex, but research shows that the insulin receptor's insensitivity to insulin is the crucial first step in the process. The clustering of obesity, hyperlipidemia, hypertension, and hyperinsulinemia in certain individuals has been termed "Syndrome X", or "the deadly quartet". Genetic predisposition and lifestyle are important in the development of Syndrome X and other examples of insulin resistance.


It is not hyperinsulinemia that is the problem, it is the receptor. It has been repeatedly shown in the medical literature that increasing the sensitivity of peripheral insulin receptors reduces hyperinsulinemia and hypertension. This process is termed insulin sensitization and is accomplished by aerobic exercise, low-fat / high-carbohydrate diet, and reduction of excessive body fat. Conversely, obesity and high fat diets have been shown to induce insulin resistance. It is important that the high carbohydrate diet have predominantly complex carbohydrates and also have a high fiber content. Overly refined, simple sugars do not appear to have the same effect as complex carbos. If one looks at epidemiological data, the traditional diet of many third world countries consists of high complex carbohydrate content, very low fat, and high fiber. Atherosclerotic disease was virtually unheard of until the introduction of the high fat Western diet. There are many published studies which show that starting insulin resistant individuals on a high carbohydrate / low fat diet reduces hyperinsulinemia, hypertension, and hyperlipidemia. For example in a study published in the American Journal of Cardiology (Am J Cardiol 1992;69:440-444), diabetic patients (NIDDM), insulin resistant patients, and normal controls were started a 3 week program of diet ( 75 - 80% carbohydrate, 10-15% protein, and <10% fat) and exercise. In all there were statistically significant reductions in blood pressure, insulin levels, and triglycerides. Additionally, high-carbohydrate / high-fiber diets have been shown to lower cholesterol and may reduce the risk of colon cancer.


There have been numerous scientific studies done relating diet and exercise. One study looked at muscle glycogen content and bicycle ergometer exercise until exhaustion on various 3 day diets (high-fat, normal, and high carbohydrate). The individuals were compared against themselves on the different diets. On the high carbohydrate diet, individuals averaged much higher glycogen content in the thigh muscles and were able to exercise 3 times longer until exhaustion as compared to when they were on the high fat diet (180 minutes vs 57 minutes). There is also evidence that chronic fatigue and staleness in endurance athletes may be due to gradual depletion of the body's carbohydrate reserves. One study that demonstrated this effect looked at runners who had muscle biopsies done -- to determine glycogen content -- while running 16 km a day for 3 days. All had diets that consisted of 40 - 60% carbohydrate, but by day #3 the thigh muscle glycogen was nearly depleted.

Apart from scientific experiments there are epidemiological observations. The Tarahumara Indians of Mexico have a diet that is very high in complex carbohydrate(75%) and is high in fiber (19g/d). They known for their incredible endurance and are reported to run 200 miles in soccer-type games lasting days. It is notable that obesity, hypertension, and death from atherosclerotic disease are extremely rare.


Hyperinsulinemia is a marker for several disease, and carbohydrate ingestion causes the release of insulin, but high carbohydrate ingestion does not cause hyperinsulinemia. The reason that the "if x=y and y=z, therefore x=z" argument doesn't work is because the problem is the insulin receptor. We are dealing with a biological organism that is extremely complex. There are more variables than meets the eye, and controlled scientific analysis of the situation reveals that a high complex carbohydrate diet lowers insulin levels. Regular aerobic exercise also enhances insulin receptor sensitivity and is protective against the development of atherosclerotic disease. I did not have to dig deeply into the medical literature to find the above information. There is a tremendous amount of information published from the U.S., Europe, and Japan about the benefits of high-carbohydrate/high-fiber diets. Likewise, there are volumes of data about the increased risk for insulin resistance, hypertension, atherosclerotic disease and some forms of cancer associated with high fat diets. These studies are the work of researchers and clinicians in many disciplines and have been conducted in a scientific manner. I could not find any evidence to support a 40% carbo/ 30%/30% diet as healthier or better for athletes. In fact, I uncovered the exact opposite of what we have heard advertised about the 40/30/30 plan. Perhaps scientific data will emerge in the future, but there currently is no scientific support for this diet plan in athletes or in sedentary individuals.

I don't expect anyone to take my word for it. In fact I encourage you to read the literature for yourself. Some of it is a bit technical but from the questions asked in this newsgroup, I think that most can digest this info. I have included a partial list of references that should serve as a starting point. In preparing this discussion, I took neither a "for" or "against" viewpoint. I was intrigued by the claims, have had patients and fellow triathletes ask me, and was just plain curious. In constructing this discussion, I simply performed a computer aided search of the medical literature and read what I found. Why am I qualified to discuss this subject? I am a board-certified general internist and am in my 5th year of clinical practice. I have treated many patients with diabetes, hypertension, and atherosclerotic disease. Currently, I am at Rice University where I serve as the Associate Director of the student health service and a team physician for the NCAA sports. I am also an athlete -- I swam NCAAs in college and have been competing in triathlons since 1987. Good health to all.

Selected references.

1. Insulin Resistance -- Not Hyperinsulinemia -- Is Pathogenic in Essential Hypertension. (Med. Hypothesis. 1994 42, 226-236)

2. Role of Diet and Exercise in the Management of Hyperinsulinemia and Associated Atherosclerotic Risk Factors. (Am J Cardiology 1992:69:440-444)

3. Insulin resistance, hyperinsulinemia, and cardiovascular disease. The need for novel dietary prevention strategies. (Basic Research in Cardiology. 1992. 87:99-105)

4. High-carbohydrate, high-fiber diets increase peripheral insulin sensitivity in healthy young and old adults. (Am J of Clinical Nutrition. 1990 ; 52:524-8)

5. Exercise Physiology: Energy, Nutrition and Human Performance. McArdle, Katch and Katch. 3rd Edition. Lea Febiger Publishers. 1991. (ISBN 0-8121-1351-9)

6. Cecil: Textbook of Medicine. Wyngaarden/Smith/Bennett. 1992. (ISBN 0-7216-2928-8)

7. Persistence of Multiple Cardiovascular Risk Clustering Related to Syndrome X From Childhood to young Adulthood: The Bogalusa Heart Study. (Archives of Internal Medicine. Vol. 154, Aug 22, 1994.)

The above is not copyrighted material, because it was a posting onto a public usenet newsgroup. It may be freely copied, used and distributed provided it is not altered. Since the original posting I have edited my response to correct spelling and grammatical mistakes, but the content has not been altered. Dr. Maffetone's posting has not been altered.