Kendrick on…the idiotic dietary advice we give to diabetes patients

07 Sep 06

Type 2 diabetes is primarily a problem of insulin resistance. Insulin becomes less effective at lowering blood glucose levels. In its early stages loss of blood glucose control occurs only after a meal.

Over time a more chronic state emerges, usually when the ß cells burn out and are unable to respond to fluctuations in blood glucose levels. At this point the advice normally given is to eat a low-fat, high-carbohydrate diet.

But let's have a think about this. In diabetes, blood glucose levels are too high. Where does blood glucose come from? Oh yes... eating carbohydrates.

Despite this, people who have lost control over glucose levels are, in effect, actively encouraged to eat glucose. The net impact of this will be to raise blood glucose levels and also insulin levels (which may cause more harm than raised blood sugar levels).

This is supposed to be the exact thing we are trying to avoid. On the face of it, this advice appears completely idiotic. But what is the reasoning behind it?

The reasoning is as follows: people with type 2 diabetes are known to have a greatly increased risk of CHD. A high-fat diet is thought to be an underlying cause of CHD.

Ergo, patients with diabetes should avoid eating fat in order to avoid CHD. This hypothesis emerged at some point in the late 1970s and very rapidly became the established dogma.

Personally, I do not believe that fat consumption has the slightest impact on heart disease in people with or without diabetes, and I would defy anyone to unearth a controlled study on restriction of dietary fat that has shown any impact on CHD.

Some of you may recall the Women's Health Initiative study published earlier this year. This was carried out in 48,835 women over eight years. In the intervention group both fat and saturated fat consumption was significantly reduced.

And the researchers concluded: 'Over a mean of 8.1 years, a dietary intervention that reduced total fat intake and increased intakes of vegetables, fruits and grains did not significantly reduce the risk of CHD, stroke or CVD in postmenopausal women and achieved only modest effects on CVD risk factors'1.

And this is not a one-off negative study. It is just the latest in a long line of dietary intervention studies that have all shown exactly the same thing.

Zero impact on CHD risk from restricting fat intake, saturated or otherwise. Yet, despite this complete and utter lack of evidence, an almost messianic belief in the diet-heart hypothesis persists.

It is true that in people who do not have diabetes, eating a high-carbohydrate diet probably does little harm – although there are some who believe that a switch from fat to carbohydrate in the diet has driven the current surge in obesity levels.

But in type 2 diabetes, a high-carbohydrate diet puts increased stress on the ß cells and will worsen insulin resistance.In fact, the only pathway the body has for getting rid of excess glucose – when liver and muscle stores are full – is to convert glucose into fat in the liver, then export this fat via VLDL/triglycerides to adipose tissue.

And this is a process driven by high insulin levels.In short, if you have type 2 diabetes and you eat a high-carbohydrate diet, you will push up blood glucose levels and blood insulin levels.

You will also create hypertriglyceridaemia and, due to protein transfer from HDL to VLDL, a low HDL level. Which is the exact metabolic state now known to be associated with CHD – metabolic syndrome, or syndrome X.

You will probably also create non-alcoholic steatohepatitis (NASH), as the liver fills with fat generated from glucose.

If, on the other hand, you eat fat, this cannot have any impact on blood glucose levels. While glucose can be converted to fat, fat cannot be converted back to glucose.

A high- fat diet also has no impact on raising insulin levels, as absorption of fat into adipose tissue requires only a low background insulin level. A high-fat diet will not raise VLDL levels or lower HDL levels either.

In short, a high-fat diet is theoretically perfect for people with type 2 diabetes.But a high-fat diet raises LDL levels, doesn't it? We are repeatedly told this, but it doesn't happen to be true.

Here, for example, is a quote from Dr William Castelli, director of the Framingham study, one of the longest-running and most widely quoted studies in the world. 'In Framingham, Massachusetts, the more saturated fat one ate, the more cholesterol one ate, the more calories one ate, the lower people's serum cholesterol' – by which he means LDL2.

And when researchers did look at a high-fat diet in type 2 diabetes, they found total cholesterol, LDL and triglycerides all fell and HDL increased.

The study by Hays, presented at the 81st annual meeting of the Endocrine Society in 1999, found total cholesterol declined from 6.1 to 5.0mmol/l, LDL fell from 3.5 to 2.7mmol/l, HDL increased from 1.16 to 1.24mmol/l and triglycerides declined from 6.0 to 4.8mmol/l.

By the end of the one-year study 90 per cent of the patients had achieved American Diabetes Association targets for HbA1c, HDL, LDL and triglycerides.

Other work has confirmed the benefits of a high-fat diet on diabetic and lipid parameters3-6.

Studies have even shown great improvements in gestational diabetes from using a high-fat diet: improved glycaemic control, less need for insulin therapy, a decrease in the incidence of large for gestational age (LGA) infants, and a decrease in Caesarean deliveries for cephalopelvic disproportion and macrosomia.

The final word in this area goes to Sylvan Weinberg, former president of the American College of Cardiology: 'The low-fat "diet heart hypothesis" has been controversial for nearly 100 years.

The low-fat, high-carbohydrate diet, promulgated vigorously by the National Cholesterol Education Programme, National Institutes of Health and American Heart Association since the Lipid Research Clinics-Primary Prevention Program in 1984, and earlier by the US Department of Agriculture food pyramid, may well have played an unintended role in the current epidemics of obesity, lipid abnormalities, type 2 diabetes and metabolic syndromes. '

This diet can no longer be defended by appeal to the authority of prestigious medical organisations or by rejecting clinical experience and a growing medical literature suggesting that the much-maligned low-carbohydrate, high-protein diet may have a salutary effect on the epidemics in question'.

Dr Malcolm Kendrick is a salaried GP in south Manchester