Summary

Cod protein intake improves insulin sensitivity in people with insulin resistance, compared with the consumption of other proteins. The mechanism can be attributed to the specific amino acid composition of codfish protein.

Introduction

We know that insulin resistance is an important early feature of type 2 diabetes. Available insulin becomes less effective in controlling the blood sugar, and one form of treatment consists of giving insulin-sensitizing drugs, such as metformin and the thiazolidinediones (the glitazones such as Actos^® and Avandia^®). These drugs can delay the transition from insulin resistance to full-blown diabetes, and may preserve beta-cell function.

Some dietary proteins can have a similar effect, they also increase insulin sensitivity. In a search for the protein most capable of this action, Quebec researchers studied the results of a diet containing cod protein in comparison to one containing other proteins (beef, pork, veal, and milk). Their findings have been published in the journal Diabetes Care, and we summarize them here.

What was done

This study had a cross-over design. Nineteen insulin-resistant subjects had their insulin sensitivity measured before and after a cod-protein diet, and also before and after a similar diet containing other lean proteins. There were 10 men and 9 women in the study; they were aged 40 to 65, and were either overweight or obese, based on their body mass index (BMI).

The cod protein diet was made from cod fillets; the 'other protein' diet consisted of lean beef, pork, veal, eggs and egg substitute, and skimmed milk and milk products. While the cod-protein diet contained more of some, and less of other amino acids, both diets had equivalent amounts of dietary fibers, monounsaturated, polyunsaturated, and saturated fatty acids, carbohydrates, and total calories. (Cod-liver oil was added to the 'other protein' diet to make up the equivalent amounts of lipids.)

After a 2-week run-in period, participants were randomly assigned to receive one or other of the two diets, for 4 weeks. They then returned to their normal pattern of eating for a wash-out period of 4 weeks before taking the alternative diet for another 4 weeks.

Oral glucose tolerance tests (75 grams of glucose given) provided blood samples for glucose, insulin, and C-peptide concentrations in the fasting and 2-hour state. These allowed determination of beta-cell function. Insulin sensitivity was determined by means of a technique called a "hyperinsulinemic-euglycemic clamp".¹ These tests were run before and at the end of each diet period.

What was found

The average age of the participants was 54; their BMIs averaged 30.9 (men) and 33.8 (women). Fasting blood glucose levels averaged 110 mg/dL, but a third of them had isolated impaired fasting and 2-hour glucose levels, indicating diabetes.

The oral glucose tolerance test showed no significant differences between the two diet groups (cod protein or 'other protein') after 4 weeks' consumption. Both diets were associated with an average weight loss of 2 pounds.

The cod diet was found to improve insulin sensitivity by 29%, whereas the 'other protein' diet reduced it by 3%. Those subjects with a low insulin sensitivity at baseline had the greatest improvements in insulin sensitivity with the cod protein diet, compared with the 'other protein' diet.

Additional analyses showed that the cod diet 'improved overall glucose homeostasis' significantly, compared with consumption of the 'other protein'. ('Overall glucose homeostasis' means the way the body works to keep the blood sugar level within an acceptable range despite the effects of large meals or glucose depletion by, say, prolonged vigorous exercise.

What the findings mean

The authors of the study claim this is the first study to show that cod protein (as an example of lean fish protein) improves insulin sensitivity in people with insulin resistance, compared with other proteins. They believe the mechanism by which cod protein works is attributable to its specific amino acid composition. In particular, the number of low-branched aminoacids (valine, leucine, isoleucine) and the higher arginine content may be important.

If this supposition is born out in further studies, it will provide impetus toward the discovery of new approaches to the prevention and/or treatment of type 2 diabetes. Studies such as this one are valuable in yielding data that can point the way to further research.