The impact of fish‐oil supplements on insulin sensitivity
Authors: Waite N. , Lodge J. , Hart K. , Robertson D. , Badley E. , Burton S. 
Published: July 2008

Background:  Insulin resistance is a condition in which the amount of insulin released is inadequate to produce a normal glucose response from fat, muscle and liver cells. Its prevalence is increasing, affecting approximately 25% of the UK population (Diabetes UK, 2006) and with it the risk of developing type 2 diabetes and cardiovascular disease (CVD). One of the major contributing factors to insulin resistance is obesity, the incidence of which is also on the rise in the UK (Zaninotto et al.,2006). Medication and lifestyle changes can be implemented to improve insulin sensitivity but whilst the use of fish oils to improve cardiovascular risk is well documented (Erkkila et al., 2003) its effect on insulin sensitivity are less well known. The aim of the study was to determine the effects of 60 days of fish oil supplementation on insulin sensitivity, plasma glucose and insulin levels as assessed by an oral glucose tolerance test (OGTT), total serum cholesterol levels and serum triglyceride (TG) levels.

Method:  Subjects (n = 5) from Surrey, UK, were selected based on BMI (≥25 kg m−2) and waist circumference (>37 inches for men and >31 inches for women) and current oily fish consumption (no fish oil supplements and <2 portions of oily fish per week). Selected subjects self‐reported to be otherwise mentally and physically healthy. This was a single‐blind intervention study with all subjects being placed on fish oil supplements [n‐3 Polyunsaturated fatty acid (PUFA) (100%); 440 mg Docosahexaenoic acid (DHA), 660 mg Eicosapentaenoic acid (EPA)] for 60 days. An oral glucose tolerance test (OGTT) over 120 min was completed to assess fasting plasma insulin and glucose levels and glucose response over time. Fasting serum triglycerides (TG's) and total cholesterol were also measured from baseline blood samples and blood pressure and anthropometric measurements taken before and after supplementation with the fish oils. Dietary intake was assessed and subjects’ insulin sensitivity and b‐cell function assessed using the homeostatic model assessment (HOMA). Data was analysed using SPSS (v14) with non‐parametric statistics, performed due to the small sample size.

Results:  Following supplementation there was a significant decrease in diastolic blood pressure (Z = −2.03; P = 0.042), but no other significant changes in anthropometric measurements (Table 1). There was an increase in insulin sensitivity following supplementation as demonstrated by a reduction in the plasma glucose response during the OGTT, and a mean reduction in serum TG's and total cholesterol, although these were not statistically significant (Table 1).

 Data for the anthropometric measurements, TAG, total cholesterol, fasting plasma glucose and insulin levels and insulin resistance (HOMA‐IR) and b‐cell function (HOMA‐B) of the subjects at baseline and following supplementation; mean (SD)

Measurements
Baseline
Following supplementation

Weight (kg)
95.3 (14.3)
95.8 (13.9)

BMI (kg m−2)
34.2 (3.9)
34.4 (3.9)

Waist (cm)
109.3 (10.3)
111.3 (8.8)

Hip (cm)
119.0 (10.3)
122.0 (10.9)

% body fat
40.2 (5.1)
41.2 (6.2)

% lean body mass
59.8 (10.3)
58.8 (6.2)

% water
44.2 (44.2)
43.6 (4.4)

Systolic blood pressure (mmHg)
137.4 (21.5)
130.2 (14.6)

Diastolic blood pressure (mmHg)
90.4* (11.5)
82.8* (8.9)

TAG
1.4 (0.6)
1.2 (0.5)

Cholesterol
    5 (0.97)
4.6 (0.5)

Fasting blood glucose levels (mM)
5.3 (0.3)
5.2 (0.5)

Fasting insulin levels (pM)
25.3 (0.3)
23.2 (0.53)

HOMA‐%S
1.0 (0.39)
0.9 (0.45)

HOMA‐%B
46.4 (9.7)
   45 (4.8)

Figures with asterisks (*) are significantly different (P < 0.05)

Discussion:  Supplementation with fish oils over 60 days improves a number of biomarkers used to assess risk of CVD in overweight individuals. Whilst only changes in diastolic BP were statistically significant, clinically significant reductions in all other biomarkers with positive implications for CVD risk and mortality were found. There were however a number of limitations, such as the small samples size, screening method, original glucose tolerance and insulin sensitivity of the subjects and the lack of a control group, that reduced the strength of these findings. Future, high dose, long‐term, double‐blind intervention studies controlling for these limitations would therefore be recommended.

Conclusion:  The findings of this pilot study show that the intake of high dose fish oil supplements over 60 days can help to reduce the risk of CHD and mortality in overweight individuals.

References  Diabetes UK. (2006) Guide to Diabetes [Online]. Available at http://www.diabetes.org.uk/Gdide‐to‐diabetes/Guide_to_Diabetes_Archived_pages/Insulin_resistance. Diabetes UK (accessed on December 2006).

Erkkila, A., Lehto, S., Pvorala, K. & Uusitupa, M. (2003) n‐3 fatty acids and 5 year risk of death and cardiovascular disease events in patients with coronary artery disease. Am. J. Clin. Nutr.78, 65–71.

Zaninotto, P., Wardle, H., Stamatakis, E., Mindell, J. & Head, J. (2006) Forecasting obesity to 2010; Prepared for the Department of Health: Joint Health Surveys Unit (National Centre for Social Research and Department of Epidemiology and Public Health at the Royal Free and University College Medical School).