The low-fat vs. low-carbohydrate diet argument has been going on for years and it's unlikely to be resolved soon. Some studies have shown that low carb is better for weight loss, while others (as we discussed here and also here) come down on the side of low fat. A new study takes another look at the issue, with a new wrinkle of adding walnuts to the mix.
Dr. Cheryl L. Rock and colleagues from the University of California, San Diego, reported in the Journal of the American Heart Association, that while low-carb (LC) and low-fat (LF) diets didn't make much difference with respect to weight loss over six months, there were differences with respect to blood lipid levels especially with the addition of walnuts to a high-fat (HF) diet.
The investigators randomly assigned each of 213 overweight or obese women to one of three diets:
- Diet 1 was 20 percent of calories from fat, 65 percent from carbs (LF)
- Diet 2 was 35 percent from fat and 45 percent from carbs (HF)
- Diet 3 was the same as the HF diet, with 18 percent of the calories coming from walnuts (HFW)
Participants were given instructions by dietitians, and the aim was for each person to consume from 500 to 1,000 calories less than they used in activity. Further, they were all advised to walk at least 10,000 steps per day.
None of the women were diabetic at the beginning of the study. Some were more sensitive to insulin than others, and the investigators also examined whether the degree of insulin sensitivity (or, conversely insulin resistance) affected the outcome of the dietary changes.
At six months into the trial, weight loss was similar for the three diet conditions: 8.5, 6.3, and 7.5 percent of initial body weight for diets 1,2,and 3 respectively. However, in each group over one-third of the initially insulin-resistant women became insulin sensitive.
In addition, women in diets 1 and 3 had significant reductions in their levels of LDL ("bad") cholesterol. Also, insulin-resistant women in all three groups lowered their levels of blood triglycerides by the six-month evaluation. The walnut-enriched diet group (HFW) also had significantly greater increases in HDL cholesterol ("good ") than either of the other groups.
In their conclusion, the authors pointed out that "walnuts are a rich source of polyunsaturated fatty acids, particularly the [omega]-3 fatty acid, a-linolenic acid, and [omega]-6 linoleic acid." They also emphasized "[t]he walnut-rich diet resulted in the most favorable changes in lipid levels while still associated with a degree of weight loss that was comparable to the lower fat diet."
It warrants noting, however, that adding walnuts to anyone's diet also adds fat and calories. Thus it would be very important, in a weight-loss scenario, to carefully substitute this source of calories for another, rather than simply adding them to a current diet. Also, although the participants in the study were educated about their dietary assignments, the lack of any means of assessing compliance is definitely a weakness of the study.