A plant-based vegetarian diet not only trumps a conventional diabetic diet when it comes to helping people with type 2 diabetes to lose weight, but because it more effectively reduces muscle fat, it also helps them to improve their metabolism.
These were the main findings of a new study published in the Journal of the American College of Nutrition by lead author Dr. Hana Kahleová, director of clinical research at the Physicians Committee for Responsible Medicine in Washington D.C., and colleagues.
Type 2 diabetes is the most common form of diabetes and develops because the body does not make or use insulin effectively. Although it can develop at any age, it most often arises in people who are middle-aged and older.
Diabetes is a significant global public health problem that affects some 150 million people worldwide. This number is expected to double by 2025, not only as a result of growing numbers of people and aging populations, but also because of modifiable factors such as sedentary lifestyles, unhealthy diets, and obesity.
In the United States, there are more than 29 million people living with diabetes and another 86 million are thought to have prediabetes, a condition in which blood sugar is higher than normal, and although not in the diabetes range, it raises the risk of developing type 2 diabetes.
It is recognized that people with prediabetes can cut their risk of developing type 2 diabetes by more than 50 percent by taking part in structured programs to change their lifestyle.
Benefits of plant-based vegetarian diet
In their study paper, Dr. Kahleová and colleagues explain that changes to diet form an important part of managing type 2 diabetes, and they discuss evidence relating to vegetarian diets.
They note, for example, that compared with a conventional diet, a vegetarian diet can achieve weight loss, improve control of blood glucose, or "glycemic control," raise insulin sensitivity, and lead to other metabolic improvements.
The authors also discuss the beneficial effects of a vegan diet - which contains only plant-based food - on health as it relates to diabetes. For example, there is evidence that in people with type 2 diabetes, a "low-fat vegan diet improves glycemic control and cardiovascular risk factors."
Thus, for their 6-month study, they decided to compare the effects of a conventional diabetic diet with those of a plant-based vegetarian diet in 74 type 2 diabetes patients, comprising 43 percent men and 57 percent women, who were on oral medication for glucose control.
The researchers randomly assigned 37 participants to the vegetarian group and 37 to the conventional diet group. Both diets were calorie-restricted to reduce intake by 500 calories per day and all meals were provided to the participants for the 6 months of the study.
Composition of the two diets
In the vegetarian diet, around 60 percent of the calories came from carbohydrates, 15 percent from protein, and 25 percent from fat. It consisted of grains, legumes, vegetables, fruits, and nuts, with animal products limited to a maximum of one serving of low-fat yogurt each day.
A typical meal plan on the vegetarian diet might comprise: a breakfast of cooked millet, plums, and almonds; a soup made with lentils, cabbage, and carrots at lunchtime; marinated tofu, bean sprouts, and brown rice for dinner; and snacks of hummus with carrot sticks.
In the conventional diabetic diet - devised according to a recognized guideline - around 50 percent of the calories came from carbohydrates, 20 percent from protein, and no more than 30 percent from fat (with a limit of 7 percent saturated fat).
A typical meal plan on the conventional diabetic diet might consist of: a breakfast of peanut butter raisin oatmeal; a wrap with tuna and cucumber for lunch; brown rice with honey lemon chicken and vegetables at dinner time; and snacks of carrot and celery sticks with a low-fat dairy dip, or low-fat plain yogurt.
For the first 3 months, the participants were asked not to change their physical exercise habits. Then, for the second 3 months, an aerobic exercise program was added to their dietary regimen. The researchers examined the participants at baseline, 3 months, and 6 months. These exams included scans using MRI to measure changes in fat composition.
Vegetarian diet achieved more weight loss and reduced muscle fat
The results showed that the average weight loss in the plant-based vegetarian diet group was 6.2 kilograms (13.7 pounds), nearly twice the 3.2 kilograms (7.1 pounds) average weight loss of the conventional diet group. This was despite the fact that both groups consumed the same amount of calories per day.
The researchers also found that while both groups showed similar reductions in subcutaneous fat, only the vegetarian group showed greater reduction in intramuscular fat and any reduction in subfascial fat.
Subcutaneous fat is a type of fat that is stored under the skin. Subfascial fat is that which lines muscles, whereas intramuscular fat is the fat that is stored inside muscles.
The researchers also found that reductions in subcutaneous and subfascial fat were in line with changes in markers of glucose metabolism and control - such as fasting blood glucose, insulin sensitivity, and glycated hemoglobin.
The differences in results in the two diet groups are significant because increase in subfascial fat has been linked to insulin resistance in people with type 2 diabetes, so reducing this type of fat could help to improve glucose control.
Dr. Kahleová explains, "By taking extra fat out of the muscle cells, we're letting insulin back in to convert sugar into energy."
She likens the effect to "a metabolic reboot, especially for people who struggle with extra weight, a sluggish metabolism, or type 2 diabetes."
The researchers also note that reduction in intramuscular fat could help to increase mobility and muscle strength, which could be of particular benefit to older people with diabetes.
"What we found is that a plant-based vegetarian diet is a helpful tool for anyone who is serious about staying healthy and lean, especially as we age."
Dr. Hana Kahleová