Weight-loss success may depend on more than just motivation and willpower, according to a study reported last week in the Journal of the American Medical Association. It appears that which diet works best may depend on your hormones.
There has been debate among scientists regarding the most effective diet for weight loss. Recent studies that have pitted diet against diet - low-fat, low-carbohydrate and low-glycemic-load - have delivered inconsistent findings, with some suggesting one diet is superior and others indicating no difference between them.
Why some people do well on a conventional low-fat diet and others do poorly is often chalked up to compliance: People just aren't motivated to stick to their diet long term.
But perhaps there's another explanation. Inherent differences in insulin secretion may affect one's ability to lose weight on a diet. (Insulin is the hormone that clears sugar from the bloodstream.)
In the new study, participants classified as "high-insulin-secreting" lost five times more weight on a low-glycemic-load diet than on a conventional low-fat diet.
Low-glycemic-load diets limit rapidly digested carbohydrates that sharply raise blood sugar (glucose) and insulin levels. High-glycemic-load foods cause blood sugar and insulin levels to rise higher than do those with a low glycemic load. In response to excess insulin secretion, blood-sugar levels drop lower over the next few hours, which can trigger hunger and overeating.
Researchers from Children's Hospital Boston assigned 73 obese adults, aged 18 to 35 years, to either a low-fat diet (55 per cent carbohydrates, 20 per cent fat, 25 per cent protein) or a low-glycemic-load diet (40 per cent carbohydrates, 35 per cent fat, 25 per cent protein) for six months. Participants were then followed for an additional 12 months.
Volunteers on the low-glycemic-load diet emphasized whole grains, nuts, legumes, fruits and non-starchy vegetables and limited their intake of high-glycemic-load foods (white bread, white rice, refined breakfast cereals, potatoes, fruit juices, sweets).
Individuals who followed the low-fat diet were counselled to eat grains, vegetables, fruits and legumes and to limit sweets and high-fat snacks. Low-fat diets generally contain more carbohydrates and raise insulin higher than low-glycemic-load diets.
The diets involved no calorie restrictions or measuring of food. Participants received ongoing counselling and were told to "eat when hungry, but before famished" and to "stop eating when satisfied, before feeling stuffed."
Initially, all volunteers were given an oral-glucose tolerance test to measure their insulin response. (The test requires drinking a liquid containing 75 grams of glucose. Blood is taken before, and again every 30 to 60 minutes after, drinking the solution.) In this study, insulin concentration was measured at 30 minutes.
After six months, changes in body weight and body fat did not differ between the two diet groups. However, when the researchers analyzed the results based on insulin levels, there were noticeable differences.
High insulin secretors lost 2.2 lbs (1 kg) per month on the low-glycemic-load diet versus only 0.9 lbs (0.4 kg) on the low-fat diet. After 18 months, total weight loss was 12.8 lbs (5.8 kg) in the low-glycemic-load group, but only 2.6 lbs (1.2 kg) in the low-fat group of high insulin secretors.
Among the low insulin secretors, body weight and fat loss did not differ significantly between the two weight-loss plans.
Regardless of insulin secretion, following the low-glycemic-load diet boosted HDL (good) cholesterol and lowered concentrations of triglyceride (a blood fat) in the blood, effects that were not seen on the low-fat diet.
This isn't the first study to suggest how much insulin you secrete might predict how well you do on a diet. A study in 2005 from Tufts-New England Medical Center in Boston found that overweight adults with higher insulin secretion lost more weight when assigned to a low-glycemic-load diet than to a conventional low-fat diet. Among people who had lower insulin secretion, weight loss did not differ on the two diets.
