GL evidence – Cardiovascular health
Two groups of overweight or obese people followed either a low GL diet or a low-fat low-calorie diet for two years. After each person had lost ten per cent of their body weight, other measures of their health were taken. Those on the low GL diet had greater improvements in insulin resistance (blood sugar control), triglycerides (fat circulating in the blood), inflammation and blood pressure compared with those on the conventional low-fat, low-calorie diet. The researchers concluded that a reduction in glycemic load may aid in the prevention or treatment of obesity, cardiovascular disease, and diabetes mellitus.
One group of people followed a low GL diet while another group followed a conventional low-fat, low-calorie diet (Canada’s Food Guide to Healthy Eating). Those following the low GL diet, not only lost more weight, they also had greater improvements in HDL cholesterol, triglycerides and fasting glucose compared to those on the conventional low-fat, low-cal diet after six months. These health gains were sustained or improved upon after twelve months. The researchers concluded that ‘implementation of a low GL diet is associated with substantial and sustained improvements in abdominal obesity, cholesterol and blood sugar control’.
A study published in the Lancet in 2004 in which two groups of mice were fed either a low-GL diet or a high GL diet and their health compared. Besides being leaner, the low-GL group had better blood sugar control, lower blood fats and did not suffer the pancreatic disruption of the high-GL group.
A study of 574 adults in Massachusetts between 1994 and 1997, found that higher total carbohydrate intake, percentage of calories from carbohydrate and glycemic index/glycemic load were related to lower levels of beneficial HDL cholesterol and higher blood triglyceride levels. These results show an unfavourable effect of increased intake of highly processed carbohydrate on fat profile, which may have implications for metabolic syndrome, diabetes and coronary heart disease.
Participants in this study were assigned to either a low-fat diet or 1 of 2 Mediterranean diets. Those in the Mediterranean diet groups received nutritional education plus either free virgin olive oil (1 litre per week) or free nuts (30g per day). Changes were evaluated after 3 months.Compared with the low-fat diet, the Mediterranean diet produced more beneficial changes in blood sugar levels, blood pressure and good HDL cholesterol. The Mediterranean diet with olive oil also reduced levels of the dangerous C-reactive protein, associated with heart disease. These results show that a Mediterranean-style diet, which promotes low GL carbohydrates and increased monounsaturated fat intake, is more beneficial than a low fat diet.
Twelve men with type two diabetes followed a low GL or high GL for 4 weeks. Blood sugar levels after a meal were significantly lower in people following the low GL diet than those in the high GL diet. Insulin levels and ‘bad’ LDL cholesterol were also lower in the low GL group.
Another study, this time from Italy, called the EPICOR trial, which involved more than 47,000 people, compared diet with cardiovascular risk over almost eight years. Those women in the top quarter of glycemic load (GL) diets, eg eating the most carbs in the form of fast-releasing sugar and refined foods, had double the risk of coronary heart disease than those in the lowest quarter. This study didn’t find the same association with men. There is no clear reason for the sex differences however it may be that women are more sensitive to the hormonal changes that accompany high GL diets.
There’s a quandary in the diet world. Evidence shows that a low-carb, high protein diet, aka Atkins, does work for weight loss, but, on the other hand, a high meat/milk intake fails to lower bad ‘LDL’ cholesterol and is associated with increased cancer risk. But what happens if you eat a lower carb diet, with more protein instead from vegetable sources, such as beans, nuts, seeds, soya, all foods associated with lowering cholesterol and cancer risk? A study published in the Archives of Internal Medicine shows excellent results. Headed by Professor David Jenkins, who invented the Glycemic Index (GI) as a way of measuring which foods raise your blood sugar level, this study gave two groups of people a reduced calorie diet – one with a low amount of carbohydrates (with low GI ratings), high protein and fat, but from vegetable sources; the other with higher carbohydrates, lower fat and protein. Both groups lost nearly 9lbs (8.8lbs) in the four weeks, but those on the low-carb diet had greater reductions in their total cholesterol and LDL cholesterol levels.
This diet was very close to the Holford Low GL Diet which provides 25% of calories from fat and 25% from mainly vegetable protein, with some fish. Dr Jenkins and colleagues also gave the right kind of carbohydrates with more foods high in soluble fibres, such as oats instead of wheat, and plenty of vegetables including aubergine and okra, again excellent sources of soluble fibres. Also, the diet included seeds, nuts and beans, excellent high protein foods that are known to lower cholesterol and provide plenty of minerals. These kind of low GL diets, based on wholefoods, not fake foods, is the perfect diet for losing weight and lowering cholesterol. Our only criticism of this diet approach is the lack of omega 3 fats, so abundant in oily fish, which are also known to reduce cardiovascular disease risk.
This meta-analysis of studies reports that “an independent association of saturated fat intake with cardiovascular disease risk has not been consistently shown in prospective epidemiologic studies”. Replacement of saturated fat by polyunsaturated or monounsaturated fat lowers both LDL and HDL cholesterol. However, replacement with a higher carbohydrate intake, particularly refined carbohydrate, can exacerbate many risk factors for cardiovascular disease including the “atherogenic dyslipidemia associated with insulin resistance and obesity, increased triglycerides, small LDL particles, and reduced HDL cholesterol”. It concludes “although substitution of dietary polyunsaturated fat for saturated fat has been shown to lower CVD risk, there are few epidemiologic or clinical trial data tosupport a benefit of replacing saturated fat with carbohydrate. Furthermore, particularly given the differential effects of dietary saturated fats and carbohydrates on concentrations of larger and smaller LDLparticles, respectively, dietary efforts to improve the increasing burden of CVD risk associated with atherogenic dyslipidemia should primarily emphasize the limitation of refined carbohydrate intakes and a reduction in excess adiposity.” In other words, it’s more important to cut back on carbs by eating a low GL diet than reducing your fat intake.
A report in the Scientific American makes it clear that it’s high carb diets, meaning high GL diets, that increase heart disease risk, not fats as we’ve been led to believe. It reports on a big meta-analysis of studies, published in the American Journal of Clinical Nutrition last month involving nearly 350,000 people which found no association between saturated fat and heart disease. Instead, eating more carbs increases risk.
While high fat diets do raise cholesterol, as Professor Meir Stampfer from Harvard School of Public Health points out “total cholesterol is not a great predictor of risk.” In a study he published in the New England Journal of Medicine those with the lowest carb intake, eating high fat, lost twice as much weight as those eating a low-fat diet.
These are just two of many studies that clearly show that eating too much sugar and refined carbohydrates, or too many carbs in total, best indicated by the ‘glycemic load’ of your diet, is a much more important health factor, and predictor of weight gain, cardiovascular risk and diabetes, than intake of fat.
As Professor David Ludwig says ““If you reduce saturated fat and replace it with high glycemic-index carbohydrates, you may not only not get benefits—you might actually produce harm. The next time you eat a piece of buttered toast, he says, consider that “butter is actually the more healthful component.”
M.Moyer, Carbs against Cardio, Scientific American, May 2010
A study of 163 overweight adults gave four diets: high carb, made of high GI foods; high carb, made of low GI foods; low carb, made of high GI foods; and low carb made of low GI foods, each for 5 weeks.
Those given a low carb, low GI food (eg low GL diet) compared to high carb, high GI, had a 20% decrease in triglycerides (blood fats), which are associated with increasing heart disease risk. Those eating low carbs and low GI also lowered triglycerides more effectively than those eating low carb and high GI foods. There was no significant changes, however, in insulin sensitivity or blood lipids or pressure between the high carb, high GI and low carb, low GI groups.
Data were extracted from 14 trials comprising 1097 participants. A median reduction in GL of 28units reduced the overall estimates for Systolic blood pressure and diastolic blood pressure by 2.0 mm Hg (95% CI:0.2, 3.8 mm Hg; P = 0.03) and 1.4 mm Hg (95% CI: 0.1, 2.6 mm Hg; P = 0.03),respectively. This review of healthy individuals indicated that a lower glycemic diet may lead to important reductions in blood pressure.