If you want to prevent a heart attack, watching your fat intake isn't enough. When it comes to diet, what seems to be more important is eating the right type of carbohydrate.
A landmark study to be published in the June issue of the American Journal of Clinical Nutrition provides direct evidence that refined, highly processed carbohydrates are worse for your heart than saturated fat.
The study – the first to investigate the link between heart disease and type of carbohydrates consumed – revealed that replacing saturated fat with refined carbs actually puts you at greater risk for heart attack than if you didn't avoid saturated fats to begin with.
For decades, the view that a high intake of saturated fat increases the risk of heart disease has been the driving force behind the recommendation to follow a diet low in fat – especially saturated fat – and high in complex carbohydrates.
It is advice that has led us to consume considerably less saturated fat. But it's also advice that's linked with unintended consequences – it has spurred an increase in our intake of refined carbohydrate and added sugars that growing evidence suggests is contributing to obesity, Type 2 diabetes and now heart disease.
Until recently, the role of carbohydrates in heart disease received little attention. Carbohydrates are traditionally classified as simple or complex, depending on their chemical structure. Simple carbohydrates such as fructose (in fruit), lactose (in milk) and sucrose (table sugar) have one or two sugars linked together.
Complex carbohydrates, or starches, are long chains of sugar (glucose) units linked together. Starchy foods have long been considered a healthy substitute for saturated fat. But we're learning that not all carbohydrates are created equal.
While all carbs ultimately end up as glucose in your bloodstream, they do so at different rates.
It turns out that many complex carbohydrates, such as baked potatoes and white bread, cause even faster spikes in blood glucose than do simple sugars.
And it seems the speed at which carbohydrate enters your bloodstream affects your risk for heart disease.
The glycemic index (GI) is a good indicator of the quality of carbohydrates you consume. The GI is a scale that ranks carbohydrate-rich foods by how fast they raise blood-sugar levels compared with a standard food, usually pure glucose, which is ranked 100.
A food's GI value depends on the amount and type of fibre and the extent of processing. Foods with a high GI (70 or more) are digested quickly and cause a rapid rise in blood glucose and, as a result, an outpouring of insulin, the hormone that removes sugar from the blood and stores it in cells.
It probably comes as no surprise that refined carbohydrates, which are ground and milled and devoid of bran, have a high GI. Many carbohydrate-rich foods we eat on a regular basis are highly processed, such as bread, rolls, pizza, white rice, instant oatmeal, cereal bars, breakfast cereals, cookies, honey, table sugar and sugary drinks.
Foods with a low GI (less than 55) release sugar more slowly into the bloodstream and don't produce a rush of insulin. Examples include grainy breads with seeds, steel-cut oats, 100 per cent bran cereal, brown rice, sweet potatoes, pasta, apples, citrus fruit, grapes, pears, legumes, nuts, milk, yogurt and soy milk.
In the latest study, researchers compared the association between saturated fat and carbohydrates with the risk of heart attack among 53,644 healthy men and women living in Denmark.
After 12 years of follow-up, 1,943 people had suffered a heart attack. Substituting some carbohydrate in the diet with saturated fat did not alter the likelihood of heart attack. In other words, consuming more saturated fat didn't increase or decrease the risk.
However, replacing some saturated fat in the diet with high-GI carbohydrate significantly boosted the risk of heart attack – by 33 per cent. Conversely, substituting low-GI carbohydrates for saturated fat lowered the risk of heart attack, although this wasn't statistically significant (so it could have been a chance finding).
Only two other studies have investigated the substitution of carbohydrates for saturated fat. While one reported a higher risk of heart disease, the other did not. However, these studies looked only at total carbohydrate, not the type consumed.
High-glycemic meals are thought to increase the risk of heart disease by increasing blood triglycerides (fat), promoting inflammation and impairing blood vessel function.
The effects of a high-glycemic diet are especially harmful for people with insulin resistance, a condition in which the body produces insulin but does not use it properly. (Excess weight and lack of physical activity contribute to insulin resistance.)
Experts warn that our obesity epidemic and growing intake of highly processed carbohydrates have created a "perfect storm" for heart disease. What's more, in a population that's predominately overweight and sedentary, refined carbohydrates likely cause even greater damage than saturated fat.
For this reason, reducing your intake of refined carbohydrate and sugars should be top priority.
Start by replacing "white" carbs with whole grains. Not only are many of these foods low on the glycemic index, they also have more fibre, vitamins, minerals and phytochemicals than their refined counterparts.
Next, use the following tips to add low-glycemic foods to your meals and snacks.
– Include at least one low-GI food per meal. In addition to the foods discussed above, you'll find the GI value of hundreds of foods at www.glycemicindex.com
– Avoid eating high-GI snacks such as pretzels, tortilla chips and refined crackers. Opt for fresh fruit, yogurt, nuts, plain popcorn or low-GI crackers such as Wasa and Finn Crisp.
– Avoid sugary drinks such as pop, fruit punch, iced tea and VitaminWater (which contains 7.5 teaspoons of sugar per 591-millilitre serving).
– Don't forget about portion size. When it comes to weight control, excess calories add up regardless of their glycemic index.
Leslie Beck, a Toronto-based dietitian at the Medcan Clinic, is on CTV's Canada AM every Wednesday. Her website is lesliebeck.com.