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Being obese does not necessarily boost your risk for heart disease, according to two studies published earlier this week in the Archives of Internal Medicine.

It seems that a surprising number of obese individuals are heart-healthy, metabolically speaking, while others are prone to heart disease, despite the same level of fatness.

On the flip side, being slim does not make you immune to heart disease. The findings also revealed that an unexpectedly high proportion of healthy-weight people have obesity-related risk factors for heart disease such as high blood pressure, elevated triglycerides (blood fats) and low HDL (good) cholesterol.

Previous research indicates that in addition to obesity - measured by body mass index - the way your body fat is distributed influences the risk for heart disease and Type 2 diabetes. BMI is calculated as weight in kilograms divided by height in metres squared. For adults, a BMI of 18.5 to 24.9 is considered healthy, 25 or more signals overweight, and 30 or more indicates obesity.)

Carrying excess body fat around your abdomen - measured by your waist circumference - is associated with metabolic risk factors such as high blood pressure, low HDL cholesterol, high blood triglycerides and insulin resistance, a condition that occurs when your cells can't properly use the blood-sugar-clearing hormone insulin. (Insulin resistance is thought to be a precursor to Type 2 diabetes.)

Waist circumference is a good measure of visceral fat, the type of deep fat that packs itself around the abdominal organs and secretes chemicals that increase insulin resistance and cause inflammation throughout the body.

Metabolic risk factors tend to appear together and are thought to double the risk of heart attack and increase the probability of developing Type 2 diabetes fivefold.

In one of the studies released this week, researchers from the Albert Einstein College of Medicine in Bronx, N.Y., assessed body weight and metabolic abnormalities in 5,400 U.S. adults aged 20 and older. Participants were considered metabolically healthy if they had none or only one metabolic risk factor and metabolically unhealthy if they had two or more.

Almost one in four (23.5 per cent) normal-weight adults had unhealthy levels of at least two metabolic risk factors whereas half (51 per cent) of overweight adults and 32 per cent of obese adults were metabolically healthy.

Normal-weight individuals with metabolic abnormalities tended to be older, less physically active and had larger waist sizes than those normal-weight individuals who were metabolically healthy. People who were obese, but considered healthy, were likely to be younger, more physically active, and have smaller waists than those with two or more metabolic risk factors.

In the second study, German researchers measured total body fat, visceral fat and subcutaneous fat (fat just beneath the skin) in 314 adults aged 18 to 69. Insulin resistance was also measured by a blood test. Individuals were then divided into four groups: normal weight, overweight, obese with insulin resistance and obese without insulin resistance.

Not surprisingly, people in the overweight and obese groups had more total body fat and more visceral fat than those in the normal weight group.

When the researchers compared people in the obese groups, they found two differences. People who were obese and had insulin resistance had more fat in their liver than obese individuals who did not have insulin resistance. They also had thicker walls in the carotid arteries of their necks, a sign of atherosclerosis (narrowing and hardening of the arteries).

People who were obese but who were not resistant to the action of insulin were similar to normal-weight individuals with respect to artery wall thickness.

These findings suggest that if you're considered obese, having a fatty liver may be more important than waist circumference in determining your risk for heart disease. Previous studies have consistently found that fat deposits in the liver influence the body's ability to use glucose and fat properly.

Fatty liver is usually detected when routine blood tests show elevated liver enzyme levels. It can also be discovered by an ultrasound examination of the abdomen. The most common cause of fatty liver disease is obesity. Having diabetes, elevated blood cholesterol, and/or high blood triglycerides are also risk factors for fatty liver.

These studies fuel the debate that BMI has limitations for measuring health risk - at least for heart disease. BMI doesn't tell you where you're carrying excess body fat and it doesn't distinguish between fat weight and muscle weight.

The results also demonstrate the harmful effects of visceral fat or waist circumference in people considered obese as well as those whose weight is normal.

That doesn't mean you don't need to worry about your weight. Having a BMI that classifies your weight as obese - or healthy - is part of the picture. This week's study findings suggest the following strategies will help reduce heart disease and diabetes risk:

Know your waist size

A waist circumference of 94 centimetres (37 inches) or greater for men and 80 cm (31.5 inches) or greater for women increases the likelihood of Type 2 diabetes, high blood pressure, elevated cholesterol, heart attack, stroke and other health problems. (Lower cut-offs for waist circumference are recommended for Asians because they are more prone to obesity-related health problems at lower BMIs and smaller waist sizes.)

Guard against fatty liver

Losing excess weight and reducing your intake of refined sugars and white starchy foods can dramatically reduce fatty deposits in your liver.

Choose low-glycemic-index foods - which are gradually converted to blood sugar - such as grainy breads, steel-cut oats, 100-per-cent bran cereal, brown rice, sweet potatoes, pasta, apples, citrus fruit, legumes, milk, yogurt and soymilk.

Increase exercise

Regular aerobic exercise such as brisk walking, jogging, cycling and stair climbing reduces the likelihood of developing metabolic risk factors for heart disease in people who are obese and of a normal weight. Moderate to high-intensity exercise - 30 minutes most days of the week - may also reduce liver fat content.

Leslie Beck, a Toronto-based dietitian at the Medcan Clinic, is on CTV's Canada AM every Wednesday. Her website is lesliebeck.com.