BMI or Body Mass Index is based on your height and weight and measures if your weight is healthy.
To calculate BMI, take your weight in kilos and divide it by your height in metres squared.
Bodyfat percentage measures how much body fat you have relative to your lean body mass. This includes not only muscle but everything in your body that is not fat, such as your organs, skin, bone, fluid levels, food weight, and stored glycogen.
However, when we are talking about what is ‘optimally’ healthy, there is an important difference between BMI and body fat percentage.
BMI does not account for body composition at all. As a result, an individual with a high level of muscle mass could be considered overweight or obese for their height. The irony here is that these types of individuals could be classed as ‘unhealthy’ whereas a typical ‘skinny fat’ person may be classed as ‘healthy’.
However, there are some important reasons why we need to consider not just our total body weight but what that weight represents.
If you dissect a pound (0.5kg) of hydrated muscle, you’ll find that it is only around a third protein, with the rest being minerals and water. As a result, a pound of muscle only contains roughly 800 kCal.
In contrast, a pound of human body fat contains around 500 kCal. Muscle mass is therefore far more metabolically costly to synthesise and maintain than body fat, which requires little additional output.
And the benefits maintaining higher levels of muscle relative to your body fat percentage is not purely aesthetic. Higher levels of muscle mass mean you can eat higher calories at rest without gaining weight. The risk of diseases such as type 2 diabetes, cardiovascular disease and high blood pressure all decrease significantly.
One disadvantage of both BMI and body fat percentage is that it is incredibly hard to measure visceral fat. Some visceral fat is essential to protect our internal organs from blunt trauma. However, in high amounts, your risk of health issues such as insulin resistance, heart disease, liver problems and chronic inflammation all increase.
If more than 10% of your total fat is held viscerally, your risk of type II diabetes, Alzheimer’s, heart disease and colorectal cancer rise dramatically.
However, the good news is that the body taps into this visceral fat first when you implement a calorie-controlled diet and exercise.
Muscle gain and fat loss occur at very different rates so it would be very difficult to change your body composition significantly without the number on the scale also decreasing. In nearly all cases, lowering your body fat percentage requires weight loss.
The great news is that the science behind weight loss is very simple; you need to create a calorie deficit or a negative energy balance. That is, you need to eat fewer calories than you burn through your daily activity. The most effective way to do this is through a combination of reduced food intake and increased physical activity.
However, without putting some other conditions in place, you may lose muscle tissue before your body starts tapping into fat. This is because muscle is more costly to the body in energy terms.
There are two ways we can encourage the body to tap into fat stores, not muscle, during a period of sustained calorie deficit. The first is resistance training, which signals to the body to retain and sometimes build lean tissue. The second is eating a high protein diet, which is scaled based on your lean body mass.
For most men, 8-10% body fat is where they can expect to see defined abdominals. In most cases, it’s likely that your body fat percentage simply isn’t low enough yet to reveal the musculature underneath. However, body fat percentages will look different on each individual so you may need to get leaner for a full six-pack to appear.
You may also need to increase the size of the abdominal muscles through direct, targeted ab training. But it is worth bearing in mind that this will not give you abs in and of themselves; dieting to a low level of body fat is nearly always necessary.
If your stomach often feels doughy or solid despite even though your body fat level is low, it could be a sign of physical stress. In some instances, stress can increase fluid retention around the belly button area.
As long as you are of a healthy body composition, this is generally not unhealthy, but your fat loss efforts may feel as though they are stalling. Ensuring you get enough high-quality sleep and manage stress are two key things you can do to improve the appearance of this area.
Yes, body fat can affect testosterone both negatively and positively. When men diet to very low levels of body fat (anything under 10% body fat), the body may start to shut down non-essential functions like reproduction.
Men will often experience low sex drive, increased muscle weakness and fatigue towards the very tail end of dieting to low levels of body fat.
However, far and away the biggest risk factor for low testosterone is having an unhealthy body composition. Being overweight or obese has consistently shown to increase the risk of hormonal dysfunctions.
Several studies have reported decreases in sperm counts in Western countries over the last two decades, fuelled by environmental and lifestyle factors that result in reduced sexual function. The severity of the situation is such that experts have even suggested there is a male fertility crisis.
Sleep, stress, and body composition play a major role in testosterone production and are particularly powerful triggers for hypogonadism or low testosterone.
Research indicates that improving body composition through fat loss has a noticeably positive impact on obesity-related low testosteronevii,viii. One study monitored T levels in male participants and found that the prevalence of low testosterone halved after losing 10% and more of their total body weight through diet and exercise.
Changes in hormonal levels due to being overweight or obese can also trigger shifts in how your body stores fat. More fat in the limbs, hips and breast area are all signs of high oestrogen levels in men. In nearly all cases, this can be reversed through diet and exercise.
No and, in effect, you wouldn’t want it to be. Fat is a critical organ in the human body – without it, you would quickly die.
Among other functions, subcutaneous fat sends vital messages about how much stored energy we have. This information helps to inform our hunger signalling. The more fat you have, the stronger the ‘fullness’ signal your body fat emits, up to a point. If you are incredibly overweight, it is also possible to ‘break’ this signal, resulting in overeating.
Visceral fat often gets a bad rap, yet we also need it in small amounts to provide cushioning to our internal organs. Without it, even relatively low impacts could cause internal bleeding and trauma to our internal organs.
We also need both brown and beige fat as this has a role in regulating energy balance and our internal body temperature. There is also some evidence that cold exposure may increase brown fat in the body, which may help us to burn more calories without changing our food intake or activity.
One study asked healthy mean to undergo a month of exposure to mild cold for at least 10 hours each night. All their food was provided and controlled for calorie and macronutrient content. At the end of the month, participants’ brown fat had increased by 42% and there was a 10% increase in fat metabolic activity and improved insulin sensitivity.
Interestingly, after the participants returned to their normal lives, all these measures reversed. The findings indicate that humans may adapt to colder temperatures by increasing brown fat, which may improve our ability to process carbohydrates.
