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.
Women naturally carry more body fat than women due to differences in their hormonal make-up and their natural biological sex differences.
Simply put, women need a certain level of body fat to fulfil reproductive functions such as pregnancy and childrearing.
Oestrogen, the primary female sex hormone, is predominantly responsible for the typical female storage fat pattern around the back of the upper arm and shoulders, breasts, pubic area, hips and thighs.
The great news is that this fat storage pattern also means that women tend to be healthier and at reduced risk of serious diseases such as type 2 diabetes compared to their male counterparts. This is because they are less likely to store visceral fat around vital internal organs, which significantly increases the risk of death from all causes.
In a word, yes, but there may be several caveats to this. For women sitting anywhere between 12-15% body fat, which may be deemed low for many, this likely shouldn’t cause disruptions to the menstrual cycle. However, consistently dieting on or eating very low calories or performing high amounts of high-intensity cardio could cause stress to the body, which may make hormonal disruptions more likely.
If you are a woman aiming for as low as 10% body fat, it is important to be aware that, because you are starting to tap into essential fat for life, your menstrual cycle is very likely to become disrupted, known as amenorrhea. In order to reverse this change, you may need to gain some body fat, but this does not mean that you need to gain unhealthy amounts.
This will depend entirely on the woman, how many calories she is eating, her natural hormonal profile, the type of training she performs and so on.
However, for the vast majority of women it is generally not recommended to go lower than 10-12% body fat for any extended period.
