Five common beliefs about losing fat all leave out the same thing, and it is muscle

Your genes set the starting point, and they do not set the finish

Weight really does run in families. Researchers in Cambridge pulled together 88 twin studies covering 140,525 twins, plus 27 family studies with 42,968 family members.¹ In the twin studies, the heritability of body mass index ranged from 0.47 to 0.90, with a middle value of 0.75. Heritability is the share of the differences between people in a group that lines up with differences in their genes, and body mass index is weight divided by height squared. Family studies came in lower, at 0.46. Heritability describes a population. It does not say how much of one person’s weight is fixed.

The more useful question is whether people with the less helpful genes can still change their result. A team pooled the individual data of 9,563 adults with overweight or obesity from eight randomised trials of diet, exercise or medication, and compared people by a variant of a gene called FTO that is linked to a higher risk of obesity.² Carriers lost just as much body mass index, weight and waist as everyone else, whatever the programme, its length, or the participants’ sex and ethnicity. The authors concluded that the genetic risk can be at least partly counteracted. It is one gene among many, so it does not speak for every gene.

Eating less and running more take some muscle along with the fat

When you eat less than you use, the body draws on fat and also on lean mass, which is everything that is not fat: muscle, organs, bone and water. How much lean mass goes is covered in our sister article on five truths about losing fat. The question here is which kind of exercise changes it.

A trial run in Houston and Albuquerque put 160 sedentary adults aged 65 to 85, all with obesity, on a weight-loss diet for six months, with three supervised exercise sessions a week.³ One group did aerobic training, one did strength training, one did both, and a control group did neither. Every exercise group lost about 9% of its body weight. The aerobic group lost 5% of its lean mass, from 55.0 to 52.3 kg. The strength group lost 2%, from 58.1 to 57.1 kg, and the combined group 3%. Strength rose 19% in the strength group and 4% in the aerobic group. Lean mass is a wider measure than muscle, and this was one age group.

Cardio still earns its place. At Duke University, 119 sedentary, middle-aged adults with overweight or obesity trained for eight months.⁴ Aerobic training took off more weight and more fat than strength training alone, while strength training, alone or combined, added more lean mass. A review of the research on muscle during weight loss reached the same balance: both kinds of exercise help preserve muscle while you lose weight, and strength training also builds strength.⁵

Muscle that goes is also slow to come back. A team in North Carolina followed 78 postmenopausal women for a year after a weight-loss trial.⁶ For every 1 kg of fat they lost, they lost 0.26 kg of lean tissue. For every 1 kg of fat that came back over the next year, only 0.12 kg of lean tissue came with it. Not every woman regained weight, and this was a follow-up of what happened, so it does not test a fix.

Your metabolism holds steadier than most people think, and muscle sets part of its pace

A large pooled study of daily energy use, in people from 8 days to 95 years old, found that once fat-free mass is allowed for, it holds steady from 20 to 60.⁷ Fat-free mass is everything in the body except fat. The sister article goes into that study in full.

So what does muscle itself burn? A team at Columbia University measured resting energy use in 131 healthy adults and the size of each organ by MRI scan, then worked out what each tissue uses per kilogram.⁸ Skeletal muscle came out at about 13 kcal per kilogram a day at rest, and fat at about 4.5. The liver uses about 200, the brain 240, and the heart and kidneys 440. In the group over 50 the figures ran about 3% lower, with muscle at 12.6.

A kilogram of muscle burns about three times what a kilogram of fat does at rest, and the numbers are modest. Losing 2 kg of muscle trims around 26 kcal a day at rest. The bigger loss is what muscle does for you every day: the strength to carry the shopping, train, play and keep doing the things you love.

A clean plate still needs protein on it

Olive oil has 884 kcal in 100 g and no protein. One tablespoon weighs 13.5 g, which comes to about 119 kcal.⁹ Romaine lettuce has 17 kcal and 1.2 g of protein in 100 g, so two cups of it, shredded, come to about 16 kcal. Three tablespoons of dressing on that bowl add about 358 kcal, and the bowl still has almost nothing in it for muscle to build with. Olive oil is good food. The point is that a salad’s energy can sit in the dressing while the protein is missing.

The word healthy also changes how much we eat. Researchers at the University of Toronto gave 99 female students the same oatmeal-raisin cookies, described either as healthy or as unhealthy.¹⁰ They ate about 35% more when the cookies were described as healthy. It was one snack, in one sitting, so it does not show what happens to weight over months.

Protein is what changes the result when you eat less. In a four-week trial, 40 young men ate about 40% less energy than they needed and did strength training and hard interval sessions six days a week.¹¹ Half ate 2.4 g of protein per kilogram of body weight a day and half ate 1.2 g. The higher-protein group gained 1.2 kg of lean mass and lost 4.8 kg of fat. The other group gained 0.1 kg of lean mass and lost 3.5 kg of fat. It was short, in young men, with far more training than most of us do, so the size of the result will not carry over, though the direction is useful. Across 49 trials and 1,863 people, adding protein to at least six weeks of strength training also added to gains in fat-free mass and strength.¹²

Strength training and protein on every plate are what the five myths leave out

Fat comes off when you eat a little less than you use. Strength training and protein help decide how much of what you lose is fat and how much is muscle. The World Health Organization suggests muscle-strengthening activity for all the major muscle groups on two or more days a week.¹³ Squats to a chair, step-ups and push-ups against a wall all count, and so does time with weights.

On the plate, protein leads and the rest still counts. Vegetables bring the fibre, and carbs and fat refuel what the training used and help you recover. The fat comes off, and far more of the muscle stays.

Here is how to lose fat and keep more muscle

Your genes set a starting point, and carriers of a common gene variant linked to obesity still lost as much weight as everyone else. So how to lose fat comes down to eating a little less than you use, strength work on two or more days a week, and protein on every plate. Eighty Thirty works out your daily protein and keeps count for you, so every plate helps you keep your muscle while you lose fat. Start with our free macro calculator.