How to get a six-pack using science

My partner, who is a bit of a fitness buff, is constantly poking fun at all the magazines and books advertising stuff like "The Best Abs Diet." His reaction is always, "I hope that diet is 'No food.'" His point is that the only way a food-based approach works toward a six-pack is by calorie restriction to create a deficit — thus eliminating the fat layer that sits on top of your abdominal muscles (which exist in everyone to highly varying degrees).

In addition to diets, I got to questioning the common wisdom around exercises for fat loss. Gym wisdom says cardio is the best way since that's how you burn the most calories (and a calorie deficit is needed to lose weight). We indeed see fat loss in people who eat hypocaloric diets and run/bike/swim a ton (we're talking 3–6 miles of treadmill running or equivalent a day — and no donuts or pizza at the end to celebrate!). You also see a plateau — you don't keep losing fat/weight forever.

It starts with a hypocaloric diet

This might be obvious to many readers, but to make it clear, fat loss requires one to take in fewer calories than one needs to metabolize for energy expenditure.

Calories In < Calories Out → six-pack! (Kinda, sort of.) Let's look at each part of that equation to break this down practically for people.

Calories IN

This is easy for most people to understand. We only get calories in through eating and drinking. What I'll add here is that most people waaaaaaaay underestimate how many calories they consume. (You will not understand your calorie intake until you get fanatical about tracking every bite.) For example, it is regrettable for me to report that a slice of pizza is 254 calories (or roughly 2.5 miles/30 minutes of running ugh!). I won't go into how low-quality the nutrients are in most pizzas — that's another detail for later.

A Major Caveat

A calorie is NOT a calorie. I like the endocrinologist and thought leader Dr. Lustig's explanation of the topic. The major takeaways:

• High-fiber foods result in fewer calories actually being metabolized (or absorbed) by your body. For example, 160 calories of almonds equates to about 130 calories actually being turned into energy and material (like fat) in your body. This is also how celery ends up being close to net zero calories.
• High-protein foods take at least 2x the energy to metabolize compared to carbohydrates. 100 calories from protein becomes about 70, but 100 calories from carbohydrates is still about 90 calories after metabolism. Not to mention, protein makes you feel full much more than carbs.
• Fats are poorly named and understood. Some fats are necessary for promoting life — omega-3 fats, for example (fish, nuts, avocados, olive oil, and eggs). Others will stack up in your arteries and kill you (trans fats in processed foods).
• Added sugar, particularly fructose, is terrible for you. This might confuse some people, as fruits have fructose and fruits are good for you, but the unique thing about fruit is that the fructose comes with fiber, which protects your body by slowing down the fructose absorption (and leaving a lot for gut bacteria to eat instead).
• Don't forget alcohol. Here is a quick "duh" note for people who forgot that their daily beer can add up to 300 calories (!!!). There's a ton of variation in calories for alcoholic drinks, but all the calories are "empty" or non-nutritious, so make sure the drink is worth it.

More about fructose, real quick

I'll probably write a post about fructose soon, but it's such an important topic that I'll note a few things here.

• Fructose is a survival nutrient in periods of near starvation. It triggers animals (including humans) to store energy and water and become insulin-resistant to preserve sugar so the body and brain have enough fuel in a famine. The downstream effects lead to diabetes, high blood pressure, and increased adiposity (fat).
• Fructose is the preferred fuel of many cancer cells, as it helps cells survive in low oxygen states (similar concept of starvation mode mechanisms — in this case, it reduces mitochondrial function via the Warburg effect and stimulates glycolysis).
• Fructose increases hunger by hijacking neuronal and hormonal pathways. Fructose increases ghrelin (the appetite-stimulating hormone) disproportionately compared to glucose. It also affects the endocannabinoid signaling pathway to reward food intake.
• Fructose decreases serotonin by decreasing serotonin reuptake transporter (SERT) protein levels in the gut, and SERT-deficient mice are used as models for depression.
• Fructose makes you stupider. No, really, studies show fructose can lead to brain insulin resistance and thus diminished cognitive function.

Calories OUT

So the summary of all the above is, calories in just means food and drinks, and not all calories are the same. Finally, let's talk about how to burn off those calories!

Baseline Metabolic Rate (BMR)

This is what our body burns over a day with minimal movement (note that this is not exactly the same as resting metabolic rate, RMR, which accounts for no movement; however, most people use the two interchangeably).

Roughly speaking, the activities contributing to BMR include:

• Breathing
• Heart beating and circulation
• Metabolism of food
• Cell function, growth, and repair

To calculate your BMR or RMR, use the commonly accepted Mifflin-St. Jeor equation:

Males: 10 × weight (in kilograms) + 6.25 × height (in centimeters) — 5 × age (in years) + 5

Females: 10 × weight (in kilograms) + 6.25 × height (in centimeters) — 5 × age (in years) — 161

Thermic Effect of Food (TEF)

We discussed this in the "Calories IN" section, but this is essentially the metabolic energy required to digest your food. Importantly:

• Fats are 9 calories/gram with a TEF of 0–3% (so you "keep" all 9 calories, basically)
• Carbs are 4 calories/gram with a TEF of 5–10% (so you keep ~3.75 of the 4 calories)
• Proteins are 4 calories/gram with an impressive TEF of 20–30% (so you only keep ~3 out of 4 calories)
• Alcohol is 7 calories/gram with a TEF of 22.5%, making it medium "bad."

Exercise Activity Thermogenesis (EAT)

Ironically, EAT is the calories you burn during exercise. More on this later.

Non-Exercise Activity Thermogenesis (NEAT)

This is a bucket of calories from activities outside workouts. This includes everything from walking to your bathroom to fidgeting.

Now let's talk about exercise

There are a few myths about what exercises are needed for a six-pack/to lose fat. Mainly, the myths follow the same pattern as the diet myths, which is that there is a belief that there is "one" perfect exercise or diet for the goal of fat loss. Let's tackle each one.

Cardio isn't enough

You heard that aerobic exercise, or cardio, burned the most calories in the shortest time. So you start training super hard, running a mile daily, then three. You're pleasantly surprised by the five pounds that slide off in the first month. But, soon, you're running ragged, and your knees kind of hurt, plus you notice that you aren't losing any more weight. Not to mention, you've been starving yourself at a caloric deficit for months and are hangrily yelling at children in the street.

Sound familiar?

This is a natural adaptive process due to homeostasis (your body's evolved mechanism to ensure everything is balanced and that you don't die).

Cardio is truly the most time-efficient way to burn calories. However, this is just step one in the arms race with your biology.

Your body, detecting that you are apparently being chased by lions on a daily basis, compensates by slowing down your metabolism. This means you have to run faster and longer to burn the same number of calories, making running more and more inefficient for weight loss.

So what do you do?

Guns out!

It turns out that while resistance training does not burn many calories during the exercise, it is vital to any fat-loss program. It is vital to any weight loss program. I tell this to all my patients who are on the fancy new medications for weight loss (GLP-1 agonists like Wegovy and Ozempic) that they must start strength training for a couple of reasons.

First of all, muscle burns more calories at rest than fat does. Secondly, when you're losing weight (via medications, hypocaloric diet, cardio training, etc.), your body will lose weight from both fat and muscle stores. Muscle mass needs training to be maintained! Or else you'll run into weight loss plateaus and grow weak!

Final word — an RCT

There was a randomized controlled study on this topic! This Spanish study did proper randomization and paid close attention to how measurements were conducted. They used DXA scans to determine fat mass and checked cardiovascular fitness with a maximal stress test to get VO2 max measurements.

The study groups were strength training (S), endurance training (E), strength and endurance training (SE), and control (C — these folks were told to adhere to physical activity recommendations generally). All groups were put on personally determined hypocaloric (25% less) diets, and the study ran for 22 weeks.

It'll probably be unsurprising to you that the winning group was the SE group. But what surprised me was that the SE group also ate the most calories (although still less than at baseline). The group ate 1848 calories daily (averaged across both male and female subjects) and lost 8.5 kg (18.74 lb).

The other groups also lost weight (including the control group, which lost 6.1 kg). However, the fat mass decrease was most significant in the SE group (-6.74%), and all experimental groups lost more than the control group (which still lost 3.94% but with a wide standard deviation).

It's a great study that I could discuss here for longer, but the key takeaway is that strength training, endurance (cardio) training, and a hypocaloric diet is the best way to lose fat mass.

"A combination of aerobic and resistance exercises, potentially supplemented with HIIT, along with a balanced, hypocaloric diet, appears to be the most effective strategy for fat loss (and gain a six-pack)."

So hit the gym!

Cheers to your health,

Hillary Lin, MD