You’ve probably heard of the basic weight loss rule: calories in vs. calories out. It’s quite simple, really. If you eat more than you burn, you gain weight. If you burn more than you eat, you lose weight.

However, many of the fad diets you see popping up online, only to eventually lose traction, seem to forget this fundamental principle. Some diets capitalise on human physiology to help you feel fuller for longer, but for any weight loss plan to work, you absolutely need to be eating fewer calories than you’re burning. There’s really no way around it.

With that being said, you might know from experience that weight loss doesn’t feel as simple as that. Factors like metabolism, hormones, and muscle-building add a level of complexity to it, and make it difficult to stick to your weight loss programme.

Let’s dive in and discuss this further!

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Back to the article!

Why Weight Loss Slows Down Over Time

Have you noticed that it’s easier to lose weight earlier on in your diet?

Every cell in our body requires energy to stay alive and carry out biochemical processes. This means that the more of you there is, and the more weight you’re carrying around with you, the more energy you will burn. Consequently, at the beginning of the weight loss process, you’re actually burning quite a bit of energy.

What nobody seems to tell you is that, as you lose weight, you burn less energy at rest. You might know this anecdotally, but to maintain your weight loss achievements, you actually need to eat less than you did to maintain your higher weight.

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Building Muscle Can Help

Now, this is where it gets interesting. Muscle (and other lean mass tissues) actually requires more energy to maintain than adipose tissue (fat) per unit mass. This means that if you build muscle while losing weight, you could preserve your metabolism. Resistance training helps to preserve lean mass during caloric restriction, resulting in a slower decline in your metabolic rate (King, 2021).

Many of us think aerobic exercise is the way to go for weight loss. After all, we know that we burn fat for energy during endurance exercise. That’s correct (King, 2021), but once you stop exercising, you almost immediately stop burning more energy. On the other hand, if you engage in resistance and strength training to build more muscle, you can burn more energy at rest. This means you can keep burning more calories even while you sit and read a good book.

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The Twist: Adaptive Thermogenesis

OK, great – build muscle to burn more calories. But there’s a catch. Your body doesn’t just burn calories in a vacuum.

The calories you burn on a daily basis will vary, regardless of your physical activity level and body composition (how much fat vs muscle you have). Your calorie intake and energy needs actually dictate, in part, how much you burn. This is known as adaptive thermogenesis, and it defends our body against weight loss.

When we eat less than we’re burning, our body produces less leptin and PYY (fullness hormones) and more ghrelin (the hunger hormone; King et al., 2011), increasing feelings of hunger and prompting us to eat more.

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Why Does This Happen? Is Our Body Fighting Against Us?

Not at all! It’s actually trying to keep us alive.

Evolutionarily, our bodies evolved over time to store energy because, for millennia, food wasn’t as readily available as it is now. Humans had to forage and hunt for their next meal, and once they were able to eat, they stored all the energy they could manage to use it until they found their next meal (Heitmann et al., 2012).

This is also why we’re drawn to high-calorie foods. The survival instinct within us draws us toward foods that will give us more energy in anticipation of periods of starvation (Heitmann et al., 2012).

While this survival mechanism was crucial in the past, it’s no longer as beneficial to us today – particularly when we’re trying to achieve that elusive bikini body before summer rolls around. But it’s what kept the human race alive and thriving for so long.

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Could Doing a Lot of Exercise Help with This?

You might be thinking that the solution is to start doing a lot more exercise. Surely, a combination of resistance and aerobic training can offer the best weight loss results: burn a lot of fat on a long run and burn more energy while resting after a gym session.

Up until recently, that was the prevailing theory. However practically, it’s actually really tough to burn enough energy through exercise to create a calorie deficit sufficient to lose weight. Aside from that, in the last 10-15 years, an anthropologist called Herman Pontzer suggested there may be an upper limit to the amount of energy we can burn daily.

Pontzer studied the Hadza hunter-gatherer tribe in Tanzania and found that, despite their higher activity levels compared to people in industrialised populations, the total daily energy expenditure (TDEE) of both groups was relatively similar (Pontzer et al., 2015).

He therefore proposed the theory of constrained TDEE, suggesting that we might compensate in some way for the increase in energy expenditure through exercise, rather than simply add the energy burnt to what we already burn at rest (see diagram below). He proposed two ways this might occur:

1. We could engage in behavioural habits, such as moving less during daily activities, to conserve energy.
2. Our basal metabolic rate (BMR) might decrease i.e. the amount of energy you burn at rest.

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But How Can BMR Decrease? Isn’t It the Minimum Energy Needed for Normal Bodily Function?

Yes, it is. The answer is that the body doesn’t function at its best when this happens!

Your body does indeed have a minimum energy requirement needed to function healthily and normally. If this is not met, your body might slow down your metabolism or reduce the energy it spends on processes it deems to be non-essential processes.

This can actually be quite dangerous. The body may prioritise resources for essential organs and take them away from growth, repair, immune function, and reproduction.

This, in part, is why mental health conditions like anorexia nervosa can prove fatal. But even on the less extreme end of the spectrum, female athletes who exercise excessively may become amenorrheic. This means that because they exercise far beyond the level necessary for health, and consume less energy than their body needs to keep up with this level of activity, they temporarily lose their menstrual period and reproductive ability. The body’s ‘reasoning’ is that if it doesn’t have enough energy for normal, healthy function, then it certainly doesn’t have any extra energy to nourish and sustain a new life. (This is all discussed in the ACSM’s position statement on the female athlete triad.)

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But It’s Not All Doom and Gloom

All this means is that there is a limit to how much of an energy deficit you can maintain on a given day. So, it’s worth making sure you’re consuming enough energy, even if you’re trying to lose weight. There’s no benefit to starving yourself – you won’t lose more weight, and it could harm you in the long run.

Instead, try to stick to a healthy, balanced diet. The NHS has plenty of tips for this, which you can find here. You might be familiar with the 1,200 kcal and 1,500 kcal minimum energy requirements for women and men, respectively. However, this is really at the lower end of the spectrum and most people can lose weight consuming more than this.

The daily recommended intake for body weight maintenance is 2,000 kcal for women and 2,500 kcal for men, and the NHS recommends a 600 kcal deficit for weight loss, putting energy intake at 1,400 kcal for women and 1,900 kcal for men.

All this to say that there doesn’t seem to be a clear-cut answer – different people have different energy requirements (e.g. tall people need more energy than shorter people). But the important thing is to have a well-rounded diet, hitting all the major food groups and targeting micronutrients too.

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Why Bother Exercising at All?

At this point, you might be wondering whether it’s even worth exercising for weight loss. If it’s so difficult to burn enough energy, why bother? Isn’t dieting alone an easier approach?

The drawback to dieting is that you tend to feel hungry almost immediately, making it very difficult to stick to your plan. On the other hand, exercise that elicits a similar energy deficit doesn’t seem to result in the same increase in hunger hormones and decrease in fullness hormones as dieting (King et al., 2011). So, it might actually complement a modest calorie-restrictive diet well, helping you avoid the hunger pangs you might expect from major calorie restriction.

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Takeaway Message

I can appreciate that this information might not be easy to distil down into actionable steps, so let me try to do that here. Please keep in mind that I’m not a doctor or a medical professional, so I recommend consulting with one before starting a diet or changing your exercise regimen.

However, if it were me, here’s the strategy I would adopt:

• Create a 600 kcal deficit through dietary changes (NHS)
• Eat 2 servings of vegetables at every meal to promote fullness (NHS)
• Aim for high-protein meals to prevent lean mass loss (at least 20g per meal, but ideally more; the protein intake recommendation to build muscle is 1.2–1.7 grams per kilogram of body weight; ACSM, 2015)
• Eat wholegrain carbohydrates (NHS)
• Eat low-calorie dairy (NHS)
• Drink 6–8 glasses of water a day
• Do at least 2 strength training sessions per week
• Do 225–420 minutes of aerobic physical activity per week (Donnelly et al., 2009)

That’s it. While it may seem like a lot, gradually making small changes can help you develop lasting habits. Start by creating a modest calorie deficit, adding more vegetables and protein to your meals, and incorporating strength training twice a week. Then gradually build from there.

I believe one of the worst mentalities to have towards weight loss is thinking it’s only for a short period of time and that, once you reach your goal, you can go back to living and eating the way you were before.

Oftentimes, this results in us putting the weight back on, making our efforts to lose it in the first place feel pointless. The best strategy is to try and change your lifestyle and turn it into a habit. Once you’ve built healthy habits, it becomes easier to maintain a healthy body!

References

American College of Sports Medicine. (2007). The Female Athlete Triad. Medicine & Science in Sports & Exercise, 39(10), 1867–1882. https://doi.org/10.1249/mss.0b013e318149f111

American College of Sports Medicine. (2015). Protein Intake for Optimal Muscle Maintenance. In American College of Sports Medicine. American College of Sports Medicine. https://www.acsm.org/docs/default-source/files-for-resource-library/protein-intake-for-optimal-muscle-maintenance.pdf

Donnelly, J. E., Blair, S. N., Jakicic, J. M., Manore, M. M., Rankin, J. W., & Smith, B. K. (2009). Appropriate Physical Activity Intervention Strategies for Weight Loss and Prevention of Weight Regain for Adults. Medicine & Science in Sports & Exercise, 41(2), 459–471. https://doi.org/10.1249/mss.0b013e3181949333

Heitmann, B. L., Westerterp, K. R., Loos, R. J. F., Sørensen, T. I. A., O’Dea, K., McLean, P., Jensen, T. K., Eisenmann, J., Speakman, J. R., Simpson, S. J., Reed, D. R., & Westerterp-Plantenga, M. S. (2012). Obesity: lessons from evolution and the environment. Obesity Reviews, 13(10), 910–922. https://doi.org/10.1111/j.1467-789x.2012.01007.x

King, J. (2021). Obesity. In D. J. Stensel, A. E. Hardman, & J. M. R. Gill (Eds.), Physical Activity and Health. Routledge. https://doi.org/10.4324/9780203095270

King, J. A., Wasse, L. K., Ewens, J., Crystallis, K., Emmanuel, J., Batterham, R. L., & Stensel, D. J. (2011). Differential Acylated Ghrelin, Peptide YY3–36, Appetite, and Food Intake Responses to Equivalent Energy Deficits Created by Exercise and Food Restriction. The Journal of Clinical Endocrinology & Metabolism, 96(4), 1114–1121. https://doi.org/10.1210/jc.2010-2735

Pontzer, H., Raichlen, D. A., Wood, B. M., Emery Thompson, M., Racette, S. B., Mabulla, A. Z. P., & Marlowe, F. W. (2015). Energy expenditure and activity among Hadza hunter-gatherers. American Journal of Human Biology, 27(5), 628–637. https://doi.org/10.1002/ajhb.22711

Pontzer, H., Raichlen, D. A., Wood, B. M., Mabulla, A. Z. P., Racette, S. B., & Marlowe, F. W. (2012). Hunter-Gatherer Energetics and Human Obesity. PLoS ONE, 7(7), e40503. https://doi.org/10.1371/journal.pone.0040503