Extreme Diets for Rapid Weight Loss: How They Work, and Should You Try Them?

Choose your macro: Meat, Fat, or Sugar?

When it comes to rapid weight loss, there’s no shortage of extreme diet strategies, but only a few of them actually work by triggering real metabolic shifts. In this article, we’ll break down three unconventional and controversial approaches that have shown short-term results:

1️⃣ the Protein-Sparing Modified Fast (PSMF)

2️⃣ the Fat Fast

3️⃣ the Sugar Diet

These diets couldn’t be more different - one is all protein, one is nearly pure fat, and one is almost entirely sugar. Yet surprisingly, they all can lead to weight loss, thanks to distinct hormonal and enzymatic mechanisms.

But should you try them? And could alternating between them improve metabolic flexibility, or is that a bad idea? Let’s take a closer look.

First, What do we mean by

"distinct hormones & enzymatic mechanisms?" 🤔

To better understand how these diets cause fat loss in such different ways, it helps to look at three powerful metabolic regulators: FGF21, AMPK, and mTOR.

Metabolic regulators are hormones or enzymes that help control how your body uses energy - such as when to burn fat, store energy, or repair cells.

FGF21 (Fibroblast Growth Factor 21) – a Hormone

Produced by the liver, FGF21 is triggered during fasting, low-protein diets, and ketosis. It helps your body switch from burning sugar to burning fat, reduces sugar cravings, and plays a key role in long-term metabolic adaptation.

Example: On a fat fast or sugar-only diet, FGF21 levels spike, mimicking the body’s response to starvation and encouraging fat breakdown while sparing protein (temporarily).

AMPK (AMP-activated Protein Kinase) – an Enzyme

AMPK acts like a fuel gauge in your cells. When energy is low (from exercise, fasting, or nutrient depletion), AMPK tells your body to increase fat burning, reduce storage, and clean up damaged cells (autophagy).

Example: The sugar diet activates AMPK without needing ketosis, while a fat fast achieves the same effect with almost no glucose.

mTOR (Mechanistic Target of Rapamycin) – a Growth Regulator

mTOR is a signaling pathway that promotes cell growth, protein synthesis (protein building), and muscle maintenance. It’s activated when you eat enough protein and calories, and suppressed during fasting or nutrient deprivation.

Example: The  Protein-Sparing Modified Fast keeps mTOR active to preserve muscle, while fat and sugar fasts suppress it, shifting the body toward breakdown and conservation mode.

These distinct hormonal and enzymatic mechanisms help explain how very different diets can all result in short-term fat loss - though none of them are equally sustainable or healthy long-term.

THE DIETS:

Protein-Sparing Modified Fast (PSMF)

What It Is:

The PSMF is a higher-protein, very low-fat, low-carb diet used to preserve muscle mass while rapidly reducing body fat. It's used in clinical obesity treatment under supervision.

How It Works:

➡ Maintains muscle due to adequate protein intake (1.5–2.0 g/kg lean mass)

➡ Puts the body into a caloric deficit, often with mild ketosis

➡ Keeps the anabolic hormone mTOR active, preventing muscle breakdown

Why It’s Effective:

This diet is ideal for people who want maximum fat loss with minimal muscle loss. It also suppresses hunger for some, although energy levels can dip due to caloric restriction. It's often used 1-2 days a week.

Note: While PSMF stimulates mTOR (the muscle-building pathway), this happens mainly after protein intake. Between meals - especially during fasting intervals - mTOR activity naturally decreases.

Fat Fasting

What It Is:

Popularized in the low-carb/keto world, the fat fast is an extremely high-fat, very low-protein, zero-carb approach - often used for 2-3 days to kickstart or deepen ketosis.

How It Works:

➡ Pushes the body into deep ketosis

➡ Suppresses appetite via ketones

➡ Triggers the starvation-response hormone FGF21

➡ Activates AMPK, a catabolic enzyme that promotes fat burning and autophagy

➡ Suppresses mTOR, slowing growth and repair

Why It’s Effective:

Despite high fat intake, the body is “tricked” into a fasting-like state due to missing protein and carbs. This stimulates a powerful fat-burning response. However, doing this for too long can lead to muscle breakdown.

The Sugar Diet

What It Is:

This extreme and counterintuitive approach involves eating mostly pure sugar (glucose or sucrose) and avoiding both protein and fat. It sounds bizarre, but some people report short-term weight loss.

How It Works:

➡ The lack of protein triggers a massive surge in FGF21, a hormone that mimics the metabolic response to fasting

➡ AMPK is activated, promoting fat oxidation and autophagy

➡ mTOR is suppressed, halting muscle building and repair

➡ Initial muscle breakdown is delayed by FGF21’s protein-sparing effects - but only temporarily

➡ The body eventually begins to break down muscle to meet essential amino acid needs

➡ Food monotony and rapid digestion may lead to unintentional calorie restriction

Why It’s Effective (Short-Term Only):

The sugar diet activates powerful starvation-like pathways despite being high in calories. This leads to temporary fat loss, often with initial muscle preservation - but muscle loss risk increases significantly the longer the diet is maintained.

Comparison Table

Diet	Fat Loss Through	Muscle Loss Risk	Hormonal Signals
PSMF	calorie deficit high protein	very low	mTOR active, FGF21 low
Fat Fast	deep ketosis FGF21 + AMPK	moderate to high	FGF21 high, AMPK active
Sugar Diet	protein deficiency AMPK + FGF21	delayed but high:short-term sparing possible	FGF21 very high, AMPK

Can Rotating These Diets Improve Metabolic Flexibility?

In theory, yes - alternating diets that use different fuels (protein, fat, carbs) could “train” the body to become more metabolically flexible. This idea mimics ancestral survival strategies: feast, famine, scarcity, and abundance.

But in practice:

➡ Protein deficiency phases (sugar or fat fasts) should be very short (1–3 days max)

➡ Refeeds with adequate protein and micronutrients are essential

➡ Frequent cycling can stress the endocrine system, especially in women or those with thyroid/adrenal issues

So while it’s possible to experiment with short bursts, these diets are not sustainable long term. The best approach is to use them strategically - for plateaus, kickstarts, or metabolic resets - under professional guidance.

So When It Comes Down to It...

Extreme diets can teach us a lot about how the body burns fat and adapts to stress. They each activate different pathways like mTOR, AMPK, and FGF21 - and while all of them can cause weight loss, not all weight loss is healthy or sustainable.

➡ For fat loss with muscle preservation, PSMF is the safest.

➡ For metabolic breakthroughs or fasted-state mimicry, a brief fat fast may help.

➡ The sugar diet is a fascinating hormonal experiment - but maybe one best left to the lab, not the kitchen.

Want Help Finding the Right Strategy for You?

Everyone’s metabolism is different, and what works for one person may not be ideal for another. If you're curious about how to use these strategies safely or want a tailored plan for fat loss, hormone health, or metabolic repair, our team at Mason & Lanz is here to help. We specialize in nutrition therapy and lifestyle coaching grounded in science and personalized for real life. Reach out to explore what’s right for your body and your goals.