Hydration, Electrolytes & Gut Health

This content is for informational purposes only and does not constitute medical or nutritional advice. Speak with your health professional before starting this protocol.

Electrolyte caution: If you take medications that affect potassium levels — including ACE inhibitors, ARBs, or potassium-sparing diuretics — do not supplement potassium without speaking to your health professional first. Hyperkalemia (excess potassium) is a serious cardiac risk. This applies regardless of the protocol's general potassium recommendations.

This page covers hydration targets, electrolyte management, and digestive changes during the sprint — combined reference for water, sodium, potassium, magnesium, and gut health.

Daily Water Targets

The FLS target is higher than maintenance because the low-carb diet depletes glycogen, and glycogen binds approximately 3 g of water per gram stored. As glycogen is depleted in the first days of the sprint, a significant volume of water is lost with it — water that needs replacing through direct intake to support kidney function, electrolyte balance, and metabolic processes.

How the app tracks this: Beverages you log on the Nutrition page count toward your Water habit. If you log a coffee, water, or broth, it auto-updates the Water card so you're not double-entering.

A practical structure for 3 litres:

• 500 ml on waking (before coffee)
• 750 ml mid-morning
• 750 ml mid-afternoon
• 500 ml with dinner
• 500 ml in the evening

Drinking water before hunger peaks (mid-morning and mid-afternoon) also serves as a first-line hunger management tactic — thirst and hunger signals overlap significantly.

Why Electrolytes Are Essential on a Low-Carb Sprint

The most important thing to understand about hydration during your sprint is that water alone is insufficient. Without electrolytes — specifically sodium, potassium, and magnesium — increased water intake can actually worsen symptoms.

Here's why:

The low-carb → electrolyte depletion cascade:

1. Carbohydrate restriction drops insulin levels
2. Insulin normally signals the kidneys to retain sodium
3. Without that signal, the kidneys excrete sodium rapidly — taking potassium and water with it
4. Glycogen depletion releases bound water, accelerating fluid loss
5. The result is a significant net loss of sodium, potassium, and magnesium — independent of how much water you drink

This is the mechanism behind "keto flu" — the headaches, fatigue, dizziness, and muscle cramps that hit in the first sprint week. These symptoms are not caused by calorie restriction. They are almost entirely caused by electrolyte depletion. And they resolve within hours of adequate electrolyte replacement.

Electrolyte Targets and Sources

Sodium

Target: 2,000–3,000 mg of additional sodium per day, on top of what's in your food.

Sodium is the most urgent electrolyte during the sprint. It is also the one most people under-consume because conventional dietary advice warns against it. During a low-carb sprint, that advice does not apply — the kidneys are actively excreting sodium at an accelerated rate.

Practical sources:

• Bouillon or broth: 500–900 mg sodium per cup — the most effective delivery method because it is warm, filling, and volume-adds to satiety
• Table salt: approximately 2,300 mg per teaspoon — add liberally to meals
• Salt on raw vegetables: particularly effective as a snack when hunger is high

If you have a headache, feel dizzy, or your muscles are cramping in the first week — increase sodium first. This resolves the symptoms in most cases within 2 hours.

Potassium

Target: 1,000–3,000 mg per day total (food plus supplement combined).

Potassium is the most critical electrolyte for cardiac safety during a severe deficit. Inadequate potassium alongside increased sodium excretion was the mechanism behind the cardiac complications seen in 1970s unsupervised liquid protein diets. The FLS protocol's intake requirements exist specifically to prevent this.

Food first. Most users hit the target from greens plus a salt substitute on meals. OTC potassium capsules cap at ~99 mg per tablet (FDA rule), so pills are a top-up — not the main vehicle.

Practical sources (in order of priority):

• Cooked spinach (1 cup): ~840 mg — two cups already lands you in range
• Avocado (½ avocado): ~485 mg
• Mushrooms (1 cup): ~356 mg
• Potato on refeed days (1 medium): ~610 mg
• Salt substitute on meals — LoSalt, NoSalt, Nu-Salt, lite-salt: ~650 mg per ¼ teaspoon. Easiest practical "supplement"
• Potassium citrate or gluconate capsules — only as a top-up if food + salt sub fall short

Magnesium

Target: 200–400 mg of elemental magnesium per day.

Magnesium participates in over 300 enzymatic reactions. During caloric restriction, magnesium deficiency causes muscle cramps, sleep disruption, anxiety, and in severe cases, cardiac arrhythmias. It is also a co-factor in melatonin synthesis, which is why the protocol recommends taking it before bed.

Recommended forms:

• Magnesium glycinate (best absorbed, minimal GI side effects)
• Magnesium citrate (well absorbed, may cause loose stools at higher doses — useful if constipation is an issue)
• Avoid magnesium oxide — poorly absorbed

A practical combination: Mix table salt, a measured amount of potassium chloride, and your daily magnesium into a litre of water with some broth. Drink over the day. Addresses all three electrolytes in a single vessel.

Digestion During the Sprint

Digestive changes in the first 1–2 weeks of an FLS sprint are common and expected. They are a side effect of the dietary shift, not a sign that something is wrong.

What Changes and Why

Lower fibre intake. The sprint diet removes grains, most legumes, and most fruit — the dominant fibre sources in a typical diet. Even with unlimited vegetables, many people enter the sprint with significantly reduced fibre compared to normal. Lower fibre = slower transit = constipation risk.

Less water bound to food. Outside the sprint, much of your daily hydration comes from high-water-content foods (grains absorb water in cooking, fruit and vegetables release it). Carbohydrate restriction reduces this incidentally.

Protein load increase. A significant increase in protein — especially if the sprint represents a large shift from usual intake — requires digestive adaptation. Some people experience bloating or discomfort in the first week as gut microbiome composition and digestive enzyme production adjust.

Constipation: The Most Common Issue

If bowel movements slow significantly or stop in the first 1–2 weeks, the protocol's first-line response is:

1. Increase vegetables. Non-starchy vegetables are your fibre source. Aim for 6–8 cups of vegetables daily — raw or cooked. Prioritise broccoli, spinach, cauliflower, and asparagus for fibre density.

2. Psyllium husk. The FLS supplement protocol includes psyllium husk (5–10 g per day) as a recommended addition for exactly this reason. Psyllium is a soluble fibre that absorbs water, increases stool bulk, and promotes regularity. Take with at least 250 ml of water and drink immediately — it gels quickly. Take between meals rather than with protein meals, as it can mildly slow protein absorption.

3. Hydration. Fibre requires water to function. If you are hitting your vegetable targets but not your water targets, constipation will persist. The two work together.

4. Magnesium citrate. If you are choosing your magnesium form, magnesium citrate has a mild laxative effect at higher doses. Switching from glycinate to citrate during a period of constipation can help. Start at 200 mg and increase if needed.

Bloating

Bloating in the first week is commonly caused by:

• Increased vegetable volume (particularly raw cruciferous vegetables — broccoli, cauliflower, Brussels sprouts)
• Psyllium adaptation
• Protein digestion adjustment

What helps: switching raw cruciferous vegetables to cooked versions, spacing meals more evenly, and ensuring adequate water intake. The bloating typically resolves within 1–2 weeks as the gut adjusts.

When It's Not a Gut Problem

Persistent severe abdominal pain (not discomfort), blood in stool, or symptoms that escalate rather than improve over 2 weeks are outside the scope of dietary adjustment and warrant a conversation with a healthcare professional.

Hydration and Hunger

Thirst is frequently misread as hunger. The signals share the same hypothalamic pathway, and for many people — especially those who chronically under-drink — hunger arrives first as the body's general distress signal.

The practical implication: before reaching for food when hungry, drink a glass of water and wait 10–15 minutes. If the hunger passes, it was dehydration. If it doesn't, it was food hunger. This is a reliable daily experiment.

Related resources

• Supplementation
• Hunger Management
• What to Eat: FLS Food List