Eating patterns
Intermittent fasting and time-restricted eating
Fasting is the voluntary abstinence from solid food for a limited time, meeting your energy needs from your own reserves. You already fast every night. Time-restricted eating (TRE) and intermittent fasting are not diets but eating patterns: they change WHEN you eat rather than only what you eat.
The purpose of fasting
Blood sugar and metabolic flexibility
Your body first runs on dietary glucose, then on stored liver glycogen and fatty acids. Eating around the clock erodes this adaptive switch, called metabolic flexibility. Fasting helps restore healthy insulin regulation and supports weight loss.
Digestive rest and the microbiome
The digestive rest of TRE or intermittent fasting can promote microbial diversity and reduce inflammation, supporting digestive health.
Ageing and oxidative stress
During fasting the body increases its antioxidant defences, helping preserve cellular function and promote longevity.
Mood and relationship to food
The biggest hurdle for most people is mind over matter, and that eases over a few weeks. Research also points to benefits for stress, anxiety, and low mood.
Fasting patterns
Fasts range from twelve hours to several days. Shorter fasts are done more often, longer fasts (24 to 36 hours) less often. Prolonged fasting requires supervision.
| TRE window | Strengths | Who it suits |
|---|---|---|
| 12:12 | Overnight fasting to improve blood sugar and sleep. Benefits show more slowly. | Everybody |
| 16:8 | Fits most lifestyles, easy to manage, noticeable benefits. Can feel long if new to fasting. | Beginners to TRE |
| 20:4 | Suits a busy lifestyle with a condensed eating time. Requires a well-balanced diet. | Those experienced in TRE |
| IF pattern | How it works | Who it suits |
|---|---|---|
| OMAD / 24-hour | One meal a day. Easy to fit a busy schedule, but can drift into chronic calorie restriction and lower resting metabolic rate over time. | Best after some TRE first. Once or twice weekly. |
| 5:2 | Two non-consecutive days at 0 to 25% of normal calories, five days eating normally. No long period without food. | Those daunted by long fasts. |
Can you eat whatever you want? No.
As you shrink your eating window, pack as many nutrients onto your plate as possible from whole, unprocessed foods. Processed foods are empty calories with blood sugar spikes and crashes that leave you tired, foggy, and hungrier during the fast, making it harder to sustain.
Focus on: vegetables (especially leafy greens, broccoli, spinach); healthy fats such as extra virgin olive oil, moderate coconut oil, avocado, nuts and seeds; protein from fish, eggs, legumes, and moderate organic meat; wholegrains such as quinoa, oats, brown rice. Eat slowly, chew well, and keep hydration optimal. A Mediterranean-style plate works well, half non-starchy vegetables.
A sample 16:8 day (eating 10am to 6pm): 10am, make the first meal the largest, e.g. a bowl of lentils, kale, spinach, and poached eggs with pumpkin seeds. 2pm, a light salad of greens, tomatoes, olives, avocado, quinoa, and seeds. 6pm, a small portion of salmon with roasted vegetables. Outside the window, water and herbal tea.
The 5:2 approach
The 5:2 limits calories to 500 (or up to 800) on two non-consecutive days, with normal eating on the other five. Done well, you can eat a substantial amount for 500 calories. A sample 500-calorie day:
Breakfast
Herbal tea (no sugar; stevia is fine), only 1 tbsp milk or nut milk for the day, plus an apple, orange, handful of strawberries, or half a grapefruit.
Lunch
100g lean veal, beef, chicken breast, white fish, or prawns, grilled with visible fat removed. One vegetable from spinach, chard, green salad, tomatoes, celery, fennel, onion, cucumber, asparagus, cabbage (2 to 3 cups).
Snack
An apple, orange, handful of strawberries, or half a grapefruit, a different choice from breakfast.
Dinner
The same style as lunch, with a different protein and vegetable. If you have calories to spare, bulk up with extra veggies. Track with a tool like Cronometer if helpful.
What breaks your fast?
During the fasting window, stick to non-caloric beverages. Water is essential, and you will need extra to make up for hydration usually drawn from food, with a squeeze of lemon for taste and electrolytes. Black coffee without sugar or milk in moderation (one or two cups, with water) may benefit your fast.
Breaks the fast
Anything that triggers an insulin response: protein powder, multivitamin gummies, prebiotic fibres. Check excipients, maltodextrin and pectin contain sugar and calories. Stevia and monk fruit do not break a fast but can trigger hunger.
Generally fine
Electrolytes without sweeteners or calories. Fat-soluble vitamins and omega-3s are better absorbed with food. Zinc on an empty stomach can cause nausea, and chromium may drop blood glucose, so time these carefully and ask your practitioner.
Body signs when you start fasting
Always listen to your body. If you feel unwell, break your fast, there is always tomorrow.
- Light-headedness: first check you are not dehydrated. If it relates to lost metabolic flexibility, return to a 12-hour window, avoid simple sugars, and lengthen the window progressively.
- Nausea or sluggishness: ensure you have regular bowel movements before opening your window; constipation holds onto toxicity and makes fasting harder.
- Fatigue: do not stop moving. Scale back intensity, but movement raises energy demand and helps burn stored fat to refuel faster.
Safety first
Always check with your doctor before starting intermittent fasting, especially if you are diabetic or on medication. Contraindications include cachexia, BMI under 18, current or past eating disorders, uncontrolled hyperthyroidism, significant mental health issues, liver or kidney insufficiency, pregnancy and lactation, and children or teens. People on medication require medical supervision. Eat a nutrient-dense, balanced diet before you begin.
Adapted for Elemental Health and Nutrition from the Orthoplex and Bio Concepts fasting guide and the EHN 5:2 handout. See also the companion resource “Fasting as a curative diet”.
Frequently asked questions
What is the difference between intermittent fasting and time-restricted eating?
Both change when you eat rather than only what you eat. Time-restricted eating (TRE) narrows your daily eating window, with common patterns being 12:12, 16:8 or the more condensed 20:4. Intermittent fasting more broadly includes patterns like one-meal-a-day or the 5:2 approach, where you eat normally for five days and keep two non-consecutive days very low in calories. Both aim to support metabolic flexibility, insulin regulation, digestive rest and the gut microbiome.
What can you eat or drink while fasting?
During the fasting window, water is essential, and herbal tea and a cup or two of black coffee are generally fine, while caloric drinks are avoided. Electrolytes without sweeteners, fat-soluble vitamins and omega-3s taken with food are usually compatible. Foods that spike insulin, such as protein powders, gummies and some prebiotic fibres, will break a fast, so it is worth checking excipients. During eating windows the emphasis is on whole foods, plenty of non-starchy vegetables and healthy fats.
Who should not try intermittent fasting?
Intermittent fasting is not suitable for everyone, and it is best to check with your doctor first. Contraindications listed include a very low BMI under 18 or cachexia, a history of eating disorders, uncontrolled hyperthyroidism, liver or kidney insufficiency, pregnancy and breastfeeding, children and teenagers, and people on medication that needs to be taken with food. Symptoms like light-headedness or fatigue can also signal that the approach needs adjusting.
Reviewed by Rohan Smith, BHSc Nutritional Medicine · Elemental Health & Nutrition, Adelaide. Last reviewed 13 June 2026.
Important: This summary is general information, not personalised medical advice, diagnosis, or a treatment protocol. Speak with a qualified practitioner about your individual situation. Book a consultation →