Building an Investment Grade Corporate Bond Portfolio: Spreads, Duration and Credit Selection

Investment grade (IG) corporate bonds occupy the core of most institutional fixed income portfolios. They offer higher yields than government bonds, diversification from equity risk, and — in most economic environments — reasonable predictability of income and capital return. For sophisticated investors constructing a fixed income allocation, understanding the mechanics of IG corporate bond investing is fundamental.

Capital is at risk. Bond prices fall when interest rates rise, and corporate bonds can lose value if credit conditions deteriorate. This guide is for information only and does not constitute regulated investment advice.

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What Is Investment Grade?

Investment grade refers to bonds rated BBB-/Baa3 or above by the major rating agencies (S&P, Moody's and Fitch). The rating spectrum for investment grade runs from AAA (highest quality, rare outside government and quasi-government issuers) down through AA, A, and BBB. The dividing line between investment grade and high yield is the BBB-/Baa3 boundary.

BBB-rated bonds — the lowest tier of investment grade — are particularly important because:

• They comprise a large share of the IG index (often 40–50% of the iBoxx £ Corporate Index by weight)
• They carry the highest spread within IG, offering more income
• They face the most "fallen angel" risk — the possibility of downgrade to high yield
• Many institutional investors face regulatory or mandate constraints on holding below-investment-grade bonds, meaning a downgrade forces selling regardless of value

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Spread Over Gilts (or Treasuries)

The fundamental metric for assessing an investment grade corporate bond's value is its spread — the additional yield it offers over the equivalent-maturity government bond (gilt in the UK, Treasury in the US). This spread compensates investors for:

• Default risk: the probability-weighted loss from a bond default
• Liquidity risk: corporate bonds are less liquid than government bonds; the spread compensates for the wider bid-offer spread and the difficulty of selling quickly in size
• Uncertainty premium: additional compensation beyond purely expected defaults

Option-adjusted spread (OAS) adjusts for the value of any embedded options — particularly call options (which allow the issuer to redeem the bond early). OAS is the preferred measure for comparing callable bonds.

Spread levels fluctuate considerably across economic cycles:

• In benign credit conditions (2020–2022 in the post-Covid period), investment grade spreads compressed to 50–100 basis points on A-rated bonds
• During the 2008 financial crisis, IG spreads widened to 300–400 basis points even for high-quality issuers as liquidity dried up
• As of 2025–2026, IG spreads have been at historically tight levels relative to historical averages, reducing the compensation for credit risk

Tight spreads do not mean defaults will spike — they reflect the market's assessment of low near-term default risk — but they do mean the incremental yield pickup over gilts is at the low end of its historical range.

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Duration Management

Duration is the measure of a bond's sensitivity to interest rate changes. A bond with a duration of seven years will fall approximately 7% in price if interest rates rise by 1 percentage point.

Investment grade corporate bonds carry two components of duration risk:

Interest rate duration (also called modified duration): exposure to moves in the underlying government bond yield curve. This can be hedged using interest rate swaps or government bond futures.

Credit spread duration: exposure to changes in credit spreads. If spreads widen, the bond's price falls. Credit spread duration cannot be fully hedged without selling the bond (or buying credit default swap protection, which is typically available for large institutional investors only).

The interaction matters: in a severe recession, government bond yields typically fall (price rises) as central banks cut rates, but credit spreads simultaneously widen (price falls). Whether the net effect on IG bonds is positive or negative depends on the relative magnitude. During the 2008 crisis, spread widening more than offset rate rally; during 2022, both effects worked in the same direction (rates rose and spreads also widened, meaning IG bonds fell sharply).

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Credit Selection vs Index Approach

Index approach (passive): an investor buys a fund or ETF that tracks a standard IG bond index such as the iBoxx £ Corporates, Bloomberg Global Aggregate or Bloomberg US Corporate Investment Grade Index. This provides diversification at low cost but means you own the entire market, including the weakest BBB- credits and the sectors you may wish to underweight.

Active credit selection: a portfolio manager analyses individual issuers, selecting bonds where the spread is attractive relative to the assessed credit quality and avoiding issuers with deteriorating fundamentals. Skilled active managers can add value by:

• Underweighting fallen angel candidates before downgrade
• Identifying relative value within sectors (e.g., one bank's bonds offering more spread than a comparable peer for similar risk)
• Managing duration proactively to reflect rate views
• Accessing new issues in the primary market at new issue concessions (a discount to secondary market pricing)

The evidence on active IG bond management is mixed. Unlike equities, where information asymmetry can be large, IG corporate bond markets are analysed by many participants and information is widely available. Active management fees (typically 0.4–0.6% per annum) need to be justified by consistent alpha generation.

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Investment Grade Bond ETFs

Low-cost IG bond ETFs have become the dominant vehicle for retail and HNW IG exposure in recent years.

iShares Core £ Corp Bond UCITS ETF (ticker: SLXX): tracks the iBoxx £ Liquid Corporates Index; sterling-denominated, hedged for UK investors.

Vanguard USD Corporate Bond UCITS ETF: tracks the Bloomberg USD Corporate Index; dollar-denominated US investment grade exposure.

iShares USD Corp Bond ESG UCITS ETF: IG bonds with ESG screens applied to the index.

Key considerations when using IG bond ETFs:

• Currency: are you buying sterling, dollar or euro-denominated bonds? Currency risk can easily overwhelm credit spread returns if unhedged
• Duration: different indices have different durations; the iBoxx £ Corporates typically runs around 7–9 years duration, which is substantial interest rate risk
• Sector concentration: IG indices are heavily weighted toward financials (banks and insurers) which are large issuers. If you have equity exposure to the banking sector, this adds correlation
• Bid-offer spread: in normal conditions, ETF dealing spreads are tight; in stressed markets they can widen materially as the underlying bond market liquidity deteriorates

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Callable Bonds and Covenant-Lite Concerns

Callable bonds give the issuer the right to redeem early, typically at par or a small premium. This means investors in callable bonds have "negative convexity" — when rates fall and the bond would otherwise appreciate, the issuer calls it, capping upside. When rates rise, the bond is not called and the investor holds a bond with poor performance characteristics. Investors require a higher yield to compensate for this option being granted to the issuer.

Covenant-lite (cov-lite) structures refer to the erosion of protective covenants (terms restricting what the borrower can do while the debt is outstanding) in leveraged loans and, increasingly, in investment grade bonds. The low-rate era of 2010–2022 allowed issuers to negotiate weaker covenant protections. This means bondholders have fewer contractual protections against management actions that could harm creditors — such as additional leverage, asset disposals, or dividend recapitalisations.

Investors should not assume that investment grade rating implies strong covenant protection. Review covenant terms, particularly for BBB-rated issuers in sectors with high M&A activity.

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Sector and Issuer Diversification

A well-constructed IG corporate bond portfolio should diversify across:

• Sectors: financials, utilities, consumer goods, healthcare, energy, technology, real estate
• Geographies: UK, US, European issuers (within a sterling-hedged framework if targeting sterling returns)
• Maturities: spreading across the yield curve reduces reinvestment and interest rate timing risk
• Individual issuers: no single issuer should represent more than 3–5% of the portfolio; industry guidelines typically suggest 10% maximum per sector

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How Global Investments Can Help

Constructing an investment grade bond portfolio that is genuinely diversified, duration-appropriate and positioned for the current credit and rate environment requires ongoing analysis and professional execution. Our fixed income team advises on the appropriate mix of active funds, ETFs and individual bonds, duration positioning relative to your liabilities and investment horizon, and currency hedging strategy. We can also assess your existing fixed income exposure for concentration, fallen angel candidates, and alignment with current market conditions.

Contact us to discuss your fixed income portfolio strategy.